--- /dev/null
+use std::rc::Rc;
+use std::cell::RefCell;
+
+#[derive(Clone)]
+pub struct Callbacks {
+ callbacks: Vec<Rc<RefCell<FnMut(i32)>>>,
+}
+
+impl Callbacks {
+ pub fn new() -> Self {
+ Callbacks { callbacks: Vec::new() } /*@*/
+ }
+
+ pub fn register<F: FnMut(i32)+'static>(&mut self, callback: F) {
+ let cell = Rc::new(RefCell::new(callback));
+ self.callbacks.push(cell); /*@*/
+ }
+
+ pub fn call(&self, val: i32) {
+ for callback in self.callbacks.iter() {
+ // We have to *explicitly* borrow the contents of a `RefCell`.
+ //@ At run-time, the cell will keep track of the number of outstanding shared and mutable borrows,
+ //@ and panic if the rules are violated. Since this function is the only one that borrow the
+ //@ environments of the closures, and this function requires a *mutable* borrow of `self`, we know this cannot
+ //@ happen. <br />
+ //@ For this check to be performed, `closure` is a *guard*: Rather than a normal borrow, `borrow_mut` returns
+ //@ a smart pointer (`RefMut`, in this case) that waits until is goes out of scope, and then
+ //@ appropriately updates the number of active borrows.
+ //@
+ //@ The function would still typecheck with an immutable borrow of `self` (since we are
+ //@ relying on the interior mutability of `self`), but then it could happen that a callback
+ //@ will in turn trigger another round of callbacks, so that `call` would indirectly call itself.
+ //@ This is called reentrancy. It would imply that we borrow the closure a second time, and
+ //@ panic at run-time. I hope this also makes it clear that there's absolutely no hope of Rust
+ //@ performing these checks statically, at compile-time: It would have to detect reentrancy!
+ let mut closure = callback.borrow_mut();
+ // Unfortunately, Rust's auto-dereference of pointers is not clever enough here. We thus have to explicitly
+ // dereference the smart pointer and obtain a mutable borrow of the target.
+ (&mut *closure)(val);
+ }
+ }
+}
+
+#[cfg(test)]
+mod tests {
+ use std::rc::Rc;
+ use std::cell::RefCell;
+ use super::*;
+
+ #[test]
+ #[should_panic]
+ fn test_reentrant() {
+ let c = Rc::new(RefCell::new(Callbacks::new()));
+ c.borrow_mut().register(|val| println!("Callback called: {}", val) );
+
+ // If we change the two "borrow" below to "borrow_mut", you can get a panic even with a "call" that requires a
+ // mutable borrow. However, that panic is then triggered by our own, external `RefCell` (so it's kind of our fault),
+ // rather than being triggered by the `RefCell` in the `Callbacks`.
+ {
+ let c2 = c.clone();
+ c.borrow_mut().register(move |val| c2.borrow().call(val+val) );
+ }
+
+ c.borrow().call(42);
+ }
+}
\ No newline at end of file
use std::thread;
#[derive(Clone)]
-struct ConcurrentCounter(Arc<RwLock<usize>>);
+pub struct ConcurrentCounter(Arc<RwLock<usize>>);
impl ConcurrentCounter {
// The constructor should not be surprising.
*counter = *counter + by;
}
+ pub fn compare_and_inc(&self, test: usize, by: usize) {
+ let mut counter = self.0.write().unwrap();
+ if *counter == test {
+ *counter += by;
+ }
+ }
+
pub fn get(&self) -> usize {
let counter = self.0.read().unwrap();
*counter
pub mod vec;
pub mod rgrep;
pub mod counter;
+pub mod callbacks;
pub fn main() {
rgrep::main();
// ---------------
//
// You will need to have Rust installed, of course. It is available for download on
-// [the Rust website](http://www.rust-lang.org/). You should go for either the "stable"
-// or the "beta" channel. More detailed installation instructions are provided in
-// [the second chapter of The Book](https://doc.rust-lang.org/stable/book/installing-rust.html).
+// [the Rust website](http://www.rust-lang.org/). Make sure you get at least version 1.2
+// (at the time of writing, that's the current beta release). More detailed installation
+// instructions are provided in [the second chapter of The Book](https://doc.rust-lang.org/stable/book/installing-rust.html).
// This will also install `cargo`, the tool responsible for building rust projects (or *crates*).
// Next, fetch the Rust-101 source code from the [git repository](http://www.ralfj.de/git/rust-101.git)
// * [Part 08: Associated Types, Modules](part08.html)
// * [Part 09: Iterators](part09.html)
// * [Part 10: Closures](part10.html)
-// * [Part 11: Trait Objects, Box, Rc, Lifetime bounds](part11.html)
-// * [Part 12: Concurrency, Arc, Send](part12.html)
-// * [Part 13: Slices, Arrays, External Dependencies](part13.html)
//
// ### Advanced Rust
//
-// * [Part 14: Mutex, Interior Mutability, Sync](part14.html)
+// * [Part 11: Trait Objects, Box, Lifetime bounds](part11.html)
+// * [Part 12: Rc, Interior Mutability, Cell, RefCell](part12.html)
+// * [Part 13: Concurrency, Arc, Send](part13.html)
+// * [Part 14: Slices, Arrays, External Dependencies](part14.html)
+// * [Part 15: Mutex, Interior Mutability (cont.), Sync](part15.html)
// * (to be continued)
//
#![allow(dead_code, unused_imports, unused_variables, unused_mut, unreachable_code)]
-// Rust-101, Part 11: Trait Objects, Box, Rc, Lifetime bounds
-// ==========================================================
+// Rust-101, Part 11: Trait Objects, Box, Lifetime bounds
+// ======================================================
//@ We will play around with closures a bit more. Let us implement some kind of generic "callback"
-//@ mechanism, providing two functions: Registering a new callback, and calling all registered callbacks. There will be two
-//@ versions, so to avoid clashes of names, we put them into modules.
-mod callbacks {
- //@ First of all, we need to find a way to store the callbacks. Clearly, there will be a `Vec` involved, so that we can
- //@ always grow the number of registered callbacks. A callback will be a closure, i.e., something implementing
- //@ `FnMut(i32)` (we want to call this multiple times, so clearly `FnOnce` would be no good). So our first attempt may be the following.
- // For now, we just decide that the callbacks have an argument of type `i32`.
- struct CallbacksV1<F: FnMut(i32)> {
- callbacks: Vec<F>,
- }
- //@ However, this will not work. Remember how the "type" of a closure is specific to the environment of captured variables. Different closures
- //@ all implementing `FnMut(i32)` may have different types. However, a `Vec<F>` is a *uniformly typed* vector.
-
- //@ We will thus need a way to store things of *different* types in the same vector. We know all these types implement `FnMut(i32)`. For this scenario,
- //@ Rust provides *trait objects*: The truth is, `FnMut(i32)` is not just a trait. It is also a type, that can be given to anything implementing
- //@ this trait. So, we may write the following.
- /* struct CallbacksV2 {
- callbacks: Vec<FnMut(i32)>,
- } */
- //@ But, Rust complains about this definition. It says something about "Sized". What's the trouble? See, for many things we want to do, it is crucial that
- //@ Rust knows the precise, fixed size of the type - that is, how large this type will be when represented in memory. For example, for a `Vec`, the
- //@ elements are stored one right after the other. How should that be possible, without a fixed size? The trouble is, `FnMut(i32)` could be of any size.
- //@ We don't know how large that "type that implemenets `FnMut(i32)`" is. Rust calls this an *unsized* type. Whenever we introduce a type variable, Rust
- //@ will implicitly add a bound to that variable, demanding that it is sized. That's why we did not have to worry about this so far. <br/>
- //@ You can opt-out of this implicit bound by saying `T: ?Sized`. Then `T` may or may not be sized.
-
- //@ So, what can we do, if we can't store the callbacks in a vector? We can put them in a box. Semantically, `Box<T>` is a lot like `T`: You fully own
- //@ the data stored there. On the machine, however, `Box<T>` is a *pointer* to `T`. It is a lot like `std::unique_ptr` in C++. In our current example,
- //@ the important bit is that since it's a pointer, `T` can be unsized, but `Box<T>` itself will always be sized. So we can put it in a `Vec`.
- pub struct Callbacks {
- callbacks: Vec<Box<FnMut(i32)>>,
- }
-
- impl Callbacks {
- // Now we can provide some functions. The constructor should be straight-forward.
- pub fn new() -> Self {
- Callbacks { callbacks: Vec::new() } /*@*/
- }
-
- // Registration simply stores the callback.
- pub fn register(&mut self, callback: Box<FnMut(i32)>) {
- self.callbacks.push(callback); /*@*/
- }
-
- // And here we call all the stored callbacks.
- pub fn call(&mut self, val: i32) {
- // Since they are of type `FnMut`, we need to mutably iterate. Notice that boxes dereference implicitly.
- for callback in self.callbacks.iter_mut() {
- callback(val); /*@*/
- }
- }
- }
-
- // Now we are ready for the demo.
- pub fn demo(c: &mut Callbacks) {
- c.register(Box::new(|val| println!("Callback 1: {}", val)));
- c.call(0);
-
- //@ We can even register callbacks that modify their environment. Rust will again attempt to borrow `count`. However,
- //@ that doesn't work out this time: Since we want to put this thing in a `Box`, it could live longer than the function
- //@ we are in. Then the borrow of `count` would become invalid. We have to explicitly tell Rust to `move` ownership of the
- //@ variable into the closure. Its environment will then contain a `usize` rather than a `&mut uszie`, and have
- //@ no effect on this local variable anymore.
- let mut count: usize = 0;
- c.register(Box::new(move |val| {
- count = count+1;
- println!("Callback 2, {}. time: {}", count, val);
- } ));
- c.call(1); c.call(2);
- }
+//@ mechanism, providing two functions: Registering a new callback, and calling all registered callbacks.
+
+//@ First of all, we need to find a way to store the callbacks. Clearly, there will be a `Vec` involved, so that we can
+//@ always grow the number of registered callbacks. A callback will be a closure, i.e., something implementing
+//@ `FnMut(i32)` (we want to call this multiple times, so clearly `FnOnce` would be no good). So our first attempt may be the following.
+// For now, we just decide that the callbacks have an argument of type `i32`.
+struct CallbacksV1<F: FnMut(i32)> {
+ callbacks: Vec<F>,
}
-
-// Remember to edit `main.rs` to run the demo.
-pub fn main() {
- let mut c = callbacks::Callbacks::new();
- callbacks::demo(&mut c);
+//@ However, this will not work. Remember how the "type" of a closure is specific to the environment of captured variables. Different closures
+//@ all implementing `FnMut(i32)` may have different types. However, a `Vec<F>` is a *uniformly typed* vector.
+
+//@ We will thus need a way to store things of *different* types in the same vector. We know all these types implement `FnMut(i32)`. For this scenario,
+//@ Rust provides *trait objects*: The truth is, `FnMut(i32)` is not just a trait. It is also a type, that can be given to anything implementing
+//@ this trait. So, we may write the following.
+/* struct CallbacksV2 {
+ callbacks: Vec<FnMut(i32)>,
+} */
+//@ But, Rust complains about this definition. It says something about "Sized". What's the trouble? See, for many things we want to do, it is crucial that
+//@ Rust knows the precise, fixed size of the type - that is, how large this type will be when represented in memory. For example, for a `Vec`, the
+//@ elements are stored one right after the other. How should that be possible, without a fixed size? The point is, `FnMut(i32)` could be of any size.
+//@ We don't know how large that "type that implemenets `FnMut(i32)`" is. Rust calls this an *unsized* type. Whenever we introduce a type variable, Rust
+//@ will implicitly add a bound to that variable, demanding that it is sized. That's why we did not have to worry about this so far. <br/>
+//@ You can opt-out of this implicit bound by saying `T: ?Sized`. Then `T` may or may not be sized.
+
+//@ So, what can we do, if we can't store the callbacks in a vector? We can put them in a box. Semantically, `Box<T>` is a lot like `T`: You fully own
+//@ the data stored there. On the machine, however, `Box<T>` is a *pointer* to a heap-allocated `T`. It is a lot like `std::unique_ptr` in C++. In our current example,
+//@ the important bit is that since it's a pointer, `T` can be unsized, but `Box<T>` itself will always be sized. So we can put it in a `Vec`.
+pub struct Callbacks {
+ callbacks: Vec<Box<FnMut(i32)>>,
}
-mod callbacks_clone {
- //@ So, this worked great, didn't it! There's one point though that I'd like to emphasize: One cannot `clone` a closure.
- //@ Hence it becomes impossible to implement `Clone` for our `Callbacks` type. What could we do about this?
-
- //@ You already learned about `Box` above. `Box` is an example of a *smart pointer*: It's like a pointer (in the C
- //@ sense), but with some additional smarts to it. For `Box`, that's the part about ownership. Once you drop the box, the
- //@ content it points to will be deleted. <br/>
- //@ Another example of a smart pointer is `Rc<T>`. This is short for *reference-counter*, so you can already guess how
- //@ this pointer is smart: It has a reference count. You can `clone` an `Rc` as often as you want, that doesn't affect the
- //@ data it contains at all. It only creates more references to the same data. Once all the references are gone, the data is deleted.
- //@
- //@ Wait a moment, you may say here. Multiple references to the same data? That's aliasing! Indeed:
- //@ Once data is stored in an `Rc`, it is read-only. By dereferencing the smart `Rc`, you can only get a shared borrow of the data.
- use std::rc::Rc;
-
- //@ Because of this read-only restriction, we cannot use `FnMut` here: We'd be unable to call the function with a mutable borrow
- //@ of it's environment! So we have to go with `Fn`. We wrap that in an `Rc`, and then Rust happily derives `Clone` for us.
- #[derive(Clone)]
- pub struct Callbacks {
- callbacks: Vec<Rc<Fn(i32)>>,
+impl Callbacks {
+ // Now we can provide some functions. The constructor should be straight-forward.
+ pub fn new() -> Self {
+ Callbacks { callbacks: Vec::new() } /*@*/
}
- impl Callbacks {
- pub fn new() -> Self {
- Callbacks { callbacks: Vec::new() } /*@*/
- }
+ // Registration simply stores the callback.
+ pub fn register(&mut self, callback: Box<FnMut(i32)>) {
+ self.callbacks.push(callback); /*@*/
+ }
- // For the `register` function, we don't actually have to use trait objects in the argument.
- //@ We can make this function generic, such that it will be instantiated with some concrete closure type `F`
- //@ and do the creation of the `Rc` and the conversion to `Fn(i32)` itself.
