-// Rust-101, Part 02: Generic types (WIP)
-// ================================
+// Rust-101, Part 02: Generic types, Traits
+// ========================================
use std;
// 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. A generic
-// `SomethingOrNothing` type looks as follows:
-enum SomethingOrNothing<T> {
+// *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,
}
-use self::SomethingOrNothing::{Something,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`, but we
-// can also write `SomethingOrNothing<bool>` or even `SomethingOrNothing<SomethingOrNothing<i32>>`.
+// `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 {
+ 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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