[rust-101.git] / src / part02.rs

// Rust-101, Part 02: Generic types, Traits
// ========================================

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.

// ## 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,
// `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.")
    }

    fn to_option(self) -> Option<T> {
        panic!("Not yet implemented.")
    }
}
// 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 {
        std::cmp::min(self, 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.

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