// Instead of writing out all the variants, we can also just import them all at once.
pub use self::SomethingOrNothing::*;
// What this does is to define an entire family of types: We can now write
-// `SomethingOrNothing<i32>` to get back our `NumberOrNothing`, but we
-// can also write `SomethingOrNothing<bool>` or even `SomethingOrNothing<SomethingOrNothing<i32>>`.
+// `SomethingOrNothing<i32>` to get back our `NumberOrNothing`.
+type NumberOrNothing = SomethingOrNothing<i32>;
+// However, we can also write `SomethingOrNothing<bool>` or even `SomethingOrNothing<SomethingOrNothing<i32>>`.
// In fact, such a type is so useful that it is already present in the standard
// library: It's called an *option type*, written `Option<T>`.
// Go check out its [documentation](http://doc.rust-lang.org/stable/std/option/index.html)!
// (And don't worry, there's indeed lots of material mentioned there that we did not cover yet.)
-// **Exercise**: Write functions converting between `SomethingOrNothing<T>` and `Option<T>`. You will have to use
-// the names of the constructor of `Option`, which you can find in the documentation I linked above.
-//
-// Here's a skeleton for your solution, you only have to fill in the function bodies.
-// (`panic!` is, again, a macro - this one terminates execution when it is reached).
+// The types are so similar, that we can provide a generic function to construct a `SomethingOrNothing<T>`
+// from an `Option<T>`, and vice versa.
+
+// **Exercise**: Implement such functions! I provided a skeleton of the solution. Here,
+// `panic!` is another macro. This one terminates execution with the given message.
//
// Notice the syntax for giving generic implementations to generic types: Think of the first `<T>`
// as *declaring* a type variable ("I am doing something for all types `T`"), and the second `<T>` as
// *using* that variable ("The thing I do, is implement `SomethingOrNothing<T>`").
+//
+// Inside an `impl`, `Self` refers to the type we are implementing things for. Here, it is
+// an alias for `SomethingOrNothing<T>`.
+// Remember that `self` is the `this` of Rust, and implicitly has type `Self`.
impl<T> SomethingOrNothing<T> {
fn new(o: Option<T>) -> Self {
panic!("Not yet implemented.")
panic!("Not yet implemented.")
}
}
-// Inside an `impl`, `Self` refers to the type we are implementing things for. Here, it is
-// an alias for `SomethingOrNothing<T>`.
-// Remember that `self` is the `this` of Rust, and implicitly has type `Self`.
-//
// Observe how `new` does *not* have a `self` parameter. This corresponds to a `static` method
// in Java or C++. In fact, `new` is the Rust convention for defining constructors: They are
// nothing special, just static functions returning `Self`.
-
-// You can call static functions, and in particular constructors, as follows:
+//
+// You can call static functions, and in particular constructors, as demonstrated in `call_constructor`.
fn call_constructor(x: i32) -> SomethingOrNothing<i32> {
SomethingOrNothing::new(Some(x))
}
// So, as a first step towards a generic `vec_min`, we define a `Minimum` trait.
// For now, just ignore the `Copy`, we will come back to this point later.
// A `trait` is a lot like interfaces in Java: You define a bunch of functions
-// you want to have implemented, and their argument and return types.
+// you want to have implemented, and their argument and return types.<br/>
+// The function `min` takes to arguments of the same type, but I made the
+// first argument the special `self` argument. I could, alternatively, have
+// made `min` a static function as follows: `fn min(a: Self, b: Self) -> Self`.
+// However, in Rust one typically prefers methods over static function wherever possible.
pub trait Minimum : Copy {
- fn min(a: Self, b: Self) -> Self;
+ fn min(self, b: Self) -> Self;
}
-// Now we can write `vec_min`, generic over a type `T` that we demand to satisfy the `Minimum` trait.
-// This is called a *trait bound*.
-// The only difference to the version from the previous part is that we call `T::min` (the `min`
-// function provided for type `T`) instead of `std::cmp::min`.
+// Next, we write `vec_min` as a generic function over a type `T` that we demand to satisfy the `Minimum` trait.
+// This requirement is called a *trait bound*.
+// The only difference to the version from the previous part is that we call `e.min(n)` instead
+// of `std::cmp::min(n, e)`. Rust automatically figures out that `n` is of type `T`, which implements
+// the `Minimum` trait, and hence we can call that function.