-
- //@ For this to work, we need to demand that the type `F` does not contain any short-lived borrows. After all, we will store it
- //@ in our list of callbacks indefinitely. If the closure contained a pointer to our caller's stackframe, that pointer
- //@ could be invalid by the time the closure is called. We can mitigate this by bounding `F` by a *lifetime*: `T: 'a` says
- //@ that all data of type `T` will *outlive* (i.e., will be valid for at least as long as) lifetime `'a`.
- //@ Here, we use the special lifetime `'static`, which is the lifetime of the entire program.
- //@ The same bound has been implicitly added in the version of `register` above, and in the definition of
- //@ `Callbacks`. This is the reason we could not have the borrowed `count` in the closure in `demo` previously.
- pub fn register<F: Fn(i32)+'static>(&mut self, callback: F) {
- self.callbacks.push(Rc::new(callback)); /*@*/
- }
+ // We can also write a generic version of `register`, such that it will be instantiated with some concrete closure type `F`
+ // and do the creation of the `Box` and the conversion from `F` to `FnMut(i32)` itself.
+
+ //@ For this to work, we need to demand that the type `F` does not contain any short-lived borrows. After all, we will store it
+ //@ in our list of callbacks indefinitely. If the closure contained a pointer to our caller's stackframe, that pointer
+ //@ could be invalid by the time the closure is called. We can mitigate this by bounding `F` by a *lifetime*: `F: 'a` says
+ //@ that all data of type `F` will *outlive* (i.e., will be valid for at least as long as) lifetime `'a`.
+ //@ Here, we use the special lifetime `'static`, which is the lifetime of the entire program.
+ //@ The same bound has been implicitly added in the version of `register` above, and in the definition of
+ //@ `Callbacks`.
+ pub fn register_generic<F: FnMut(i32)+'static>(&mut self, callback: F) {
+ self.callbacks.push(Box::new(callback)); /*@*/
+ }
- pub fn call(&mut self, val: i32) {
- // We only need a shared iterator here. `Rc` also implicitly dereferences, so we can simply call the callback.
- for callback in self.callbacks.iter() {
- callback(val); /*@*/
- }
+ // And here we call all the stored callbacks.
+ pub fn call(&mut self, val: i32) {
+ // Since they are of type `FnMut`, we need to mutably iterate.
+ for callback in self.callbacks.iter_mut() {
+ //@ Here, `callback` has type `&mut Box<FnMut(i32)>`. We can make use of the fact that `Box` is a *smart pointer*: In
+ //@ particular, we can use it as if it were a normal pointer, and use `*` to get to its contents. Then we mutably borrow
+ //@ these contents, because we call a `FnMut`.
+ (&mut *callback)(val); /*@*/
+ //@ Just like it is the case with normal borrows, this typically happens implicitly, so we can also directly call the function.
+ //@ Try removing the `&mut *`.
+ //@
+ //@ The difference to a normal pointer is that `Box` implies ownership: Once you drop the box (i.e., when the entire `Callbacks` instance is
+ //@ dropped), the content it points to on the heap will be deleted.
}
}
+}
- // The demo works just as above. Our counting callback doesn't work anymore though, because we are using `Fn` now.
- fn demo(c: &mut Callbacks) {
- c.register(|val| println!("Callback 1: {}", val));
- c.call(0); c.call(1);
+// Now we are ready for the demo. Remember to edit `main.rs` to run it.
+pub fn main() {
+ let mut c = Callbacks::new();
+ c.register(Box::new(|val| println!("Callback 1: {}", val)));
+ c.call(0);
+
+ {
+ //@ We can even register callbacks that modify their environment. Per default, Rust will attempt to borrow `count`. However,
+ //@ that doesn't work out this time. Remember the `'static` bound above? Borrowing `count` in the environment would
+ //@ violate that bound, as the borrow is only valid for this block. If the callbacks are triggered later, we'd be in trouble.
+ //@ We have to explicitly tell Rust to `move` ownership of the variable into the closure. Its environment will then contain a
+ //@ `usize` rather than a `&mut uszie`, and the closure has no effect on this local variable anymore.
+ let mut count: usize = 0;
+ c.register_generic(move |val| {
+ count = count+1;
+ println!("Callback 2: {} ({}. time)", val, count);
+ } );
}
+ c.call(1); c.call(2);
}
-// **Exercise 11.1**: We made the arbitrary choice of using `i32` for the arguments. Generalize the data-structures above
-// to work with an arbitrary type `T` that's passed to the callbacks. Since you need to call multiple callbacks with the
-// same `t: T`, you will either have to restrict `T` to `Copy` types, or pass a borrow.
-
//@ ## Run-time behavior
//@ When you run the program above, how does Rust know what to do with the callbacks? Since an unsized type lacks some information,
-//@ a *pointer* to such a type (be it a `Box`, an `Rc` or a borrow) will need to complete this information. We say that pointers to
+//@ a *pointer* to such a type (be it a `Box` or a borrow) will need to complete this information. We say that pointers to
//@ trait objects are *fat*. They store not only the address of the object, but (in the case of trait objects) also a *vtable*: A
//@ table of function pointers, determining the code that's run when a trait method is called. There are some restrictions for traits to be usable
//@ as trait objects. This is called *object safety* and described in [the documentation](http://doc.rust-lang.org/stable/book/trait-objects.html) and [the reference](http://doc.rust-lang.org/reference.html#trait-objects).
+//@ In case of the `FnMut` trait, there's only a single action to be performed: Calling the closure. You can thus think of a pointer to `FnMut` as
+//@ a pointer to the code, and a pointer to the environment. This is how Rust recovers the typical encoding of closures as a special case of a more
+//@ general concept.
//@
-//@ Whenever you write a generic function, you have a choice: You can make it polymorphic, like our `vec_min`. Or you
-//@ can use trait objects, like the first `register` above. The latter will result in only a single compiled version (rather
+//@ Whenever you write a generic function, you have a choice: You can make it generic, like `register_generic`. Or you
+//@ can use trait objects, like `register`. The latter will result in only a single compiled version (rather
//@ than one version per type it is instantiated with). This makes for smaller code, but you pay the overhead of the virtual function calls.
-//@ Isn't it beautiful how traits can handle both of these cases (and much more, as we saw, like closures and operator overloading) nicely?
+//@ (Of course, in the case of `register` above, there's no function called on the trait object.)
+//@ Isn't it beautiful how traits can nicely handle this tradeoff (and much more, as we saw, like closures and operator overloading)?
+
+// **Exercise 11.1**: We made the arbitrary choice of using `i32` for the arguments. Generalize the data-structures above
+// to work with an arbitrary type `T` that's passed to the callbacks. Since you need to call multiple callbacks with the
+// same `t: T`, you will either have to restrict `T` to `Copy` types, or pass a borrow.
//@ [index](main.html) | [previous](part10.html) | [next](part12.html)
-// Rust-101, Part 12: Concurrency, Arc, Send
-// =========================================
-
-use std::io::prelude::*;
-use std::{io, fs, thread};
-use std::sync::mpsc::{sync_channel, SyncSender, Receiver};
-use std::sync::Arc;
-
-//@ Our next stop are the concurrency features of Rust. We are going to write our own small version of "grep",
-//@ called *rgrep*, and it is going to make use of concurrency: One thread reads the input files, one thread does
-//@ the actual matching, and one thread writes the output. I already mentioned in the beginning of the course that
-//@ Rust's type system (more precisely, the discipline of ownership and borrowing) will help us to avoid a common
-//@ pitfall of concurrent programming: data races.
-
-// Before we come to the actual code, we define a data-structure `Options` to store all the information we need
-// to complete the job: Which files to work on, which pattern to look for, and how to output. <br/>
-//@ Besides just printing all the matching lines, we will also offer to count them, or alternatively to sort them.
-#[derive(Clone,Copy)]
-pub enum OutputMode {
- Print,
- SortAndPrint,
- Count,
-}
-use self::OutputMode::*;
+// Rust-101, Part 12: Rc, Interior Mutability, Cell, RefCell
+// =========================================================
+
+use std::rc::Rc;
+use std::cell::{Cell, RefCell};
-pub struct Options {
- pub files: Vec<String>,
- pub pattern: String,
- pub output_mode: OutputMode,
+//@ Our generic callback mechanism is already working quite nicely. However, there's one point we may want to fix:
+//@ `Callbacks` does not implement `Clone`. The problem is that closures (or rather, their environment) can never be cloned.
+//@ (There's not even an automatic derivation happening for the cases where it would be possible.)
+//@ This restriction propagates up to `Callbacks` itself. What could we do about this?
+
+//@ The solution is to find some way of cloning `Callbacks` without cloning the environments. This can be achieved with
+//@ `Rc<T>`, a *reference-counted* pointer. This is is another example of a smart pointer. You can `clone` an `Rc` as often
+//@ as you want, that doesn't affect the data it contains. It only creates more references to the same data. Once all the
+//@ references are gone, the data is deleted.
+//@
+//@ Wait a moment, you may say here. Multiple references to the same data? That's aliasing! Indeed:
+//@ Once data is stored in an `Rc`, it is read-only and you can only ever get a shared borrow of the data again.
+
+//@ Because of this read-only restriction, we cannot use `FnMut` here: We'd be unable to call the function with a mutable borrow
+//@ of it's environment! So we have to go with `Fn`. We wrap that in an `Rc`, and then Rust happily derives `Clone` for us.
+#[derive(Clone)]
+struct Callbacks {
+ callbacks: Vec<Rc<Fn(i32)>>,
}
-//@ Now we can write three functions to do the actual job of reading, matching, and printing, respectively.
-//@ To get the data from one thread to the next, we will use *message passing*: We will establish communication
-//@ channels between the threads, with one thread *sending* data, and the other one *receiving* it. `SyncSender<T>`
-//@ is the type of the sending end of a synchronous channel transmitting data of type `T`. *Synchronous* here
-//@ means that the `send` operation could block, waiting for the other side to make progress. We don't want to
-//@ end up with the entire file being stored in the buffer of the channels, and the output not being fast enough
-//@ to keep up with the speed of input.
-//@
-//@ We also need all the threads to have access to the options of the job they are supposed to do. Since it would
-//@ be rather unnecessary to actually copy these options around, we will use reference-counting to share them between
-//@ all threads. `Arc` is the thread-safe version of `Rc`, using atomic operations to keep the reference count up-to-date.
-
-// The first function reads the files, and sends every line over the `out_channel`.
-fn read_files(options: Arc<Options>, out_channel: SyncSender<String>) {
- for file in options.files.iter() {
- // First, we open the file, ignoring any errors.
- let file = fs::File::open(file).unwrap();
- // Then we obtain a `BufReader` for it, which provides the `lines` function.
- let file = io::BufReader::new(file);
- for line in file.lines() {
- let line = line.unwrap();
- // Now we send the line over the channel, ignoring the possibility of `send` failing.
- out_channel.send(line).unwrap();
- }
+impl Callbacks {
+ pub fn new() -> Self {
+ Callbacks { callbacks: Vec::new() } /*@*/
}
- // When we drop the `out_channel`, it will be closed, which the other end can notice.
-}
-// The second function filters the lines it receives through `in_channel` with the pattern, and sends
-// matches via `out_channel`.
-fn filter_lines(options: Arc<Options>,
- in_channel: Receiver<String>,
- out_channel: SyncSender<String>) {
- // We can simply iterate over the channel, which will stop when the channel is closed.
- for line in in_channel.iter() {
- // `contains` works on lots of types of patterns, but in particular, we can use it to test whether
- // one string is contained in another. This is another example of Rust using traits as substitute for overloading.
- if line.contains(&options.pattern) {
- out_channel.send(line).unwrap(); /*@*/
- }
+ // Registration works just like last time, except that we are creating an `Rc` now.
+ pub fn register<F: Fn(i32)+'static>(&mut self, callback: F) {
+ self.callbacks.push(Rc::new(callback)); /*@*/
}
-}
-// The third function performs the output operations, receiving the relevant lines on its `in_channel`.
-fn output_lines(options: Arc<Options>, in_channel: Receiver<String>) {
- match options.output_mode {
- Print => {
- // Here, we just print every line we see.
- for line in in_channel.iter() {
- println!("{}", line); /*@*/
- }
- },
- Count => {
- // We are supposed to count the number of matching lines. There's a convenient iterator adapter that
- // we can use for this job.
- let count = in_channel.iter().count(); /*@*/
- println!("{} hits for {}.", count, options.pattern); /*@*/
- },
- SortAndPrint => {
- // We are asked to sort the matching lines before printing. So let's collect them all in a local vector...
- let mut data: Vec<String> = in_channel.iter().collect();
- // ...and implement the actual sorting later.
- unimplemented!()
+ pub fn call(&self, val: i32) {
+ // We only need a shared iterator here. Since `Rc` is a smart pointer, we can directly call the callback.
+ for callback in self.callbacks.iter() {
+ callback(val); /*@*/
}
}
}
-// With the operations of the three threads defined, we can now implement a function that performs grepping according
-// to some given options.
-pub fn run(options: Options) {
- // We move the `options` into an `Arc`, as that's what the thread workers expect.
- let options = Arc::new(options);
-
- // This sets up the channels. We use a `sync_channel` with buffer-size of 16 to avoid needlessly filling RAM.
- let (line_sender, line_receiver) = sync_channel(16);
- let (filtered_sender, filtered_receiver) = sync_channel(16);
-
- // Spawn the read thread: `thread::spawn` takes a closure that is run in a new thread.
- //@ The `move` keyword again tells Rust that we want ownership of captured variables to be moved into the
- //@ closure. This means we need to do the `clone` *first*, otherwise we would lose our `options` to the
- //@ new thread!
- let options1 = options.clone();
- let handle1 = thread::spawn(move || read_files(options1, line_sender));
-
- // Same with the filter thread.
- let options2 = options.clone();
- let handle2 = thread::spawn(move || {
- filter_lines(options2, line_receiver, filtered_sender)
- });
-
- // And the output thread.
- let options3 = options.clone();
- let handle3 = thread::spawn(move || output_lines(options3, filtered_receiver));
-
- // Finally, wait until all three threads did their job.
- //@ Joining a thread waits for its termination. This can fail if that thread panicked: In this case, we could get
- //@ access to the data that it provided to `panic!`. Here, we just assert that they did not panic - so we will panic ourselves
- //@ if that happened.
- handle1.join().unwrap();
- handle2.join().unwrap();
- handle3.join().unwrap();
+// Time for a demo!
+fn demo(c: &mut Callbacks) {
+ c.register(|val| println!("Callback 1: {}", val));
+ c.call(0); c.clone().call(1);
}
-// Now we have all the pieces together for testing our rgrep with some hard-coded options.
