-// Rust-101, Part 02: Generic types (WIP)
-// ================================
+// 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.
-// 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 `SomethingOrNothing` type.
-enum SomethingOrNothing<T> {
+//@ 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,
}
-use self::SomethingOrNothing::{Something,Nothing};
-// 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>>`.
-// 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.)
-
-// **Exercise**: Write functions converting between `SomethingOrNothing<T>` and `Option<T>`. You will have to use
-// the names of the constructor of `Option`, which you can find in the documentation I linked above.
-
-// Here's a skeleton for your solution, you only have to fill in the function bodies.
-// (`panic!` is, again, a macro - this one terminates execution when it is reached).
-//
-// 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>`").
+// 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, 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 {
- panic!("Not yet implemented.");
+ match o { None => Nothing, Some(t) => Something(t) } /*@*/
}
fn to_option(self) -> Option<T> {
- panic!("Not yet implemented.");
+ match self { Nothing => None, Something(t) => Some(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`.
-//
-// 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 follows:
+//@ 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))
}
-
-// [index](main.html) | [previous](part01.html) | next
+// ## 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 `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,
+ // 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 you 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!
+
+//@ [index](main.html) | [previous](part01.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part02.rs) | [next](part03.html)
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