//
-// Notice a crucial difference to templates in C++: We actually have to declare which traits
+// There is a crucial difference to templates in C++: We actually have to declare which traits
// we want the type to satisfy. If we left away the `Minimum`, Rust would have complained that
-// we cannot call `min`. Just try it! There is no reason to believe that `T` provides such an operation.
+// we cannot call `min`. Just try it!<br/>
// This is in strong contrast to C++, where the compiler only checks such details when the
// function is actually used.
pub fn vec_min<T: Minimum>(v: Vec<T>) -> SomethingOrNothing<T> {
for e in v {
min = Something(match min {
Nothing => e,
- Something(n) => T::min(n, e)
+ Something(n) => e.min(n)
});
}
min
}
+// Before going on, take a moment to ponder the flexibility of Rust's take on abstraction:
+// We just defined our own, custom trait (interface), and then implemented that trait
+// *for an existing type*. With the hierarchical approach of, e.g., C++ or Java,
+// that's not possible: We cannot make an existing type suddenly also inherit from our abstract base class.
+//
+// In case you are worried about performance, note that Rust performs *monomorphisation*
+// of generic functions: When you call `vec_min` with `T` being `i32`, Rust essentially goes
+// ahead and creates a copy of the function for this particular type, filling in all the blanks.
+// In this case, the call to `T::min` will become a call to our implementation *statically*. There is
+// no dynamic dispatch, like there would be for Java interface methods or C++ `virtual` methods.
+// This behavior is similar to C++ templates. The optimizer (Rust is using LLVM) then has all the
+// information it could want to, e.g., inline function calls.
// To make the function usable with a `Vec<i32>`, we implement the `Minimum` trait for `i32`.
impl Minimum for i32 {
- fn min(a: Self, b: Self) -> Self {
- std::cmp::min(a, b)
+ fn min(self, b: Self) -> Self {
+ std::cmp::min(self, b)
}
}
// In order to run our code and see the result, we again provide a `print` function.
-// This also shows that we can have multiple `impl` blocks for the same type, and we
+// This also shows that we can have multiple `impl` blocks for the same type (remember
+// that `NumberOrNothing` is just a type alias for `SomethingOrNothing<i32>`), and we
// can provide some methods only for certain instances of a generic type.
-impl SomethingOrNothing<i32> {
+impl NumberOrNothing{
pub fn print(self) {
match self {
Nothing => println!("The number is: <nothing>"),
// Now we are again ready to run our code. Remember to change `main.rs` appropriately.
// Rust figures out automatically that we want the `T` of `vec_min` to be `i32`, and
// that `i32` implements `Minimum` and hence all is good.
-//
-// In case you are worried about performance, note that Rust performs *monomorphisation*
-// of generic functions: When you call `vec_min` with `T` being `i32`, Rust essentially goes
-// ahead and creates a copy of the function for this particular type, filling in all the blanks.
-// In this case, the call to `T::min` will become a call to our implementation *statically*. There is
-// no dynamic dispatch, like there would be for Java interface methods or C++ `virtual` methods.
-// This behavior is similar to C++ templates. The optimizer (Rust is using LLVM) then has all the
-// information it could want to, e.g., inline function calls.
fn read_vec() -> Vec<i32> {
vec![18,5,7,3,9,27]
}
min.print();
}
-// If this printed `3`, then you generic `vec_min` is working!
-//
-// Before going on, take a moment to ponder the flexibility of Rust's take on abstraction:
-// We just defined our own, custom trait (interface), and then implemented that trait
-// *for an existing type*. With the hierarchical approach of, e.g., C++ or Java,
-// that's not possible: We cannot make an existing type suddenly also inherit from our abstract base class.
-
-// **Exercise**: Define a trait `Print` to write a generic version of `SomethingOrNothing::print`.
-// Implement that trait for `i32`, and change the code above to use it.
-// I will again provide a skeleton for this solution. It also shows how to attach bounds to generic
-// implementations (just compare it to the `impl` block from the previous exercise).
-// You can read this as "For all types `T` satisfying the `Print` trait, I provide an implementation
-// for `SomethingOrNothing<T>`".