-//@ We need to call `to_string` on string literals to convert them to a fully-owned `String`.
pub fn main() {
- let options = Options {
- files: vec!["src/part10.rs".to_string(),
- "src/part11.rs".to_string(),
- "src/part12.rs".to_string()],
- pattern: "let".to_string(),
- output_mode: Print
- };
- run(options);
+ let mut c = Callbacks::new();
+ demo(&mut c);
}
-// **Exercise 12.1**: Change rgrep such that it prints not only the matching lines, but also the name of the file
-// and the number of the line in the file. You will have to change the type of the channels from `String` to something
-// that records this extra information.
-
-//@ ## Ownership, Borrowing, and Concurrency
-//@ The little demo above showed that concurrency in Rust has a fairly simple API. Considering Rust has closures,
-//@ that should not be entirely surprising. However, as it turns out, Rust goes well beyond this and actually ensures
-//@ the absence of data races. <br/>
-//@ A data race is typically defined as having two concurrent, unsynchronized
-//@ accesses to the same memory location, at least one of which is a write. In other words, a data race is mutation in
-//@ the presence of aliasing, which Rust reliably rules out! It turns out that the same mechanism that makes our single-threaded
-//@ programs memory safe, and that prevents us from invalidating iterators, also helps secure our multi-threaded code against
-//@ data races. For example, notice how `read_files` sends a `String` to `filter_lines`. At run-time, only the pointer to
-//@ the character data will actually be moved around (just like when a `String` is passed to a function with full ownership). However,
-//@ `read_files` has to *give up* ownership of the string to perform `send`, to it is impossible for an outstanding borrow to
-//@ still be around. After it sent the string to the other side, `read_files` has no pointer into the string content
-//@ anymore, and hence no way to race on the data with someone else.
-//@
-//@ There is a little more to this. Remember the `'static` bound we had to add to `register` in the previous part, to make
-//@ sure that the callbacks do not reference any pointers that might become invalid? This is just as crucial for spawning
-//@ a thread: In general, that thread could last for much longer than the current stack frame. Thus, it must not use
-//@ any pointers to data in that stack frame. This is achieved by requiring the `FnOnce` closure passed to `thread::spawn`
-//@ to be valid for lifetime `'static`, as you can see in [its documentation](http://doc.rust-lang.org/stable/std/thread/fn.spawn.html).
-//@ This avoids another kind of data race, where the thread's access races with the callee deallocating its stack frame.
-//@ It is only thanks to the concept of lifetimes that this can be expressed as part of the type of `spawn`.
-
-//@ ## Send
-//@ However, the story goes even further. I said above that `Arc` is a thread-safe version of `Rc`, which uses atomic operations
-//@ to manipulate the reference count. It is thus crucial that we don't use `Rc` across multiple threads, or the reference count may
-//@ become invalid. And indeed, if you replace `Arc` by `Rc` (and add the appropriate imports), Rust will tell you that something
-//@ is wrong. That's great, of course, but how did it do that?
-//@
-//@ The answer is already hinted at in the error: It will say something about `Send`. You may have noticed that the closure in
-//@ `thread::spawn` does not just have a `'static` bound, but also has to satisfy `Send`. `Send` is a trait, and just like `Copy`,
-//@ it's just a marker - there are no functions provided by `Send`. What the trait says is that types which are `Send`, can be
-//@ safely sent to another thread without causing trouble. Of course, all the primitive data-types are `Send`. So is `Arc`,
-//@ which is why Rust accepted our code. But `Rc` is not `Send`, and for a good reason!
+// ## Interior Mutability
+//@ Of course, the counting example from last time does not work anymore: It needs to mutate the environment, which a `Fn`
+//@ cannot do. The strict borrowing Rules of Rust are getting into our way. However, when it comes to mutating a mere number
+//@ (`usize`), there's not really any chance of problems coming up. Everybody can read and write that variable just as they want.
+//@ So it would be rather sad if we were not able to write this program. Lucky enough, Rust's standard library provides a
+//@ solution in the form of `Cell<T>`. This type represents a memory cell of some type `T`, providing the two basic operations
+//@ `get` and `set`. `get` returns a *copy* of the content of the cell, so all this works only if `T` is `Copy`.
+//@ `set`, which overrides the content, only needs a *shared borrow* of the cell. The phenomenon of a type that permits mutation through
+//@ shared borrows (i.e., mutation despite the possibility of aliasing) is called *interior mutability*. You can think
+//@ of `set` changing only the *contents* of the cell, not its *identity*. In contrast, the kind of mutation we saw so far was
+//@ about replacing one piece of data by something else of the same type. This is called *exterior mutability*. <br/>
+//@ Notice that it is impossible to *borrow* the contents of the cell, and that is actually the key to why this is safe.
+
+// So, let us put our counter in a `Cell`, and replicate the example from the previous part.
+fn demo_cell(c: &mut Callbacks) {
+ {
+ let count = Cell::new(0);
+ // Again, we have to move ownership if the `count` into the environment closure.
+ c.register(move |val| {
+ // In here, all we have is a shared borrow of our environment. But that's good enough for the `get` and `set` of the cell!
+ //@ At run-time, the `Cell` will be almost entirely compiled away, so this becomes pretty much equivalent to the version
+ //@ we wrote in the previous part.
+ let new_count = count.get()+1;
+ count.set(new_count);
+ println!("Callback 2: {} ({}. time)", val, new_count);
+ } );
+ }
+
+ c.call(2); c.clone().call(3);
+}
+
+//@ It is worth mentioning that `Rc` itself also has to make use of interior mutability: When you `clone` an `Rc`, all it has available
+//@ is a shared borrow. However, it has to increment the reference count! Internally, `Rc` uses `Cell` for the count, such that it
+//@ can be updated during `clone`.
+
+// ## `RefCell`
+//@ As the next step in the evolution of `Callbacks`, we could try to solve this problem of mutability once and for all, by adding `Cell`
+//@ to `Callbacks` such that clients don't have to worry about this. However, that won't end up working: Remember that `Cell` only works
+//@ with types that are `Copy`, which the environment of a closure will never be. We need a variant of `Cell` that allows borrowing its
+//@ contents, such that we can provide a `FnMut` with its environment. But if `Cell` would allow that, we could write down all those
+//@ crashing C++ programs that we wanted to get rid of.
//@
-//@ Now, `Send` as a trait is fairly special. It has a so-called *default implementation*. This means that *every type* implements
-//@ `Send`, unless it opts out. Opting out is viral: If your type contains a type that opted out, then you don't have `Send`, either.
-//@ So if the environment of your closure contains an `Rc`, it won't be `Send`, preventing it from causing trouble. If however every
-//@ captured variable *is* `Send`, then so is the entire environment, and you are good.
+//@ This is the point where our program got too complex for Rust to guarantee at compile-time that nothing bad will happen. Since we don't
+//@ want to give up the safety guarantee, we are going to need some code that actually checks at run-time that the borrowing rules
+//@ are not violated. Such a check is provided by `RefCell<T>`: Unlike `Cell<T>`, this lets us borrow the contents, and it works for
+//@ non-`Copy` `T`. But, as we will see, it incurs some run-time overhead.
+
+// Our final version of `Callbacks` puts the closure environment into a `RefCell`.
+#[derive(Clone)]
+struct CallbacksMut {
+ callbacks: Vec<Rc<RefCell<FnMut(i32)>>>,
+}
+
+impl CallbacksMut {
+ pub fn new() -> Self {
+ CallbacksMut { callbacks: Vec::new() } /*@*/
+ }
+
+ pub fn register<F: FnMut(i32)+'static>(&mut self, callback: F) {
+ let cell = Rc::new(RefCell::new(callback));
+ self.callbacks.push(cell); /*@*/
+ }
+
+ pub fn call(&mut self, val: i32) {
+ for callback in self.callbacks.iter() {
+ // We have to *explicitly* borrow the contents of a `RefCell` by calling `borrow` or `borrow_mut`.
+ //@ At run-time, the cell will keep track of the number of outstanding shared and mutable borrows,
+ //@ and panic if the rules are violated. <br />
+ //@ For this check to be performed, `closure` is a *guard*: Rather than a normal borrow, `borrow_mut` returns
+ //@ a smart pointer (`RefMut`, in this case) that waits until is goes out of scope, and then
+ //@ appropriately updates the number of active borrows.
+ //@
+ //@ Since `call` is the only place that borrows the environments of the closures, we should expect that
+ //@ the check will always succeed. However, this function would still typecheck with an immutable borrow of `self` (since we are
+ //@ relying on the interior mutability of `RefCell`). Under this condition, it could happen that a callback
+ //@ will in turn trigger another round of callbacks, so that `call` would indirectly call itself.
+ //@ This is called reentrancy. It would imply that we borrow the closure a second time, and
+ //@ panic at run-time. I hope this also makes it clear that there's absolutely no hope of Rust
+ //@ performing these checks statically, at compile-time: It would have to detect reentrancy!
+ let mut closure = callback.borrow_mut();
+ // Unfortunately, Rust's auto-dereference of pointers is not clever enough here. We thus have to explicitly
+ // dereference the smart pointer and obtain a mutable borrow of the content.
+ (&mut *closure)(val);
+ }
+ }
+}
+
+// Now we can repeat the demo from the previous part - but this time, our `CallbacksMut` type
+// can be cloned.
+fn demo_mut(c: &mut CallbacksMut) {
+ c.register(|val| println!("Callback 1: {}", val));
+ c.call(0);
+
+ {
+ let mut count: usize = 0;
+ c.register(move |val| {
+ count = count+1;
+ println!("Callback 2: {} ({}. time)", val, count);
+ } );
+ }
+ c.call(1); c.clone().call(2);
+}
+
+// **Exercise 12.1**: Change the type of `call` to ask only for a shared borrow. Then write some piece of code using only the available, public
+// interface of `CallbacksMut` such that a reentrant call to `call` is happening, and the program aborts because the `RefCell` refuses to hand
+// out a second mutable borrow to its content.
//@ [index](main.html) | [previous](part11.html) | [next](part13.html)
-// Rust-101, Part 13: Slices, Arrays, External Dependencies
-// ========================================================
-
-//@ To complete rgrep, there are two pieces we still need to implement: Sorting, and taking the job options
-//@ as argument to the program, rather than hard-coding them. Let's start with sorting.
-
-// ## Slices
-//@ Again, we first have to think about the type we want to give to our sorting function. We may be inclined to
-//@ pass it a `Vec<T>`. Of course, sorting does not actually consume the argument, so we should make that a `&mut Vec<T>`.
-//@ But there's a problem with that: If we want to implement some divide-and-conquer sorting algorithm (say,
-//@ Quicksort), then we will have to *split* our argument at some point, and operate recursively on the two parts.
-//@ But we can't split a `Vec`! We could now extend the function signature to also take some indices, marking the
-//@ part of the vector we are supposed to sort, but that's all rather clumsy. Rust offers a nicer solution.
-
-//@ `[T]` is the type of an (unsized) *array*, with elements of type `T`. All this means is that there's a contiguous
-//@ region of memory, where a bunch of `T` are stored. How many? We can't tell! This is an unsized type. Just like for
-//@ trait objects, this means we can only operate on pointers to that type, and these pointers will carry the missing
-//@ information - namely, the length. Such a pointer is called a *slice*. As we will see, a slice can be split.
-//@ Our function can thus take a borrowed slice, and promise to sort all elements in there.
-pub fn sort<T: PartialOrd>(data: &mut [T]) {
- if data.len() < 2 { return; }
-
- // We decide that the element at 0 is our pivot, and then we move our cursors through the rest of the slice,
- // making sure that everything on the left is no larger than the pivot, and everything on the right is no smaller.
- let mut lpos = 1;
- let mut rpos = data.len();
- /* Invariant: pivot is data[0]; everything with index (0,lpos) is <= pivot;
- [rpos,len) is >= pivot; lpos < rpos */
- loop {
- // **Exercise 13.1**: Complete this Quicksort loop. You can use `swap` on slices to swap two elements. Write a
- // test function for `sort`.
- unimplemented!()
- }
-
- // Once our cursors met, we need to put the pivot in the right place.
- data.swap(0, lpos-1);
-
- // Finally, we split our slice to sort the two halves. The nice part about slices is that splitting them is cheap:
- //@ They are just a pointer to a start address, and a length. We can thus get two pointers, one at the beginning and
- //@ one in the middle, and set the lengths appropriately such that they don't overlap. This is what `split_at_mut` does.
- //@ Since the two slices don't overlap, there is no aliasing and we can have them both mutably borrowed.
- let (part1, part2) = data.split_at_mut(lpos);
- //@ The index operation can not only be used to address certain elements, it can also be used for *slicing*: Giving a range
- //@ of indices, and obtaining an appropriate part of the slice we started with. Here, we remove the last element from
- //@ `part1`, which is the pivot. This makes sure both recursive calls work on strictly smaller slices.
- sort(&mut part1[..lpos-1]); /*@*/
- sort(part2); /*@*/
+// Rust-101, Part 13: Concurrency, Arc, Send
+// =========================================
+
+use std::io::prelude::*;
+use std::{io, fs, thread};
+use std::sync::mpsc::{sync_channel, SyncSender, Receiver};
+use std::sync::Arc;
+
+//@ Our next stop are the concurrency features of Rust. We are going to write our own small version of "grep",
+//@ called *rgrep*, and it is going to make use of concurrency: One thread reads the input files, one thread does
+//@ the actual matching, and one thread writes the output. I already mentioned in the beginning of the course that
+//@ Rust's type system (more precisely, the discipline of ownership and borrowing) will help us to avoid a common
+//@ pitfall of concurrent programming: data races.
+
+// Before we come to the actual code, we define a data-structure `Options` to store all the information we need
+// to complete the job: Which files to work on, which pattern to look for, and how to output. <br/>
+//@ Besides just printing all the matching lines, we will also offer to count them, or alternatively to sort them.
+#[derive(Clone,Copy)]
+pub enum OutputMode {
+ Print,
+ SortAndPrint,
+ Count,
}
+use self::OutputMode::*;
-// **Exercise 13.2**: Since `String` implements `PartialEq`, you can now change the function `output_lines` in the previous part
-// to call the sort function above. If you did exercise 12.1, you will have slightly more work. Make sure you sort by the matched line
-// only, not by filename or line number!
-
-// Now, we can sort, e.g., an vector of numbers.
-fn sort_nums(data: &mut Vec<i32>) {
- //@ Vectors support slicing, just like slices do. Here, `..` denotes the full range, which means we want to slice the entire vector.
