More comments in solution for cyclic Callbacks

[rust-101.git] / src / part12.rs

diff --git a/src/part12.rs b/src/part12.rs
index 40d9ad52a0a1f7f0648f64239c2f8adb65bada10..c2331eaf618b82aa2ee8e1308512c52b364401cd 100644 (file)
--- a/src/part12.rs
+++ b/src/part12.rs
@@ -9,16 +9,17 @@ use std::cell::{Cell, RefCell};
 //@ (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?
 
 //@ (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?
 

+//@ ## `Rc`

 //@ 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:
 //@ 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.
+//@ Once data is stored in an `Rc`, it is read-only and you can only ever get a shared reference to 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.
+//@ Because of this read-only restriction, we cannot use `FnMut` here: We'd be unable to call the function with a mutable reference
+//@ to 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)>>,
 #[derive(Clone)]
 struct Callbacks {
     callbacks: Vec<Rc<Fn(i32)>>,


@@ -60,19 +61,19 @@ pub fn main() {
 //@ 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`.
 //@ 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
+//@ `set`, which overrides the content, only needs a *shared reference* to the cell. The phenomenon of a type that permits mutation through
+//@ shared references (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
 //@ 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/>
+//@ about replacing one piece of data by something else of the same type. This is called *inherited 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);
 //@ 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.
+        // Again, we have to move ownership of the `count` into the environment closure.

         c.register(move |val| {
         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!
+            // In here, all we have is a shared reference 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;
             //@ 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;


@@ -85,8 +86,13 @@ fn demo_cell(c: &mut Callbacks) {
 }
 
 //@ It is worth mentioning that `Rc` itself also has to make use of interior mutability: When you `clone` an `Rc`, all it has available
 }
 
 //@ 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
+//@ is a shared reference. However, it has to increment the reference count! Internally, `Rc` uses `Cell` for the count, such that it

 //@ can be updated during `clone`.
 //@ can be updated during `clone`.

+//@ 
+//@ Putting it all together, the story around mutation and ownership through references looks as follows: There are *unique* references,
+//@ which - because of their exclusivity - are always safe to mutate through. And there are *shared* references, where the compiler cannot
+//@ generally promise that mutation is safe. However, if extra circumstances guarantee that mutation *is* safe, then it can happen even
+//@ through a shared reference - as we saw with `Cell`.

 
 // ## `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`
 
 // ## `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`


@@ -119,22 +125,23 @@ impl CallbacksMut {
     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`.
     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,
+            //@ At run-time, the cell will keep track of the number of outstanding shared and mutable references,

             //@ and panic if the rules are violated. <br />
             //@ 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.
+            //@ For this check to be performed, `closure` is a *guard*: Rather than a normal reference, `borrow_mut` returns
+            //@ a smart pointer ([`RefMut`](https://doc.rust-lang.org/stable/std/cell/struct.RefMut.html), in this case) that waits until is goes out of scope, and then
+            //@ appropriately updates the number of active references.

             //@ 
             //@ Since `call` is the only place that borrows the environments of the closures, we should expect that
             //@ 
             //@ 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!
+            //@ the check will always succeed, as is actually entirely useless. However, this is not actually true. Several different `CallbacksMut` could share
+            //@ a callback (as they were created with `clone`), and calling one callback here could trigger calling
+            //@ all callbacks of the other `CallbacksMut`, which would end up calling the initial callback again. This issue of functions accidentally recursively calling
+            //@ themselves is called *reentrancy*, and it can lead to subtle bugs. Here, it would mean that the closure runs twice, each time thinking it has a
+            //@ unique, mutable reference to its environment - so it may end up dereferencing a dangling pointer. Ouch! Lucky enough,
+            //@ Rust detects this at run-time and panics once we try to borrow the same environment again. 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
             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.
+            // dereference the smart pointer and obtain a mutable reference to the content.

             (&mut *closure)(val);
         }
     }
             (&mut *closure)(val);
         }
     }


@@ -156,8 +163,7 @@ fn demo_mut(c: &mut CallbacksMut) {
     c.call(1); c.clone().call(2);
 }
 
     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.
+// **Exercise 12.1**: Write some piece of code using only the available, public interface of `CallbacksMut` such that a reentrant call to a closure
+// is happening, and the program panics because the `RefCell` refuses to hand out a second mutable borrow of the closure's environment.

 
 

-//@ [index](main.html) | [previous](part11.html) | [next](part13.html)
+//@ [index](main.html) | [previous](part11.html) | [raw source](workspace/src/part12.rs) | [next](part13.html)