-//
-// Notice that I called the function on `SomethingOrNothing` `print2` to disambiguate from the `print` defined above.
-//
-// *Hint*: There is a macro `print!` for printing without appending a newline.
-trait Print {
- /* Add things here */
-}
-impl<T: Print> SomethingOrNothing<T> {
- fn print2(self) {
- panic!("Not yet implemented.")
- }
-}
+// If this printed `3`, then you generic `vec_min` is working! So get ready for the next part.
// [index](main.html) | [previous](part01.html) | [next](part03.html)
// In part 00, I promised that we would eventually replace `read_vec` by a function
// that actually asks the user to enter a bunch of numbers. Unfortunately,
-// I/O is a complicated topic, so the code to do that is not pretty - but well,
+// I/O is a complicated topic, so the code to do that is not exactly pretty - but well,
// let's get that behind us.
-// IO/ is provided by the module `std::io`, so we first import that.
+// I/O is provided by the module `std::io`, so we first import that.
// We also import the I/O *prelude*, which brings a bunch of commonly used I/O stuff
// directly available.
use std::io::prelude::*;
use std::io;
-// Let's now go over this function line-by-line.
+// Let's now go over this function line-by-line. First, we call the constructor of `Vec`
+// to create an empty vector. As mentioned in the previous part, `new` here is just
+// a static function with no special treatment. While it is possible to call `new`
+// for a particular type (`Vec::<i32>::new()`), the common way to make sure we
+// get the right type is to annotate a type at the *variable*. It is this variable
+// that we interact with for the rest of the function, so having its type available
+// (and visible!) is much more useful. Without knowing the return type of `Vec::new`,
+// specifying its type parameter doesn't tell us all that much.
fn read_vec() -> Vec<i32> {
- let mut vec = Vec::new();
+ let mut vec: Vec<i32> = Vec::<i32>::new();
// The central handle to the standard input is made available by `io::stdin()`.
let stdin = io::stdin();
println!("Enter a list of numbers, one per line. End with Ctrl-D.");
// it. (See [the documentation](http://doc.rust-lang.org/stable/std/io/struct.Stdin.html) for more
// details.)
for line in stdin.lock().lines() {
- // The `line` we have here is not yet of type `String`. The problem with I/O is that it can always
- // go wrong, so `line` has type `io::Result<String>`. This is a lot like `Option<String>` ("a `String` or
+ // Rust's type for (dynamic, growable) strings is `String`. However, our variable `line`
+ // here is not yet of that type. The problem with I/O is that it can always go wrong, so
+ // `line` has type `io::Result<String>`. This is a lot like `Option<String>` ("a `String` or
// nothing"), but in the case of "nothing", there is additional information about the error.
// Again, I recommend to check [the documentation](http://doc.rust-lang.org/stable/std/io/type.Result.html).
// You will see that `io::Result` is actually just an alias for `Result`, so click on that to obtain
// the list of all constructors and methods of the type.
// We will be lazy here and just assume that nothing goes wrong: `unwrap()` returns the `String` if there is one,
- // and halts the program (with an appropriate error message) otherwise. Can you find the documentation
- // of `Result::unwrap()`?
+ // and panics the program otherwise. Since a `Result` carries some details about the error that occurred,
+ // there will be a somewhat reasonable error message. Still, you would not want a user to see such
+ // an error, so in a "real" program, we would have to do proper error handling.
+ // Can you find the documentation of `Result::unwrap()`?
+ //
+ // I chose the same name (`line`) for the new variable to ensure that I will never, accidentally,
+ // access the "old" `line` again.
let line = line.unwrap();
// Now that we have our `String`, we want to make it an `i32`. `parse` is a method on `String` that
// can convert a string to anything. Try finding it's documentation!
// In this case, Rust *could* figure out automatically that we need an `i32` (because of the return type
- // of the function), but that's a bit too much magic for my taste. So I use this opportunity to
- // introduce the syntax for explicitly giving the type parameter of a generic function: `parse::<i32>` is `parse`
- // with its generic type set to `i32`.
+ // of the function), but that's a bit too much magic for my taste. We are being more explicit here:
+ // `parse::<i32>` is `parse` with its generic type set to `i32`.
match line.parse::<i32>() {
// `parse` returns again a `Result`, and this time we use a `match` to handle errors (like, the user entering
// something that is not a number).