- //@ It is then passed to the `sort` function, which doesn't even know that it is working on data inside a vector.
- sort(&mut data[..]);
+pub struct Options {
+ pub files: Vec<String>,
+ pub pattern: String,
+ pub output_mode: OutputMode,
}
-// ## Arrays
-//@ An *array* in Rust is given be the type `[T; n]`, where `n` is some *fixed* number. So, `[f64; 10]` is an array of 10 floating-point
-//@ numbers, all one right next to the other in memory. Arrays are sized, and hence can be used like any other type. But we can also
-//@ borrow them as slices, e.g., to sort them.
-fn sort_array() {
- let mut array_of_data: [f64; 5] = [1.0, 3.4, 12.7, -9.12, 0.1];
- sort(&mut array_of_data);
+//@ Now we can write three functions to do the actual job of reading, matching, and printing, respectively.
+//@ To get the data from one thread to the next, we will use *message passing*: We will establish communication
+//@ channels between the threads, with one thread *sending* data, and the other one *receiving* it. `SyncSender<T>`
+//@ is the type of the sending end of a synchronous channel transmitting data of type `T`. *Synchronous* here
+//@ means that the `send` operation could block, waiting for the other side to make progress. We don't want to
+//@ end up with the entire file being stored in the buffer of the channels, and the output not being fast enough
+//@ to keep up with the speed of input.
+//@
+//@ We also need all the threads to have access to the options of the job they are supposed to do. Since it would
+//@ be rather unnecessary to actually copy these options around, we will use reference-counting to share them between
+//@ all threads. `Arc` is the thread-safe version of `Rc`, using atomic operations to keep the reference count up-to-date.
+
+// The first function reads the files, and sends every line over the `out_channel`.
+fn read_files(options: Arc<Options>, out_channel: SyncSender<String>) {
+ for file in options.files.iter() {
+ // First, we open the file, ignoring any errors.
+ let file = fs::File::open(file).unwrap();
+ // Then we obtain a `BufReader` for it, which provides the `lines` function.
+ let file = io::BufReader::new(file);
+ for line in file.lines() {
+ let line = line.unwrap();
+ // Now we send the line over the channel, ignoring the possibility of `send` failing.
+ out_channel.send(line).unwrap();
+ }
+ }
+ // When we drop the `out_channel`, it will be closed, which the other end can notice.
}
-// ## External Dependencies
-//@ This leaves us with just one more piece to complete rgrep: Taking arguments from the command-line. We could now directly work on
-//@ [`std::env::args`](http://doc.rust-lang.org/stable/std/env/fn.args.html) to gain access to those arguments, and this would become
-//@ a pretty boring lesson in string manipulation. Instead, I want to use this opportunity to show how easy it is to benefit from
-//@ other people's work in your program.
-//@
-//@ For sure, we are not the first to equip a Rust program with support for command-line arguments. Someone must have written a library
-//@ for the job, right? Indeed, someone has. Rust has a central repository of published libraries, called [crates.io](https://crates.io/).
-//@ It's a bit like [PyPI](https://pypi.python.org/pypi) or the [Ruby Gems](https://rubygems.org/): Everybody can upload their code,
-//@ and there's tooling for importing that code into your project. This tooling is provided by `cargo`, the tool we are already using to
-//@ build this tutorial. (`cargo` also has support for *publishing* your crate on crates.io, I refer you to [the documentation](http://doc.crates.io/crates-io.html) for more details.)
-//@ In this case, we are going to use the [`docopt` crate](https://crates.io/crates/docopt), which creates a parser for command-line
-//@ arguments based on the usage string. External dependencies are declared in the `Cargo.toml` file.
-
-//@ I already prepared that file, but the declaration of the dependency is still commented out. So please open `Cargo.toml` of your workspace
-//@ now, and enabled the two commented-out lines. Then do `cargo build`. Cargo will now download the crate from crates.io, compile it,
-//@ and link it to your program. In the future, you can do `cargo update` to make it download new versions of crates you depend on.
-//@ Note that crates.io is only the default location for dependencies, you can also give it the URL of a git repository or some local
-//@ path. All of this is explained in the [Cargo Guide](http://doc.crates.io/guide.html).
-
-// I disabled the following module (using a rather bad hack), because it only compiles if `docopt` is linked.
-// Remove the attribute of the `rgrep` module to enable compilation.
-#[cfg(feature = "disabled")]
-pub mod rgrep {
- // Now that `docopt` is linked, we can first root it in the namespace and then import it with `use`. We also import some other pieces that we will need.
- extern crate docopt;
- use self::docopt::Docopt;
- use part12::{run, Options, OutputMode};
- use std::process;
-
- // The `USAGE` string documents how the program is to be called. It's written in a format that `docopt` can parse.
- static USAGE: &'static str = "
-Usage: rgrep [-c] [-s] <pattern> <file>...
-
-Options:
- -c, --count Count number of matching lines (rather than printing them).
- -s, --sort Sort the lines before printing.
-";
-
- // This function extracts the rgrep options from the command-line arguments.
- fn get_options() -> Options {
- // Parse `argv` and exit the program with an error message if it fails. This is taken from the [`docopt` documentation](http://burntsushi.net/rustdoc/docopt/).
- //@ The function `and_then` takes a closure from `T` to `Result<U, E>`, and uses it to transform a `Result<T, E>` to a
- //@ `Result<U, E>`. This way, we can chain computations that only happen if the previous one succeeded (and the error
- //@ type has to stay the same). In case you know about monads, this style of programming will be familiar to you.
- //@ There's a similar function for `Option`. `unwrap_or_else` is a bit like `unwrap`, but rather than panicking in
- //@ case of an `Err`, it calls the closure.
- let args = Docopt::new(USAGE).and_then(|d| d.parse()).unwrap_or_else(|e| e.exit());
- // Now we can get all the values out.
- let count = args.get_bool("-c");
- let sort = args.get_bool("-s");
- let pattern = args.get_str("<pattern>");
- let files = args.get_vec("<file>");
- if count && sort {
- println!("Setting both '-c' and '-s' at the same time does not make any sense.");
- process::exit(1);
+// The second function filters the lines it receives through `in_channel` with the pattern, and sends
+// matches via `out_channel`.
+fn filter_lines(options: Arc<Options>,
+ in_channel: Receiver<String>,
+ out_channel: SyncSender<String>) {
+ // We can simply iterate over the channel, which will stop when the channel is closed.
+ for line in in_channel.iter() {
+ // `contains` works on lots of types of patterns, but in particular, we can use it to test whether
+ // one string is contained in another. This is another example of Rust using traits as substitute for overloading.
+ if line.contains(&options.pattern) {
+ out_channel.send(line).unwrap(); /*@*/
}
+ }
+}
- // We need to make the strings owned to construct the `Options` instance.
- //@ If you check all the types carefully, you will notice that `pattern` above is of type `&str`. `str` is the type of a UTF-8
- //@ encoded string, that is, a bunch of bytes in memory (`[u8]`) that are valid according of UTF-8. `str` is unsized. `&str`
- //@ stores the address of the character data, and their length. String literals like "this one" are
- //@ of type `&'static str`: They point right to the constant section of the binary, so
- //@ However, the borrow is valid for as long as the program runs, hence it has lifetime `'static`. Calling
- //@ `to_string` will copy the string data into an owned buffer on the heap, and thus convert it to `String`.
- let mode = if count {
- OutputMode::Count
- } else if sort {
- OutputMode::SortAndPrint
- } else {
- OutputMode::Print
- };
- Options {
- files: files.iter().map(|file| file.to_string()).collect(),
- pattern: pattern.to_string(),
- output_mode: mode,
+// The third function performs the output operations, receiving the relevant lines on its `in_channel`.
+fn output_lines(options: Arc<Options>, in_channel: Receiver<String>) {
+ match options.output_mode {
+ Print => {
+ // Here, we just print every line we see.
+ for line in in_channel.iter() {
+ println!("{}", line); /*@*/
+ }
+ },
+ Count => {
+ // We are supposed to count the number of matching lines. There's a convenient iterator adapter that
+ // we can use for this job.
+ let count = in_channel.iter().count(); /*@*/
+ println!("{} hits for {}.", count, options.pattern); /*@*/
+ },
+ SortAndPrint => {
+ // We are asked to sort the matching lines before printing. So let's collect them all in a local vector...
+ let mut data: Vec<String> = in_channel.iter().collect();
+ // ...and implement the actual sorting later.
+ unimplemented!()
}
}
+}
- // Finally, we can call the `run` function from the previous part on the options extracted using `get_options`. Edit `main.rs` to call this function.
- // You can now use `cargo run -- <pattern> <files>` to call your program, and see the argument parser and the threads we wrote previously in action!
- pub fn main() {
- run(get_options()); /*@*/
- }
+// With the operations of the three threads defined, we can now implement a function that performs grepping according
+// to some given options.
+pub fn run(options: Options) {
+ // We move the `options` into an `Arc`, as that's what the thread workers expect.
+ let options = Arc::new(options);
+
+ // This sets up the channels. We use a `sync_channel` with buffer-size of 16 to avoid needlessly filling RAM.
+ let (line_sender, line_receiver) = sync_channel(16);
+ let (filtered_sender, filtered_receiver) = sync_channel(16);
+
+ // Spawn the read thread: `thread::spawn` takes a closure that is run in a new thread.
+ //@ The `move` keyword again tells Rust that we want ownership of captured variables to be moved into the
+ //@ closure. This means we need to do the `clone` *first*, otherwise we would lose our `options` to the
+ //@ new thread!
+ let options1 = options.clone();
+ let handle1 = thread::spawn(move || read_files(options1, line_sender));
+
+ // Same with the filter thread.
+ let options2 = options.clone();
+ let handle2 = thread::spawn(move || {
+ filter_lines(options2, line_receiver, filtered_sender)
+ });
+
+ // And the output thread.
+ let options3 = options.clone();
+ let handle3 = thread::spawn(move || output_lines(options3, filtered_receiver));
+
+ // Finally, wait until all three threads did their job.
+ //@ Joining a thread waits for its termination. This can fail if that thread panicked: In this case, we could get
+ //@ access to the data that it provided to `panic!`. Here, we just assert that they did not panic - so we will panic ourselves
+ //@ if that happened.
+ handle1.join().unwrap();
+ handle2.join().unwrap();
+ handle3.join().unwrap();
+}
+
+// Now we have all the pieces together for testing our rgrep with some hard-coded options.
+//@ We need to call `to_string` on string literals to convert them to a fully-owned `String`.
+pub fn main() {
+ let options = Options {
+ files: vec!["src/part10.rs".to_string(),
+ "src/part11.rs".to_string(),
+ "src/part12.rs".to_string()],
+ pattern: "let".to_string(),
+ output_mode: Print
+ };
+ run(options);
}
-// **Exercise 13.3**: Wouldn't it be nice if rgrep supported regular expressions? There's already a crate that does all the parsing and matching on regular
-// expression, it's called [regex](https://crates.io/crates/regex). Add this crate to the dependencies of your workspace, add an option ("-r") to switch
-// the pattern to regular-expression mode, and change `filter_lines` to honor this option. The documentation of regex is available from its crates.io site.
-// (You won't be able to use the `regex!` macro if you are on the stable or beta channel of Rust. But it wouldn't help for our use-case anyway.)
+// **Exercise 12.1**: Change rgrep such that it prints not only the matching lines, but also the name of the file
+// and the number of the line in the file. You will have to change the type of the channels from `String` to something
+// that records this extra information.
+
+//@ ## Ownership, Borrowing, and Concurrency
+//@ The little demo above showed that concurrency in Rust has a fairly simple API. Considering Rust has closures,
+//@ that should not be entirely surprising. However, as it turns out, Rust goes well beyond this and actually ensures
+//@ the absence of data races. <br/>
+//@ A data race is typically defined as having two concurrent, unsynchronized
+//@ accesses to the same memory location, at least one of which is a write. In other words, a data race is mutation in
+//@ the presence of aliasing, which Rust reliably rules out! It turns out that the same mechanism that makes our single-threaded
+//@ programs memory safe, and that prevents us from invalidating iterators, also helps secure our multi-threaded code against
+//@ data races. For example, notice how `read_files` sends a `String` to `filter_lines`. At run-time, only the pointer to
+//@ the character data will actually be moved around (just like when a `String` is passed to a function with full ownership). However,
+//@ `read_files` has to *give up* ownership of the string to perform `send`, to it is impossible for an outstanding borrow to
+//@ still be around. After it sent the string to the other side, `read_files` has no pointer into the string content
+//@ anymore, and hence no way to race on the data with someone else.
+//@
+//@ There is a little more to this. Remember the `'static` bound we had to add to `register` in the previous part, to make
+//@ sure that the callbacks do not reference any pointers that might become invalid? This is just as crucial for spawning
+//@ a thread: In general, that thread could last for much longer than the current stack frame. Thus, it must not use
+//@ any pointers to data in that stack frame. This is achieved by requiring the `FnOnce` closure passed to `thread::spawn`
+//@ to be valid for lifetime `'static`, as you can see in [its documentation](http://doc.rust-lang.org/stable/std/thread/fn.spawn.html).
+//@ This avoids another kind of data race, where the thread's access races with the callee deallocating its stack frame.
+//@ It is only thanks to the concept of lifetimes that this can be expressed as part of the type of `spawn`.
+
+//@ ## Send
+//@ However, the story goes even further. I said above that `Arc` is a thread-safe version of `Rc`, which uses atomic operations
+//@ to manipulate the reference count. It is thus crucial that we don't use `Rc` across multiple threads, or the reference count may
+//@ become invalid. And indeed, if you replace `Arc` by `Rc` (and add the appropriate imports), Rust will tell you that something
+//@ is wrong. That's great, of course, but how did it do that?
+//@
+//@ The answer is already hinted at in the error: It will say something about `Send`. You may have noticed that the closure in
+//@ `thread::spawn` does not just have a `'static` bound, but also has to satisfy `Send`. `Send` is a trait, and just like `Copy`,
+//@ it's just a marker - there are no functions provided by `Send`. What the trait says is that types which are `Send`, can be
+//@ safely sent to another thread without causing trouble. Of course, all the primitive data-types are `Send`. So is `Arc`,
+//@ which is why Rust accepted our code. But `Rc` is not `Send`, and for a good reason!
+//@
+//@ Now, `Send` as a trait is fairly special. It has a so-called *default implementation*. This means that *every type* implements
+//@ `Send`, unless it opts out. Opting out is viral: If your type contains a type that opted out, then you don't have `Send`, either.
