X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/53260b4082f1c76b657845560c273b853a166fab..ab942598491747509a9586bf3c0c1c65785b9dd3:/src/part02.rs?ds=inline diff --git a/src/part02.rs b/src/part02.rs index 32fbe0a..44a20d4 100644 --- a/src/part02.rs +++ b/src/part02.rs @@ -1,28 +1,149 @@ -// Rust-101, Part 02: Generic types (WIP) -// ================================ - -use std; - -// 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. -// -// 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. A generic -// `SomethingOrNothing` type looks as follows: -enum SomethingOrNothing { +// 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, } -use self::SomethingOrNothing::{Something,Nothing}; -// What this does is to define an entire family of types: We can now write -// `SomethingOrNothing` to get back our `NumberOrNothing`, but 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.) +// Instead of writing out all the variants, we can also just import them all at once. +pub use self::SomethingOrNothing::*; +//@ What this does is 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, a type like `SomethingOrNothing` 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](https://doc.rust-lang.org/stable/std/option/index.html)! +//@ (And don't worry, there's indeed lots of material mentioned there that we have not covered 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. +//@ +//@ 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 { + match o { None => Nothing, Some(t) => Something(t) } /*@*/ + } + + fn to_option(self) -> Option { + match self { Nothing => None, Something(t) => Some(t) } /*@*/ + } +} +//@ 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 have 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 two 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 functions 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, + // Here, we can now call the `min` function of the trait. + 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 also inherit from our abstract base class after the fact. +//@ +//@ 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 `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 } /*@*/ + } +} + +// 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. +impl NumberOrNothing { + pub fn print(self) { + match self { + Nothing => println!("The number is: "), + Something(n) => println!("The number is: {}", n), + }; + } +} + +// Now we are ready to run our new 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. +fn read_vec() -> Vec { + vec![18,5,7,3,9,27] +} +pub fn main() { + let vec = read_vec(); + let min = vec_min(vec); + min.print(); +} + +//@ If this printed `3`, then your generic `vec_min` is working! So get ready for the next part. + +// **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) +//@ [index](main.html) | [previous](part01.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part02.rs) | [next](part03.html)