}
// So much for `read_vec`. If there are any questions left, the documentation of the respective function
-// should be very helpful. I will not always provide the links, as the documentation is quite easy to navigate
-// and you should get used to that.
+// should be very helpful. Try finding the one for `Vec::push`. I will not always provide the links,
+// as the documentation is quite easy to navigate and you should get used to that.
// For the rest of the code, we just re-use part 02 by importing it with `use`.
// I already sneaked a bunch of `pub` in the other module to make this possible: Only
min.print();
}
-// After all this nit-picking about I/O details, let me show you quickly something unrelated,
-// but really nice: Rust's built-in support for testing.
-// Now that the user can run our program on loads of inputs, we better make sure that it is correct.
-// To be able to test the result of `vec_min`, we first have to write a function that
-// is able to test equality if `SimethingOrNothing`. So let's quickly do that.
-
-// `equals` performs pattern-matching on both `self` and `other` to test the two for being
-// equal. Because we are lazy, we want to write only one `match`. so we group the two into a
-// pair such that we can match on both of them at once. You can read the first arm of the match
-// as testing whether `(self, other)` is `(Nothing, Nothing)`, which is the case exactly if
-// both `self` and `other` are `Nothing`. Similar so for the second arm.
-impl SomethingOrNothing<i32> {
- pub fn equals(self, other: Self) -> bool {
- match (self, other) {
- (Nothing , Nothing ) => true,
- (Something(n), Something(m)) => n == m,
- // `_` is the syntax for "I don't care", so this is how you add a default case to your `match`.
- _ => false,
- }
- }
-}
-
-// Now we are almost done! Writing a test in Rust is shockingly simple. Just write a function
-// that takes no arguments as returns nothing, and add `#[test]` right in front of it.
-// That's called an *attribute*, and the `test` attribute, well, declares the function to
-// be a test.
-
-// Within the function, we can then use `panic!` to indicate test failure. Helpfully, there's
-// a macro `assert!` that panics if its argument becomes `false`.
-// Using `assert!` and our brand-new `equals`, we can now call `vec_min` with some lists
-// and make sure it returns The Right Thing.
-#[test]
-fn test_vec_min() {
- assert!(vec_min(vec![6,325,33,532,5,7]).equals(Something(5)));
- assert!(vec_min(vec![6,325,33,532]).equals(Something(6)));
-}
-// To execute the test, run `cargo test`. It should tell you that everything is all right.
-// Now that was simple, wasn't it?
-//
-// **Exercise**: Add a case to `test_vec_min` that checks the behavior on empty lists.
+// **Exercise**: Define a trait `Print` to write a generic version of `SomethingOrNothing::print`.
+// Implement that trait for `i32`, and change the code above to use it.
+// I will again provide a skeleton for this solution. It also shows how to attach bounds to generic
+// implementations (just compare it to the `impl` block from the previous exercise).
+// You can read this as "For all types `T` satisfying the `Print` trait, I provide an implementation
+// for `SomethingOrNothing<T>`".
//
-// **Exercise**: Change `vec_min` such that everything still compiles, but the test fails.
+// Notice that I called the function on `SomethingOrNothing` `print2` to disambiguate from the `print` defined previously.
//
-// **Bonus Exercise**: Because `String::parse` is itself generic, you can change `read_vec` to
-// be a generic function that works for any type, not just for `i32`. However, you will have to add
-// a trait bound to `read_vec`, as not every type supports being parsed. <br/>
-// Once you made `vec_min` generic, copy your generic `print` from the previous part. Implement all
-// our traits (`Minimum` and `Print`) for `f32` (32-bit floating-point numbers), and change `part_main()`
-// such that your program now computes the minimum of a list of floating-point numbers. <br/>
-// *Hint*: You can figure out the trait bound `read_vec` needs from the documentation of `String::parse`.
-// Furthermore, `std::cmp::min` works not just for `i32`, but also for `f32`.
+// *Hint*: There is a macro `print!` for printing without appending a newline.
+trait Print {
+ /* Add things here */
+}
+impl<T: Print> SomethingOrNothing<T> {
+ fn print2(self) {
+ panic!("Not yet implemented.")
+ }
+}
// [index](main.html) | [previous](part02.html) | [next](part04.html)
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