+//@ So if the environment of your closure contains an `Rc`, it won't be `Send`, preventing it from causing trouble. If however every
+//@ captured variable *is* `Send`, then so is the entire environment, and you are good.
//@ [index](main.html) | [previous](part12.html) | [next](part14.html)
-// Rust-101, Part 14: Mutex, Interior Mutability, Sync
-// ===================================================
+// Rust-101, Part 14: Slices, Arrays, External Dependencies
+// ========================================================
-use std::sync::{Arc, Mutex};
-use std::thread;
+//@ To complete rgrep, there are two pieces we still need to implement: Sorting, and taking the job options
+//@ as argument to the program, rather than hard-coding them. Let's start with sorting.
-//@ We already saw that we can use `Arc` to share memory between threads. However, `Arc` can only provide *read-only*
-//@ access to memory: Since there is aliasing, Rust cannot, in general, permit mutation. If however,
-//@ some care would be taken at run-time, then mutation would still be all right: We have to ensure that whenever
-//@ someone changes the data, nobody else is looking at it. In other words, we need a *critical section* or (as it
-//@ is called in Rust) a [`Mutex`](http://doc.rust-lang.org/stable/std/sync/struct.Mutex.html). Some other languages also call this a *lock*.
-//@
-//@ As an example, let us write a concurrent counter. As usual in Rust, we first have to think about our data layout.
-//@ In case of the mutex, this means we have to declare the type of the data that we want to be protected. In Rust,
-//@ a `Mutex` protects data, not code - and it is impossible to access the data in any other way. This is generally considered
-//@ good style, but other languages typically lack the ability to actually enforce this.
-//@ Of course, we want multiple threads to have access to this `Mutex`, so we wrap it in an `Arc`.
-//@
-//@ Rather than giving every field a name, a struct can also be defined by just giving a sequence of types (similar
-//@ to how a variant of an `enum` is defined). This is called a *tuple struct*. It is often used when constructing
-//@ a *newtype*, as we do here: `ConcurrentCounter` is essentially just a new name for `Arc<Mutex<usize>>`. However,
-//@ is is a locally declared types, so we can give it an inherent implementation and implement traits for it. Since the
-//@ field is private, nobody outside this module can even know the type we are wrapping.
-
-// The derived `Clone` implementation will clone the `Arc`, so all clones will actually talk about the same counter.
-#[derive(Clone)]
-struct ConcurrentCounter(Arc<Mutex<usize>>);
-
-impl ConcurrentCounter {
- // The constructor just wraps the constructors of `Arc` and `Mutex`.
- pub fn new(val: usize) -> Self {
- ConcurrentCounter(Arc::new(Mutex::new(val))) /*@*/
- }
+// ## Slices
+//@ Again, we first have to think about the type we want to give to our sorting function. We may be inclined to
+//@ pass it a `Vec<T>`. Of course, sorting does not actually consume the argument, so we should make that a `&mut Vec<T>`.
+//@ But there's a problem with that: If we want to implement some divide-and-conquer sorting algorithm (say,
+//@ Quicksort), then we will have to *split* our argument at some point, and operate recursively on the two parts.
+//@ But we can't split a `Vec`! We could now extend the function signature to also take some indices, marking the
+//@ part of the vector we are supposed to sort, but that's all rather clumsy. Rust offers a nicer solution.
- //@ The core operation is, of course, `increment`. The type may be surprising at first: A shared borrow?
- //@ How can this be, since `increment` definitely modifies the counter? We already discussed above that `Mutex` is
- //@ a way to get around this restriction in Rust. This phenomenon of data that can be mutated through a shared
- //@ borrow is called *interior mutability*: We are changing the inner parts of the object, but seen from the outside,
- //@ this does not count as "mutation". This stands in contrast to *exterior mutability*, which is the kind of
- //@ mutability we saw so far, where one piece of data is replaced by something else of the same type. If you are familiar
- //@ with languages like ML, you can compare this to how something of type `ref` permits mutation, even though it is
- //@ itself a functional value (more precisely, a location) like all the others.
- //@
- //@ Interior mutability breaks the rules of Rust that I outlined earlier: There is aliasing (a shared borrow) and mutation.
- //@ The reason that this still works is careful programming of the primitives for interior mutability - in this case, that's
- //@ `Mutex`. It has to ensure with dynamic checks, at run-time, that things don't fall apart. In particular, it has to ensure
- //@ that the data covered by the mutex can only ever be accessed from inside a critical section. This is where Rust's type
- //@ system comes into play: With its discipline of ownership and borrowing, it can enforce such rules. Let's see how this goes.
- pub fn increment(&self, by: usize) {
- // `lock` on a mutex returns a *guard*, giving access to the data contained in the mutex.
- //@ (We will discuss the `unwrap` soon.) `.0` is how we access the first component of a tuple or a struct.
- let mut counter = self.0.lock().unwrap();
- //@ The guard is another example of a smart pointer, and it can be used as if it were a pointer to the data protected
- //@ by the lock.
- *counter = *counter + by;
- //@ At the end of the function, `counter` is dropped and the mutex is available again.
- //@ This can only happen when full ownership of the guard is given up. In particular, it is impossible for us
- //@ to borrow some of its content, release the lock of the mutex, and subsequently access the protected data without holding
- //@ the lock. Enforcing the locking discipline is expressible in the Rust type system, so we don't have to worry
- //@ about data races *even though* we are mutating shared memory!
- //@
- //@ One of the subtle aspects of locking is *poisoning*. If a thread panics while it holds a lock, it could leave the
- //@ data-structure in a bad state. The lock is hence considered *poisoned*. Future attempts to `lock` it will fail.
- //@ Above, we simply assert via `unwrap` that this will never happen. Alternatively, we could have a look at the poisoned
- //@ state and attempt to recover from it.
- }
+//@ `[T]` is the type of an (unsized) *array*, with elements of type `T`. All this means is that there's a contiguous
+//@ region of memory, where a bunch of `T` are stored. How many? We can't tell! This is an unsized type. Just like for
+//@ trait objects, this means we can only operate on pointers to that type, and these pointers will carry the missing
+//@ information - namely, the length. Such a pointer is called a *slice*. As we will see, a slice can be split.
+//@ Our function can thus take a borrowed slice, and promise to sort all elements in there.
+pub fn sort<T: PartialOrd>(data: &mut [T]) {
+ if data.len() < 2 { return; }
- // The function `get` returns the current value of the counter.
- pub fn get(&self) -> usize {
- let counter = self.0.lock().unwrap(); /*@*/
- *counter /*@*/
+ // We decide that the element at 0 is our pivot, and then we move our cursors through the rest of the slice,
+ // making sure that everything on the left is no larger than the pivot, and everything on the right is no smaller.
+ let mut lpos = 1;
+ let mut rpos = data.len();
+ /* Invariant: pivot is data[0]; everything with index (0,lpos) is <= pivot;
+ [rpos,len) is >= pivot; lpos < rpos */
+ loop {
+ // **Exercise 13.1**: Complete this Quicksort loop. You can use `swap` on slices to swap two elements. Write a
+ // test function for `sort`.
+ unimplemented!()
}
+
+ // Once our cursors met, we need to put the pivot in the right place.
+ data.swap(0, lpos-1);
+
+ // Finally, we split our slice to sort the two halves. The nice part about slices is that splitting them is cheap:
+ //@ They are just a pointer to a start address, and a length. We can thus get two pointers, one at the beginning and
+ //@ one in the middle, and set the lengths appropriately such that they don't overlap. This is what `split_at_mut` does.
+ //@ Since the two slices don't overlap, there is no aliasing and we can have them both mutably borrowed.
+ let (part1, part2) = data.split_at_mut(lpos);
+ //@ The index operation can not only be used to address certain elements, it can also be used for *slicing*: Giving a range
+ //@ of indices, and obtaining an appropriate part of the slice we started with. Here, we remove the last element from
+ //@ `part1`, which is the pivot. This makes sure both recursive calls work on strictly smaller slices.
+ sort(&mut part1[..lpos-1]); /*@*/
+ sort(part2); /*@*/
}
-// Now our counter is ready for action.
-pub fn main() {
- let counter = ConcurrentCounter::new(0);
+// **Exercise 13.2**: Since `String` implements `PartialEq`, you can now change the function `output_lines` in the previous part
+// to call the sort function above. If you did exercise 12.1, you will have slightly more work. Make sure you sort by the matched line
+// only, not by filename or line number!
- // We clone the counter for the first thread, which increments it by 2 every 15ms.
- let counter1 = counter.clone();
- let handle1 = thread::spawn(move || {
- for _ in 0..10 {
- thread::sleep_ms(15);
- counter1.increment(2);
- }
- });
-
- // The second thread increments the counter by 3 every 20ms.
- let counter2 = counter.clone();
- let handle2 = thread::spawn(move || {
- for _ in 0..10 {
- thread::sleep_ms(20);
- counter2.increment(3);
+// Now, we can sort, e.g., an vector of numbers.
+fn sort_nums(data: &mut Vec<i32>) {
+ //@ Vectors support slicing, just like slices do. Here, `..` denotes the full range, which means we want to slice the entire vector.
+ //@ It is then passed to the `sort` function, which doesn't even know that it is working on data inside a vector.
+ sort(&mut data[..]);
+}
+
+// ## Arrays
+//@ An *array* in Rust is given be the type `[T; n]`, where `n` is some *fixed* number. So, `[f64; 10]` is an array of 10 floating-point
+//@ numbers, all one right next to the other in memory. Arrays are sized, and hence can be used like any other type. But we can also
+//@ borrow them as slices, e.g., to sort them.
+fn sort_array() {
+ let mut array_of_data: [f64; 5] = [1.0, 3.4, 12.7, -9.12, 0.1];
+ sort(&mut array_of_data);
+}
+
+// ## External Dependencies
+//@ This leaves us with just one more piece to complete rgrep: Taking arguments from the command-line. We could now directly work on
+//@ [`std::env::args`](http://doc.rust-lang.org/stable/std/env/fn.args.html) to gain access to those arguments, and this would become
+//@ a pretty boring lesson in string manipulation. Instead, I want to use this opportunity to show how easy it is to benefit from
+//@ other people's work in your program.
+//@
+//@ For sure, we are not the first to equip a Rust program with support for command-line arguments. Someone must have written a library
+//@ for the job, right? Indeed, someone has. Rust has a central repository of published libraries, called [crates.io](https://crates.io/).
+//@ It's a bit like [PyPI](https://pypi.python.org/pypi) or the [Ruby Gems](https://rubygems.org/): Everybody can upload their code,
+//@ and there's tooling for importing that code into your project. This tooling is provided by `cargo`, the tool we are already using to
+//@ build this tutorial. (`cargo` also has support for *publishing* your crate on crates.io, I refer you to [the documentation](http://doc.crates.io/crates-io.html) for more details.)
+//@ In this case, we are going to use the [`docopt` crate](https://crates.io/crates/docopt), which creates a parser for command-line
+//@ arguments based on the usage string. External dependencies are declared in the `Cargo.toml` file.
+
+//@ I already prepared that file, but the declaration of the dependency is still commented out. So please open `Cargo.toml` of your workspace
+//@ now, and enabled the two commented-out lines. Then do `cargo build`. Cargo will now download the crate from crates.io, compile it,
+//@ and link it to your program. In the future, you can do `cargo update` to make it download new versions of crates you depend on.
+//@ Note that crates.io is only the default location for dependencies, you can also give it the URL of a git repository or some local
+//@ path. All of this is explained in the [Cargo Guide](http://doc.crates.io/guide.html).
+
+// I disabled the following module (using a rather bad hack), because it only compiles if `docopt` is linked.
+// Remove the attribute of the `rgrep` module to enable compilation.
+#[cfg(feature = "disabled")]
+pub mod rgrep {
+ // Now that `docopt` is linked, we can first add it to the namespace and then import shorter names with `use`. We also import some other pieces that we will need.
+ extern crate docopt;
+ use self::docopt::Docopt;
+ use part12::{run, Options, OutputMode};
+ use std::process;
+
+ // The `USAGE` string documents how the program is to be called. It's written in a format that `docopt` can parse.
+ static USAGE: &'static str = "
+Usage: rgrep [-c] [-s] <pattern> <file>...
+
+Options:
+ -c, --count Count number of matching lines (rather than printing them).
+ -s, --sort Sort the lines before printing.
+";
+
+ // This function extracts the rgrep options from the command-line arguments.
+ fn get_options() -> Options {
+ // Parse `argv` and exit the program with an error message if it fails. This is taken from the [`docopt` documentation](http://burntsushi.net/rustdoc/docopt/).
+ //@ The function `and_then` takes a closure from `T` to `Result<U, E>`, and uses it to transform a `Result<T, E>` to a
+ //@ `Result<U, E>`. This way, we can chain computations that only happen if the previous one succeeded (and the error
+ //@ type has to stay the same). In case you know about monads, this style of programming will be familiar to you.
+ //@ There's a similar function for `Option`. `unwrap_or_else` is a bit like `unwrap`, but rather than panicking in
+ //@ case of an `Err`, it calls the closure.
+ let args = Docopt::new(USAGE).and_then(|d| d.parse()).unwrap_or_else(|e| e.exit());
+ // Now we can get all the values out.
+ let count = args.get_bool("-c");
+ let sort = args.get_bool("-s");
+ let pattern = args.get_str("<pattern>");
+ let files = args.get_vec("<file>");
+ if count && sort {
+ println!("Setting both '-c' and '-s' at the same time does not make any sense.");
+ process::exit(1);
}
- });
- // Now we watch the threads working on the counter.
- for _ in 0..50 {
- thread::sleep_ms(5);
- println!("Current value: {}", counter.get());
+ // We need to make the strings owned to construct the `Options` instance.
+ //@ If you check all the types carefully, you will notice that `pattern` above is of type `&str`. `str` is the type of a UTF-8
+ //@ encoded string, that is, a bunch of bytes in memory (`[u8]`) that are valid according of UTF-8. `str` is unsized. `&str`
+ //@ stores the address of the character data, and their length. String literals like "this one" are
+ //@ of type `&'static str`: They point right to the constant section of the binary, so
+ //@ However, the borrow is valid for as long as the program runs, hence it has lifetime `'static`. Calling
+ //@ `to_string` will copy the string data into an owned buffer on the heap, and thus convert it to `String`.
+ let mode = if count {
+ OutputMode::Count
+ } else if sort {
+ OutputMode::SortAndPrint
+ } else {
+ OutputMode::Print
+ };
+ Options {
+ files: files.iter().map(|file| file.to_string()).collect(),
+ pattern: pattern.to_string(),
+ output_mode: mode,
+ }
}
- // Finally, we wait for all the threads to finish to be sure we can catch the counter's final value.
