X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/229b86d07e94cd3ec175051a44b3f3cb45b40b65..57fd78e47f3f9d935f62a5dc05e8676ae035ad3d:/workspace/src/part02.rs diff --git a/workspace/src/part02.rs b/workspace/src/part02.rs index 908bb2a..357c5a0 100644 --- a/workspace/src/part02.rs +++ b/workspace/src/part02.rs @@ -1,46 +1,20 @@ -// ***Remember to enable/add this part in `main.rs`!*** - // Rust-101, Part 02: Generic types, Traits // ======================================== -// Let us for a moment reconsider the type `NumberOrNothing`. Isn't it a bit annoying that we -// had to hard-code the type `i32` in there? What if tomorrow, we want a `CharOrNothing`, and -// later a `FloatOrNothing`? Certainly we don't want to re-write the type and all its inherent methods. // ## Generic datatypes -// The solution to this is called *generics* or *polymorphism* (the latter is Greek, -// meaning "many shapes"). You may know something similar from C++ (where it's called -// *templates*) or Java, or one of the many functional languages. So here, we define -// a generic type `SomethingOrNothing`. pub enum SomethingOrNothing { Something(T), Nothing, } // 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` to get back our `NumberOrNothing`. type NumberOrNothing = SomethingOrNothing; -// However, we can also write `SomethingOrNothing` or even `SomethingOrNothing>`. -// 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`. 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.) // ## Generic `impl`, Static functions -// The types are so similar, that we can provide a generic function to construct a `SomethingOrNothing` -// from an `Option`, and vice versa. -// **Exercise 02.1**: Implement such functions! I provided a skeleton of the solution. Here, -// `unimplemented!` is another macro. This one terminates execution saying that something has not yet -// been implemented. -// -// Notice the syntax for giving generic implementations to generic types: Think of the first `` -// as *declaring* a type variable ("I am doing something for all types `T`"), and the second `` as -// *using* that variable ("The thing I do, is implement `SomethingOrNothing`"). -// // Inside an `impl`, `Self` refers to the type we are implementing things for. Here, it is // an alias for `SomethingOrNothing`. -// Remember that `self` is the `this` of Rust, and implicitly has type `Self`. impl SomethingOrNothing { fn new(o: Option) -> Self { unimplemented!() @@ -50,77 +24,40 @@ impl SomethingOrNothing { unimplemented!() } } -// 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 demonstrated in `call_constructor`. fn call_constructor(x: i32) -> SomethingOrNothing { SomethingOrNothing::new(Some(x)) } // ## Traits -// Now that we have a generic `SomethingOrNothing`, wouldn't it be nice to also gave a generic -// `vec_min`? Of course, we can't take the minimum of a vector of *any* type. It has to be a type -// supporting a `min` operation. Rust calls such properties that we may demand of types *traits*. -// 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.
-// 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(self, b: Self) -> Self; } -// 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. -// -// 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!
-// This is in strong contrast to C++, where the compiler only checks such details when the -// function is actually used. pub fn vec_min(v: Vec) -> SomethingOrNothing { let mut min = Nothing; for e in v { min = Something(match min { Nothing => e, - Something(n) => e.min(n) + // Here, we can now call the `min` function of the trait. + Something(n) => { + unimplemented!() + } }); } 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. // ## Trait implementations -// To make the function usable with a `Vec`, we implement the `Minimum` trait for `i32`. +// To make `vec_min` usable with a `Vec`, we implement the `Minimum` trait for `i32`. impl Minimum for i32 { fn min(self, b: Self) -> Self { - if self < b { self } else { b } + unimplemented!() } } -// We again provide a `print` function. This also shows that we can have multiple `impl` blocks -// for the same type (remember that `NumberOrNothing` is just a type alias for `SomethingOrNothing`), -// and we can provide some methods only for certain instances of a generic type. +// We again provide a `print` function. impl NumberOrNothing { pub fn print(self) { match self { @@ -130,9 +67,7 @@ impl NumberOrNothing { } } -// 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. +// Now we are ready to run our new code. Remember to change `main.rs` appropriately. fn read_vec() -> Vec { vec![18,5,7,3,9,27] } @@ -142,9 +77,7 @@ pub fn main() { min.print(); } -// If this printed `3`, then you generic `vec_min` is working! So get ready for the next part. -// **Exercise 02.2**: Change your program such that it computes the minimum of a `Vec` (where `f32` is the type +// **Exercise 02.1**: Change your program such that it computes the minimum of a `Vec` (where `f32` is the type // of 32-bit floating-point numbers). You should not change `vec_min` in any way, obviously! -// [index](main.html) | [previous](part01.html) | [next](part03.html)