X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/c3cbdbbd2fff81d2509b22d614343e6ca0250c09..229b86d07e94cd3ec175051a44b3f3cb45b40b65:/workspace/src/part02.rs?ds=sidebyside diff --git a/workspace/src/part02.rs b/workspace/src/part02.rs index 8d1c8b6..908bb2a 100644 --- a/workspace/src/part02.rs +++ b/workspace/src/part02.rs @@ -1 +1,150 @@ - +// ***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!() + } + + fn to_option(self) -> Option { + 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) + }); + } + 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`. +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 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. +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 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 +// 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)