- handle1.join().unwrap();
- handle2.join().unwrap();
- println!("Final value: {}", counter.get());
+ // Finally, we can call the `run` function from the previous part on the options extracted using `get_options`. Edit `main.rs` to call this function.
+ // You can now use `cargo run -- <pattern> <files>` to call your program, and see the argument parser and the threads we wrote previously in action!
+ pub fn main() {
+ run(get_options()); /*@*/
+ }
}
-// **Exercise 14.1**: Besides `Mutex`, there's also [`RwLock`](http://doc.rust-lang.org/stable/std/sync/struct.RwLock.html), which
-// provides two ways of locking: One that grants only read-only access, to any number of concurrent readers, and another one
-// for exclusive write access. (Notice that this is the same pattern we already saw with shared vs. mutable borrows.) Change
-// the code above to use `RwLock`, such that multiple calls to `get` can be executed at the same time.
-//
-// **Exercise 14.2**: Add an operation `compare_and_inc(&self, test: usize, by: usize)` that increments the counter by
-// `by` *only if* the current value is `test`.
-
-//@ ## Sync
-//@ In part 12, we talked about types that are marked `Send` and thus can be moved to another thread. However, we did *not*
-//@ talk about the question whether a borrow is `Send`. For `&mut T`, the answer is: It is `Send` whenever `T` is send.
-//@ `&mut` allows moving values back and forth, it is even possible to [`swap`](http://doc.rust-lang.org/beta/std/mem/fn.swap.html)
-//@ the contents of two mutably borrowed values. So in terms of concurrency, sending a mutable borrow is very much like
-//@ sending full ownership, in the sense that it can be used to move the object to another thread.
-//@
-//@ But what about `&T`, a shared borrow? Without interior mutability, it would always be all-right to send such values.
-//@ After all, no mutation can be performed, so there can be as many threads accessing the data as we like. In the
-//@ presence of interior mutability though, the story gets more complicated. Rust introduces another marker trait for
-//@ this purpose: `Sync`. A type `T` is `Sync` if `&T` is `Send`. Just like `Send`, `Sync` has a default implementation
-//@ and is thus automatically implemented for a data-structure *if* all its members implement it.
-//@
-//@ Almost all the types we saw so far are `Sync`, with the exception of `Rc`. Remember that a shared borrow is good enough
-//@ for cloning, and we don't want other threads to clone our local `Rc`, so it must not be `Sync`. The rule of `Mutex`
-//@ is to enforce synchronization, so it should not be entirely surprising that `Mutex<T>` is `Send` *and* `Sync` provided that
-//@ `T` is `Send`.
-//@
-//@ In the next part, we will learn about a type called `RefCell` that is `Send`, but not `Sync`.
-//@
-//@ You may be curious whether there is a type that's `Sync`, but not `Send`. There are indeed rather esoteric examples
-//@ of such types, but that's not a topic I want to go into. In case you are curious, there's a
-//@ [Rust RFC](https://github.com/rust-lang/rfcs/blob/master/text/0458-send-improvements.md), which contains a type `RcMut` that would be `Sync` and not `Send`.
-//@ You may also be interested in [this blog post](https://huonw.github.io/blog/2015/02/some-notes-on-send-and-sync/) on the topic.
+// **Exercise 13.3**: Wouldn't it be nice if rgrep supported regular expressions? There's already a crate that does all the parsing and matching on regular
+// expression, it's called [regex](https://crates.io/crates/regex). Add this crate to the dependencies of your workspace, add an option ("-r") to switch
+// the pattern to regular-expression mode, and change `filter_lines` to honor this option. The documentation of regex is available from its crates.io site.
+// (You won't be able to use the `regex!` macro if you are on the stable or beta channel of Rust. But it wouldn't help for our use-case anyway.)
-//@ [index](main.html) | [previous](part13.html) | [next](main.html)
+//@ [index](main.html) | [previous](part13.html) | [next](part15.html)
-// Rust-101, Part 15: Interior Mutability (cont.), RefCell, Cell, Drop
-// ===================================================================
+// Rust-101, Part 15: Mutex, Interior Mutability (cont.), Sync
+// ===========================================================
+
+use std::sync::{Arc, Mutex};
+use std::thread;
+
+//@ We already saw that we can use `Arc` to share memory between threads. However, `Arc` can only provide *read-only*
+//@ access to memory: Since there is aliasing, Rust cannot, in general, permit mutation. If however,
+//@ some care would be taken at run-time, then mutation would still be all right: We have to ensure that whenever
+//@ someone changes the data, nobody else is looking at it. In other words, we need a *critical section* or (as it
+//@ is called in Rust) a [`Mutex`](http://doc.rust-lang.org/stable/std/sync/struct.Mutex.html). Some other languages also call this a *lock*.
+//@
+//@ As an example, let us write a concurrent counter. As usual in Rust, we first have to think about our data layout.
+//@ In case of the mutex, this means we have to declare the type of the data that we want to be protected. In Rust,
+//@ a `Mutex` protects data, not code - and it is impossible to access the data in any other way. This is generally considered
+//@ good style, but other languages typically lack the ability to actually enforce this.
+//@ Of course, we want multiple threads to have access to this `Mutex`, so we wrap it in an `Arc`.
+//@
+//@ Rather than giving every field a name, a struct can also be defined by just giving a sequence of types (similar
+//@ to how a variant of an `enum` is defined). This is called a *tuple struct*. It is often used when constructing
+//@ a *newtype*, as we do here: `ConcurrentCounter` is essentially just a new name for `Arc<Mutex<usize>>`. However,
+//@ is is a locally declared types, so we can give it an inherent implementation and implement traits for it. Since the
+//@ field is private, nobody outside this module can even know the type we are wrapping.
+
+// The derived `Clone` implementation will clone the `Arc`, so all clones will actually talk about the same counter.
+#[derive(Clone)]
+struct ConcurrentCounter(Arc<Mutex<usize>>);
+
+impl ConcurrentCounter {
+ // The constructor just wraps the constructors of `Arc` and `Mutex`.
+ pub fn new(val: usize) -> Self {
+ ConcurrentCounter(Arc::new(Mutex::new(val))) /*@*/
+ }
+
+ //@ The core operation is, of course, `increment`. The type may be surprising at first: A shared borrow?
+ //@ How can this be, since `increment` definitely modifies the counter? We already discussed above that `Mutex` is
+ //@ a way to get around this restriction in Rust. This phenomenon of data that can be mutated through a shared
+ //@ borrow is called *interior mutability*: We are changing the inner parts of the object, but seen from the outside,
+ //@ this does not count as "mutation". This stands in contrast to *exterior mutability*, which is the kind of
+ //@ mutability we saw so far, where one piece of data is replaced by something else of the same type. If you are familiar
+ //@ with languages like ML, you can compare this to how something of type `ref` permits mutation, even though it is
+ //@ itself a functional value (more precisely, a location) like all the others.
+ //@
+ //@ Interior mutability breaks the rules of Rust that I outlined earlier: There is aliasing (a shared borrow) and mutation.
+ //@ The reason that this still works is careful programming of the primitives for interior mutability - in this case, that's
+ //@ `Mutex`. It has to ensure with dynamic checks, at run-time, that things don't fall apart. In particular, it has to ensure
+ //@ that the data covered by the mutex can only ever be accessed from inside a critical section. This is where Rust's type
+ //@ system comes into play: With its discipline of ownership and borrowing, it can enforce such rules. Let's see how this goes.
+ pub fn increment(&self, by: usize) {
+ // `lock` on a mutex returns a *guard*, giving access to the data contained in the mutex.
+ //@ (We will discuss the `unwrap` soon.) `.0` is how we access the first component of a tuple or a struct.
+ let mut counter = self.0.lock().unwrap();
+ //@ The guard is another example of a smart pointer, and it can be used as if it were a pointer to the data protected
+ //@ by the lock.
+ *counter = *counter + by;
+ //@ At the end of the function, `counter` is dropped and the mutex is available again.
+ //@ This can only happen when full ownership of the guard is given up. In particular, it is impossible for us
+ //@ to borrow some of its content, release the lock of the mutex, and subsequently access the protected data without holding
+ //@ the lock. Enforcing the locking discipline is expressible in the Rust type system, so we don't have to worry
+ //@ about data races *even though* we are mutating shared memory!
+ //@
+ //@ One of the subtle aspects of locking is *poisoning*. If a thread panics while it holds a lock, it could leave the
+ //@ data-structure in a bad state. The lock is hence considered *poisoned*. Future attempts to `lock` it will fail.
+ //@ Above, we simply assert via `unwrap` that this will never happen. Alternatively, we could have a look at the poisoned
+ //@ state and attempt to recover from it.
+ }
+
+ // The function `get` returns the current value of the counter.
+ pub fn get(&self) -> usize {
+ let counter = self.0.lock().unwrap(); /*@*/
+ *counter /*@*/
+ }
+}
+
+// Now our counter is ready for action.
+pub fn main() {
+ let counter = ConcurrentCounter::new(0);
+
+ // We clone the counter for the first thread, which increments it by 2 every 15ms.
+ let counter1 = counter.clone();
+ let handle1 = thread::spawn(move || {
+ for _ in 0..10 {
+ thread::sleep_ms(15);
+ counter1.increment(2);
+ }
+ });
+
+ // The second thread increments the counter by 3 every 20ms.
+ let counter2 = counter.clone();
+ let handle2 = thread::spawn(move || {
+ for _ in 0..10 {
+ thread::sleep_ms(20);
+ counter2.increment(3);
+ }
+ });
+
+ // Now we watch the threads working on the counter.
+ for _ in 0..50 {
+ thread::sleep_ms(5);
+ println!("Current value: {}", counter.get());
+ }
+
+ // Finally, we wait for all the threads to finish to be sure we can catch the counter's final value.
+ handle1.join().unwrap();
+ handle2.join().unwrap();
+ println!("Final value: {}", counter.get());
+}
+
+// **Exercise 14.1**: Besides `Mutex`, there's also [`RwLock`](http://doc.rust-lang.org/stable/std/sync/struct.RwLock.html), which
+// provides two ways of locking: One that grants only read-only access, to any number of concurrent readers, and another one
+// for exclusive write access. (Notice that this is the same pattern we already saw with shared vs. mutable borrows.) Change
+// the code above to use `RwLock`, such that multiple calls to `get` can be executed at the same time.
+//
+// **Exercise 14.2**: Add an operation `compare_and_inc(&self, test: usize, by: usize)` that increments the counter by
+// `by` *only if* the current value is `test`.
+
+//@ ## Sync
+//@ In part 12, we talked about types that are marked `Send` and thus can be moved to another thread. However, we did *not*
+//@ talk about the question whether a borrow is `Send`. For `&mut T`, the answer is: It is `Send` whenever `T` is send.
+//@ `&mut` allows moving values back and forth, it is even possible to [`swap`](http://doc.rust-lang.org/beta/std/mem/fn.swap.html)
+//@ the contents of two mutably borrowed values. So in terms of concurrency, sending a mutable borrow is very much like
+//@ sending full ownership, in the sense that it can be used to move the object to another thread.
+//@
+//@ But what about `&T`, a shared borrow? Without interior mutability, it would always be all-right to send such values.
+//@ After all, no mutation can be performed, so there can be as many threads accessing the data as we like. In the
+//@ presence of interior mutability though, the story gets more complicated. Rust introduces another marker trait for
+//@ this purpose: `Sync`. A type `T` is `Sync` if `&T` is `Send`. Just like `Send`, `Sync` has a default implementation
+//@ and is thus automatically implemented for a data-structure *if* all its members implement it.
+//@
+//@ Almost all the types we saw so far are `Sync`, with the exception of `Rc`. Remember that a shared borrow is good enough
+//@ for cloning, and we don't want other threads to clone our local `Rc`, so it must not be `Sync`. The rule of `Mutex`
+//@ is to enforce synchronization, so it should not be entirely surprising that `Mutex<T>` is `Send` *and* `Sync` provided that
+//@ `T` is `Send`.
+//@
+//@ In the next part, we will learn about a type called `RefCell` that is `Send`, but not `Sync`.
+//@
+//@ You may be curious whether there is a type that's `Sync`, but not `Send`. There are indeed rather esoteric examples
+//@ of such types, but that's not a topic I want to go into. In case you are curious, there's a
+//@ [Rust RFC](https://github.com/rust-lang/rfcs/blob/master/text/0458-send-improvements.md), which contains a type `RcMut` that would be `Sync` and not `Send`.
+//@ You may also be interested in [this blog post](https://huonw.github.io/blog/2015/02/some-notes-on-send-and-sync/) on the topic.
+
+// FIXME TODO some old outdated explanation FIXME TODO
//@ [`RefCell`](http://doc.rust-lang.org/beta/std/cell/struct.RefCell.html)
//@ [`is very much like `RwLock`, but it's not thread-safe: "Locking" is done without atomic operations.
//@ the thread-safe `RwLock`. And finally, in case a distinction between readers and writers is not helpful, one can use the
//@ more efficient `Mutex`.
+
//@ [index](main.html) | [previous](part14.html) | [next](main.html)
-// Rust-101, Part 11: Trait Objects, Box, Rc, Lifetime bounds
-// ==========================================================
+// Rust-101, Part 11: Trait Objects, Box, Lifetime bounds
+// ======================================================
-mod callbacks {
- // For now, we just decide that the callbacks have an argument of type `i32`.
- struct CallbacksV1<F: FnMut(i32)> {
- callbacks: Vec<F>,
- }
-
- /* struct CallbacksV2 {
- callbacks: Vec<FnMut(i32)>,
- } */
- pub struct Callbacks {
- callbacks: Vec<Box<FnMut(i32)>>,
- }
+// For now, we just decide that the callbacks have an argument of type `i32`.
+struct CallbacksV1<F: FnMut(i32)> {
+ callbacks: Vec<F>,
+}
- impl Callbacks {
- // Now we can provide some functions. The constructor should be straight-forward.
- pub fn new() -> Self {
- unimplemented!()
- }
+/* struct CallbacksV2 {
+ callbacks: Vec<FnMut(i32)>,
+} */
- // Registration simply stores the callback.
- pub fn register(&mut self, callback: Box<FnMut(i32)>) {
- unimplemented!()
- }
+pub struct Callbacks {
+ callbacks: Vec<Box<FnMut(i32)>>,
+}
- // And here we call all the stored callbacks.
- pub fn call(&mut self, val: i32) {
- // Since they are of type `FnMut`, we need to mutably iterate. Notice that boxes dereference implicitly.
