+++ /dev/null
-// ***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<T> {
- 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<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.)
-
-// ## Generic `impl`, Static functions
-// 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 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 `<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 {
- unimplemented!()
- }
-
- fn to_option(self) -> Option<T> {
- 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<i32> {
- 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.<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(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!<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> {
- 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<i32>`, 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<i32>`),
-// 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: <nothing>"),
- 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<i32> {
- 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<f32>` (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)
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