Fix the variable type

[rust-101.git] / src / part02.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 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, a type like `SomethingOrNothing` 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](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<T>` from an `Option<T>`, and vice versa.
//@ 
//@ 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 {
        match o { None => Nothing, Some(t) => Something(t) }        /*@*/
    }

    fn to_option(self) -> Option<T> {
        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<i32> {
    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. <br/>
//@ 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 `min_i32(n, e)`. Rust automatically figures out that `e` 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,
            // 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<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 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<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 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<f32>` (where
// `f32` is the type of 32-bit floating-point numbers). You should not change `vec_min` in any
// way, obviously!

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