- for callback in self.callbacks.iter_mut() {
- unimplemented!()
- }
- }
+impl Callbacks {
+ // Now we can provide some functions. The constructor should be straight-forward.
+ pub fn new() -> Self {
+ unimplemented!()
}
- // Now we are ready for the demo.
- pub fn demo(c: &mut Callbacks) {
- c.register(Box::new(|val| println!("Callback 1: {}", val)));
- c.call(0);
-
- let mut count: usize = 0;
- c.register(Box::new(move |val| {
- count = count+1;
- println!("Callback 2, {}. time: {}", count, val);
- } ));
- c.call(1); c.call(2);
+ // Registration simply stores the callback.
+ pub fn register(&mut self, callback: Box<FnMut(i32)>) {
+ unimplemented!()
}
-}
-// Remember to edit `main.rs` to run the demo.
-pub fn main() {
- let mut c = callbacks::Callbacks::new();
- callbacks::demo(&mut c);
-}
-
-mod callbacks_clone {
-
- use std::rc::Rc;
-
- #[derive(Clone)]
- pub struct Callbacks {
- callbacks: Vec<Rc<Fn(i32)>>,
+ // We can also write a generic version of `register`, such that it will be instantiated with some concrete closure type `F`
+ // and do the creation of the `Box` and the conversion from `F` to `FnMut(i32)` itself.
+
+ pub fn register_generic<F: FnMut(i32)+'static>(&mut self, callback: F) {
+ unimplemented!()
}
- impl Callbacks {
- pub fn new() -> Self {
- unimplemented!()
- }
-
- // For the `register` function, we don't actually have to use trait objects in the argument.
-
- pub fn register<F: Fn(i32)+'static>(&mut self, callback: F) {
+ // And here we call all the stored callbacks.
+ pub fn call(&mut self, val: i32) {
+ // Since they are of type `FnMut`, we need to mutably iterate.
+ for callback in self.callbacks.iter_mut() {
unimplemented!()
}
-
- pub fn call(&mut self, val: i32) {
- // We only need a shared iterator here. `Rc` also implicitly dereferences, so we can simply call the callback.
- for callback in self.callbacks.iter() {
- unimplemented!()
- }
- }
}
+}
+
+// Now we are ready for the demo. Remember to edit `main.rs` to run it.
+pub fn main() {
+ let mut c = Callbacks::new();
+ c.register(Box::new(|val| println!("Callback 1: {}", val)));
+ c.call(0);
- // The demo works just as above. Our counting callback doesn't work anymore though, because we are using `Fn` now.
- fn demo(c: &mut Callbacks) {
- c.register(|val| println!("Callback 1: {}", val));
- c.call(0); c.call(1);
+ {
+ let mut count: usize = 0;
+ c.register_generic(move |val| {
+ count = count+1;
+ println!("Callback 2: {} ({}. time)", val, count);
+ } );
}
+ c.call(1); c.call(2);
}
+
// **Exercise 11.1**: We made the arbitrary choice of using `i32` for the arguments. Generalize the data-structures above
// to work with an arbitrary type `T` that's passed to the callbacks. Since you need to call multiple callbacks with the
// same `t: T`, you will either have to restrict `T` to `Copy` types, or pass a borrow.
-
-// Rust-101, Part 12: Concurrency, Arc, Send
-// =========================================
-
-use std::io::prelude::*;
-use std::{io, fs, thread};
-use std::sync::mpsc::{sync_channel, SyncSender, Receiver};
-use std::sync::Arc;
-
-
-// Before we come to the actual code, we define a data-structure `Options` to store all the information we need
-// to complete the job: Which files to work on, which pattern to look for, and how to output. <br/>
-#[derive(Clone,Copy)]
-pub enum OutputMode {
- Print,
- SortAndPrint,
- Count,
-}
-use self::OutputMode::*;
+// Rust-101, Part 12: Rc, Interior Mutability, Cell, RefCell
+// =========================================================
-pub struct Options {
- pub files: Vec<String>,
- pub pattern: String,
- pub output_mode: OutputMode,
-}
+use std::rc::Rc;
+use std::cell::{Cell, RefCell};
-// The first function reads the files, and sends every line over the `out_channel`.
-fn read_files(options: Arc<Options>, out_channel: SyncSender<String>) {
- for file in options.files.iter() {
- // First, we open the file, ignoring any errors.
- let file = fs::File::open(file).unwrap();
- // Then we obtain a `BufReader` for it, which provides the `lines` function.
- let file = io::BufReader::new(file);
- for line in file.lines() {
- let line = line.unwrap();
- // Now we send the line over the channel, ignoring the possibility of `send` failing.
- out_channel.send(line).unwrap();
- }
- }
- // When we drop the `out_channel`, it will be closed, which the other end can notice.
+
+#[derive(Clone)]
+struct Callbacks {
+ callbacks: Vec<Rc<Fn(i32)>>,
}
-// The second function filters the lines it receives through `in_channel` with the pattern, and sends
-// matches via `out_channel`.
-fn filter_lines(options: Arc<Options>,
- in_channel: Receiver<String>,
- out_channel: SyncSender<String>) {
- // We can simply iterate over the channel, which will stop when the channel is closed.
- for line in in_channel.iter() {
- // `contains` works on lots of types of patterns, but in particular, we can use it to test whether
- // one string is contained in another. This is another example of Rust using traits as substitute for overloading.
- if line.contains(&options.pattern) {
- unimplemented!()
- }
+impl Callbacks {
+ pub fn new() -> Self {
+ unimplemented!()
}
-}
-// The third function performs the output operations, receiving the relevant lines on its `in_channel`.
-fn output_lines(options: Arc<Options>, in_channel: Receiver<String>) {
- match options.output_mode {
- Print => {
- // Here, we just print every line we see.
- for line in in_channel.iter() {
- unimplemented!()
- }
- },
- Count => {
- // We are supposed to count the number of matching lines. There's a convenient iterator adapter that
- // we can use for this job.
- unimplemented!()
- },
- SortAndPrint => {
- // We are asked to sort the matching lines before printing. So let's collect them all in a local vector...
- let mut data: Vec<String> = in_channel.iter().collect();
- // ...and implement the actual sorting later.
+ // Registration works just like last time, except that we are creating an `Rc` now.
+ pub fn register<F: Fn(i32)+'static>(&mut self, callback: F) {
+ unimplemented!()
+ }
+
+ pub fn call(&self, val: i32) {
+ // We only need a shared iterator here. Since `Rc` is a smart pointer, we can directly call the callback.
+ for callback in self.callbacks.iter() {
unimplemented!()
}
}
}
-// With the operations of the three threads defined, we can now implement a function that performs grepping according
-// to some given options.
-pub fn run(options: Options) {
- // We move the `options` into an `Arc`, as that's what the thread workers expect.
- let options = Arc::new(options);
-
- // This sets up the channels. We use a `sync_channel` with buffer-size of 16 to avoid needlessly filling RAM.
- let (line_sender, line_receiver) = sync_channel(16);
- let (filtered_sender, filtered_receiver) = sync_channel(16);
-
- // Spawn the read thread: `thread::spawn` takes a closure that is run in a new thread.
- let options1 = options.clone();
- let handle1 = thread::spawn(move || read_files(options1, line_sender));
-
- // Same with the filter thread.
- let options2 = options.clone();
- let handle2 = thread::spawn(move || {
- filter_lines(options2, line_receiver, filtered_sender)
- });
-
- // And the output thread.
- let options3 = options.clone();
- let handle3 = thread::spawn(move || output_lines(options3, filtered_receiver));
-
- // Finally, wait until all three threads did their job.
- handle1.join().unwrap();
- handle2.join().unwrap();
- handle3.join().unwrap();
+// Time for a demo!
+fn demo(c: &mut Callbacks) {
+ c.register(|val| println!("Callback 1: {}", val));
+ c.call(0); c.clone().call(1);
}
-// Now we have all the pieces together for testing our rgrep with some hard-coded options.
pub fn main() {
- let options = Options {
- files: vec!["src/part10.rs".to_string(),
- "src/part11.rs".to_string(),
- "src/part12.rs".to_string()],
- pattern: "let".to_string(),
- output_mode: Print
- };
- run(options);
+ let mut c = Callbacks::new();
+ demo(&mut c);
+}
+
+// ## Interior Mutability
+
+// So, let us put our counter in a `Cell`, and replicate the example from the previous part.
+fn demo_cell(c: &mut Callbacks) {
+ {
+ let count = Cell::new(0);
+ // Again, we have to move ownership if the `count` into the environment closure.
+ c.register(move |val| {
+ // In here, all we have is a shared borrow of our environment. But that's good enough for the `get` and `set` of the cell!
+ let new_count = count.get()+1;
+ count.set(new_count);
+ println!("Callback 2: {} ({}. time)", val, new_count);
+ } );
+ }
+
+ c.call(2); c.clone().call(3);
}
-// **Exercise 12.1**: Change rgrep such that it prints not only the matching lines, but also the name of the file
-// and the number of the line in the file. You will have to change the type of the channels from `String` to something
-// that records this extra information.
+// ## `RefCell`
+
+// Our final version of `Callbacks` puts the closure environment into a `RefCell`.
+#[derive(Clone)]
+struct CallbacksMut {
+ callbacks: Vec<Rc<RefCell<FnMut(i32)>>>,
+}
+
+impl CallbacksMut {
+ pub fn new() -> Self {
+ unimplemented!()
+ }
+
+ pub fn register<F: FnMut(i32)+'static>(&mut self, callback: F) {
+ let cell = Rc::new(RefCell::new(callback));
+ unimplemented!()
+ }
+
+ pub fn call(&mut self, val: i32) {
+ for callback in self.callbacks.iter() {
+ // We have to *explicitly* borrow the contents of a `RefCell` by calling `borrow` or `borrow_mut`.
+ let mut closure = callback.borrow_mut();
+ // Unfortunately, Rust's auto-dereference of pointers is not clever enough here. We thus have to explicitly
+ // dereference the smart pointer and obtain a mutable borrow of the content.
+ (&mut *closure)(val);
+ }
+ }
+}
+
+// Now we can repeat the demo from the previous part - but this time, our `CallbacksMut` type
+// can be cloned.
+fn demo_mut(c: &mut CallbacksMut) {
+ c.register(|val| println!("Callback 1: {}", val));
+ c.call(0);
+
+ {
+ let mut count: usize = 0;
+ c.register(move |val| {
+ count = count+1;
+ println!("Callback 2: {} ({}. time)", val, count);
+ } );
+ }
+ c.call(1); c.clone().call(2);
+}
+// **Exercise 12.1**: Change the type of `call` to ask only for a shared borrow. Then write some piece of code using only the available, public
+// interface of `CallbacksMut` such that a reentrant call to `call` is happening, and the program aborts because the `RefCell` refuses to hand
+// out a second mutable borrow to its content.
-// Rust-101, Part 13: Slices, Arrays, External Dependencies
-// ========================================================
-
+// Rust-101, Part 13: Concurrency, Arc, Send
+// =========================================
+
+use std::io::prelude::*;
+use std::{io, fs, thread};
+use std::sync::mpsc::{sync_channel, SyncSender, Receiver};
+use std::sync::Arc;
+
+
+// Before we come to the actual code, we define a data-structure `Options` to store all the information we need
+// to complete the job: Which files to work on, which pattern to look for, and how to output. <br/>
+#[derive(Clone,Copy)]
+pub enum OutputMode {
+ Print,
+ SortAndPrint,
+ Count,
+}
+use self::OutputMode::*;
-// ## Slices
+pub struct Options {
+ pub files: Vec<String>,
+ pub pattern: String,
+ pub output_mode: OutputMode,
+}
-pub fn sort<T: PartialOrd>(data: &mut [T]) {
- if data.len() < 2 { return; }
- // We decide that the element at 0 is our pivot, and then we move our cursors through the rest of the slice,
- // making sure that everything on the left is no larger than the pivot, and everything on the right is no smaller.
- let mut lpos = 1;
- let mut rpos = data.len();
- /* Invariant: pivot is data[0]; everything with index (0,lpos) is <= pivot;
- [rpos,len) is >= pivot; lpos < rpos */
- loop {
- // **Exercise 13.1**: Complete this Quicksort loop. You can use `swap` on slices to swap two elements. Write a
- // test function for `sort`.
- unimplemented!()
+// The first function reads the files, and sends every line over the `out_channel`.
+fn read_files(options: Arc<Options>, out_channel: SyncSender<String>) {
+ for file in options.files.iter() {
+ // First, we open the file, ignoring any errors.
+ let file = fs::File::open(file).unwrap();
+ // Then we obtain a `BufReader` for it, which provides the `lines` function.
+ let file = io::BufReader::new(file);
+ for line in file.lines() {
+ let line = line.unwrap();
+ // Now we send the line over the channel, ignoring the possibility of `send` failing.
+ out_channel.send(line).unwrap();
+ }
}
-
- // Once our cursors met, we need to put the pivot in the right place.
- data.swap(0, lpos-1);
-
- // Finally, we split our slice to sort the two halves. The nice part about slices is that splitting them is cheap:
- let (part1, part2) = data.split_at_mut(lpos);
- unimplemented!()
+ // When we drop the `out_channel`, it will be closed, which the other end can notice.
}
-// **Exercise 13.2**: Since `String` implements `PartialEq`, you can now change the function `output_lines` in the previous part
-// to call the sort function above. If you did exercise 12.1, you will have slightly more work. Make sure you sort by the matched line
-// only, not by filename or line number!
+// The second function filters the lines it receives through `in_channel` with the pattern, and sends
+// matches via `out_channel`.
+fn filter_lines(options: Arc<Options>,
+ in_channel: Receiver<String>,
+ out_channel: SyncSender<String>) {
+ // We can simply iterate over the channel, which will stop when the channel is closed.
+ for line in in_channel.iter() {
+ // `contains` works on lots of types of patterns, but in particular, we can use it to test whether
+ // one string is contained in another. This is another example of Rust using traits as substitute for overloading.
+ if line.contains(&options.pattern) {
+ unimplemented!()
+ }
+ }
+}
-// Now, we can sort, e.g., an vector of numbers.
-fn sort_nums(data: &mut Vec<i32>) {
- sort(&mut data[..]);
+// The third function performs the output operations, receiving the relevant lines on its `in_channel`.
+fn output_lines(options: Arc<Options>, in_channel: Receiver<String>) {
+ match options.output_mode {
+ Print => {
+ // Here, we just print every line we see.
+ for line in in_channel.iter() {
+ unimplemented!()
+ }
+ },
+ Count => {
+ // We are supposed to count the number of matching lines. There's a convenient iterator adapter that
+ // we can use for this job.
+ unimplemented!()
+ },
+ SortAndPrint => {
+ // We are asked to sort the matching lines before printing. So let's collect them all in a local vector...
+ let mut data: Vec<String> = in_channel.iter().collect();
+ // ...and implement the actual sorting later.
+ unimplemented!()
+ }
+ }
}
-// ## Arrays
-fn sort_array() {
- let mut array_of_data: [f64; 5] = [1.0, 3.4, 12.7, -9.12, 0.1];
- sort(&mut array_of_data);
+// With the operations of the three threads defined, we can now implement a function that performs grepping according
+// to some given options.
+pub fn run(options: Options) {
+ // We move the `options` into an `Arc`, as that's what the thread workers expect.
+ let options = Arc::new(options);
+
+ // This sets up the channels. We use a `sync_channel` with buffer-size of 16 to avoid needlessly filling RAM.
+ let (line_sender, line_receiver) = sync_channel(16);
+ let (filtered_sender, filtered_receiver) = sync_channel(16);
+
+ // Spawn the read thread: `thread::spawn` takes a closure that is run in a new thread.
+ let options1 = options.clone();
+ let handle1 = thread::spawn(move || read_files(options1, line_sender));
+
+ // Same with the filter thread.
+ let options2 = options.clone();
+ let handle2 = thread::spawn(move || {
+ filter_lines(options2, line_receiver, filtered_sender)
+ });
+
+ // And the output thread.
+ let options3 = options.clone();
+ let handle3 = thread::spawn(move || output_lines(options3, filtered_receiver));
+
+ // Finally, wait until all three threads did their job.
+ handle1.join().unwrap();
+ handle2.join().unwrap();
+ handle3.join().unwrap();
}
-// ## External Dependencies
-
-
-// I disabled the following module (using a rather bad hack), because it only compiles if `docopt` is linked.
-// Remove the attribute of the `rgrep` module to enable compilation.
-#[cfg(feature = "disabled")]
-pub mod rgrep {
- // Now that `docopt` is linked, we can first root it in the namespace and then import it with `use`. We also import some other pieces that we will need.
- extern crate docopt;
- use self::docopt::Docopt;
- use part12::{run, Options, OutputMode};
- use std::process;
-
- // The `USAGE` string documents how the program is to be called. It's written in a format that `docopt` can parse.
- static USAGE: &'static str = "
-Usage: rgrep [-c] [-s] <pattern> <file>...
-
-Options:
- -c, --count Count number of matching lines (rather than printing them).
- -s, --sort Sort the lines before printing.
-";
-
- // This function extracts the rgrep options from the command-line arguments.
- fn get_options() -> Options {
- // Parse `argv` and exit the program with an error message if it fails. This is taken from the [`docopt` documentation](http://burntsushi.net/rustdoc/docopt/).
- let args = Docopt::new(USAGE).and_then(|d| d.parse()).unwrap_or_else(|e| e.exit());
- // Now we can get all the values out.
- let count = args.get_bool("-c");
- let sort = args.get_bool("-s");
- let pattern = args.get_str("<pattern>");
- let files = args.get_vec("<file>");
- if count && sort {
- println!("Setting both '-c' and '-s' at the same time does not make any sense.");
- process::exit(1);
- }
+// Now we have all the pieces together for testing our rgrep with some hard-coded options.
+pub fn main() {
+ let options = Options {
+ files: vec!["src/part10.rs".to_string(),
+ "src/part11.rs".to_string(),
+ "src/part12.rs".to_string()],
+ pattern: "let".to_string(),
+ output_mode: Print
+ };
+ run(options);
+}
- // We need to make the strings owned to construct the `Options` instance.
- let mode = if count {
- OutputMode::Count
- } else if sort {
- OutputMode::SortAndPrint
- } else {
- OutputMode::Print
- };
- Options {
- files: files.iter().map(|file| file.to_string()).collect(),
- pattern: pattern.to_string(),
- output_mode: mode,
- }
- }
+// **Exercise 12.1**: Change rgrep such that it prints not only the matching lines, but also the name of the file
+// and the number of the line in the file. You will have to change the type of the channels from `String` to something
+// that records this extra information.
- // Finally, we can call the `run` function from the previous part on the options extracted using `get_options`. Edit `main.rs` to call this function.
- // You can now use `cargo run -- <pattern> <files>` to call your program, and see the argument parser and the threads we wrote previously in action!
- pub fn main() {
- unimplemented!()
- }
-}
-// **Exercise 13.3**: Wouldn't it be nice if rgrep supported regular expressions? There's already a crate that does all the parsing and matching on regular
-// expression, it's called [regex](https://crates.io/crates/regex). Add this crate to the dependencies of your workspace, add an option ("-r") to switch
-// the pattern to regular-expression mode, and change `filter_lines` to honor this option. The documentation of regex is available from its crates.io site.
-// (You won't be able to use the `regex!` macro if you are on the stable or beta channel of Rust. But it wouldn't help for our use-case anyway.)
-// Rust-101, Part 14: Mutex, Interior Mutability, Sync
-// ===================================================
+// Rust-101, Part 14: Slices, Arrays, External Dependencies
+// ========================================================
-use std::sync::{Arc, Mutex};
-use std::thread;
+// ## Slices
-// The derived `Clone` implementation will clone the `Arc`, so all clones will actually talk about the same counter.
-#[derive(Clone)]
-struct ConcurrentCounter(Arc<Mutex<usize>>);
+pub fn sort<T: PartialOrd>(data: &mut [T]) {
+ if data.len() < 2 { return; }
-impl ConcurrentCounter {
- // The constructor just wraps the constructors of `Arc` and `Mutex`.
- pub fn new(val: usize) -> Self {
+ // We decide that the element at 0 is our pivot, and then we move our cursors through the rest of the slice,
+ // making sure that everything on the left is no larger than the pivot, and everything on the right is no smaller.
+ let mut lpos = 1;
+ let mut rpos = data.len();
+ /* Invariant: pivot is data[0]; everything with index (0,lpos) is <= pivot;
+ [rpos,len) is >= pivot; lpos < rpos */
+ loop {
+ // **Exercise 13.1**: Complete this Quicksort loop. You can use `swap` on slices to swap two elements. Write a
+ // test function for `sort`.
unimplemented!()
}
- pub fn increment(&self, by: usize) {
- // `lock` on a mutex returns a *guard*, giving access to the data contained in the mutex.
- let mut counter = self.0.lock().unwrap();
- *counter = *counter + by;
- }
+ // Once our cursors met, we need to put the pivot in the right place.
+ data.swap(0, lpos-1);
- // The function `get` returns the current value of the counter.
- pub fn get(&self) -> usize {
- unimplemented!()
- }
+ // Finally, we split our slice to sort the two halves. The nice part about slices is that splitting them is cheap:
+ let (part1, part2) = data.split_at_mut(lpos);
+ unimplemented!()
}
-// Now our counter is ready for action.
-pub fn main() {
- let counter = ConcurrentCounter::new(0);
+// **Exercise 13.2**: Since `String` implements `PartialEq`, you can now change the function `output_lines` in the previous part
+// to call the sort function above. If you did exercise 12.1, you will have slightly more work. Make sure you sort by the matched line
+// only, not by filename or line number!
- // We clone the counter for the first thread, which increments it by 2 every 15ms.
- let counter1 = counter.clone();
- let handle1 = thread::spawn(move || {
- for _ in 0..10 {
- thread::sleep_ms(15);
- counter1.increment(2);
- }
- });
-
- // The second thread increments the counter by 3 every 20ms.
- let counter2 = counter.clone();
- let handle2 = thread::spawn(move || {
- for _ in 0..10 {
- thread::sleep_ms(20);
- counter2.increment(3);
+// Now, we can sort, e.g., an vector of numbers.
+fn sort_nums(data: &mut Vec<i32>) {
+ sort(&mut data[..]);
+}
+
+// ## Arrays
+fn sort_array() {
+ let mut array_of_data: [f64; 5] = [1.0, 3.4, 12.7, -9.12, 0.1];
+ sort(&mut array_of_data);
+}
+
+// ## External Dependencies
+
+
+// I disabled the following module (using a rather bad hack), because it only compiles if `docopt` is linked.
+// Remove the attribute of the `rgrep` module to enable compilation.
+#[cfg(feature = "disabled")]
+pub mod rgrep {
+ // Now that `docopt` is linked, we can first add it to the namespace and then import shorter names with `use`. We also import some other pieces that we will need.
+ extern crate docopt;
+ use self::docopt::Docopt;
+ use part12::{run, Options, OutputMode};
+ use std::process;
+
+ // The `USAGE` string documents how the program is to be called. It's written in a format that `docopt` can parse.
+ static USAGE: &'static str = "
+Usage: rgrep [-c] [-s] <pattern> <file>...
+
+Options:
+ -c, --count Count number of matching lines (rather than printing them).
+ -s, --sort Sort the lines before printing.
+";
+
+ // This function extracts the rgrep options from the command-line arguments.
+ fn get_options() -> Options {
+ // Parse `argv` and exit the program with an error message if it fails. This is taken from the [`docopt` documentation](http://burntsushi.net/rustdoc/docopt/).
+ let args = Docopt::new(USAGE).and_then(|d| d.parse()).unwrap_or_else(|e| e.exit());
+ // Now we can get all the values out.
+ let count = args.get_bool("-c");
+ let sort = args.get_bool("-s");
+ let pattern = args.get_str("<pattern>");
+ let files = args.get_vec("<file>");
+ if count && sort {
+ println!("Setting both '-c' and '-s' at the same time does not make any sense.");
+ process::exit(1);
}
- });
- // Now we watch the threads working on the counter.
- for _ in 0..50 {
- thread::sleep_ms(5);
- println!("Current value: {}", counter.get());
+ // We need to make the strings owned to construct the `Options` instance.
+ let mode = if count {
+ OutputMode::Count
+ } else if sort {
+ OutputMode::SortAndPrint
+ } else {
+ OutputMode::Print
+ };
+ Options {
+ files: files.iter().map(|file| file.to_string()).collect(),
+ pattern: pattern.to_string(),
+ output_mode: mode,
+ }
}
- // Finally, we wait for all the threads to finish to be sure we can catch the counter's final value.
- handle1.join().unwrap();
- handle2.join().unwrap();
- println!("Final value: {}", counter.get());
+ // Finally, we can call the `run` function from the previous part on the options extracted using `get_options`. Edit `main.rs` to call this function.
+ // You can now use `cargo run -- <pattern> <files>` to call your program, and see the argument parser and the threads we wrote previously in action!
+ pub fn main() {
+ unimplemented!()
+ }
}
-// **Exercise 14.1**: Besides `Mutex`, there's also [`RwLock`](http://doc.rust-lang.org/stable/std/sync/struct.RwLock.html), which
-// provides two ways of locking: One that grants only read-only access, to any number of concurrent readers, and another one
-// for exclusive write access. (Notice that this is the same pattern we already saw with shared vs. mutable borrows.) Change
-// the code above to use `RwLock`, such that multiple calls to `get` can be executed at the same time.
-//
-// **Exercise 14.2**: Add an operation `compare_and_inc(&self, test: usize, by: usize)` that increments the counter by
-// `by` *only if* the current value is `test`.
-
+// **Exercise 13.3**: Wouldn't it be nice if rgrep supported regular expressions? There's already a crate that does all the parsing and matching on regular
+// expression, it's called [regex](https://crates.io/crates/regex). Add this crate to the dependencies of your workspace, add an option ("-r") to switch
+// the pattern to regular-expression mode, and change `filter_lines` to honor this option. The documentation of regex is available from its crates.io site.
+// (You won't be able to use the `regex!` macro if you are on the stable or beta channel of Rust. But it wouldn't help for our use-case anyway.)
-// Rust-101, Part 15: Interior Mutability (cont.), RefCell, Cell, Drop
-// ===================================================================
+// Rust-101, Part 15: Mutex, Interior Mutability (cont.), Sync
+// ===========================================================
+
+use std::sync::{Arc, Mutex};
+use std::thread;
+
+
+// The derived `Clone` implementation will clone the `Arc`, so all clones will actually talk about the same counter.
+#[derive(Clone)]
+struct ConcurrentCounter(Arc<Mutex<usize>>);
+
+impl ConcurrentCounter {
+ // The constructor just wraps the constructors of `Arc` and `Mutex`.
+ pub fn new(val: usize) -> Self {
+ unimplemented!()
+ }
+
+ pub fn increment(&self, by: usize) {
+ // `lock` on a mutex returns a *guard*, giving access to the data contained in the mutex.
+ let mut counter = self.0.lock().unwrap();
+ *counter = *counter + by;
+ }
+
+ // The function `get` returns the current value of the counter.
+ pub fn get(&self) -> usize {
+ unimplemented!()
+ }
+}
+
+// Now our counter is ready for action.
+pub fn main() {
+ let counter = ConcurrentCounter::new(0);
+
+ // We clone the counter for the first thread, which increments it by 2 every 15ms.
+ let counter1 = counter.clone();
+ let handle1 = thread::spawn(move || {
+ for _ in 0..10 {
+ thread::sleep_ms(15);
+ counter1.increment(2);
+ }
+ });
+
+ // The second thread increments the counter by 3 every 20ms.
+ let counter2 = counter.clone();
+ let handle2 = thread::spawn(move || {
+ for _ in 0..10 {
+ thread::sleep_ms(20);
+ counter2.increment(3);
+ }
+ });
+
+ // Now we watch the threads working on the counter.
+ for _ in 0..50 {
+ thread::sleep_ms(5);
+ println!("Current value: {}", counter.get());
+ }
+
+ // Finally, we wait for all the threads to finish to be sure we can catch the counter's final value.
+ handle1.join().unwrap();
+ handle2.join().unwrap();
+ println!("Final value: {}", counter.get());
+}
+
+// **Exercise 14.1**: Besides `Mutex`, there's also [`RwLock`](http://doc.rust-lang.org/stable/std/sync/struct.RwLock.html), which
+// provides two ways of locking: One that grants only read-only access, to any number of concurrent readers, and another one
+// for exclusive write access. (Notice that this is the same pattern we already saw with shared vs. mutable borrows.) Change
+// the code above to use `RwLock`, such that multiple calls to `get` can be executed at the same time.
+//
+// **Exercise 14.2**: Add an operation `compare_and_inc(&self, test: usize, by: usize)` that increments the counter by
+// `by` *only if* the current value is `test`.
+
+
+// FIXME TODO some old outdated explanation FIXME TODO
+
projects / rust-101.git / commitdiff