X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/53260b4082f1c76b657845560c273b853a166fab..41c94f98f6cf8b5a05a8c3eb8d13bb73fc4fda3f:/src/part02.rs?ds=sidebyside diff --git a/src/part02.rs b/src/part02.rs index 32fbe0a..7261f19 100644 --- a/src/part02.rs +++ b/src/part02.rs @@ -1,5 +1,5 @@ -// Rust-101, Part 02: Generic types (WIP) -// ================================ +// Rust-101, Part 02: Generic types, Traits +// ======================================== use std; @@ -7,11 +7,11 @@ 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. -// + // 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: +// *templates*) or Java, or one of the many functional languages. So here, we define +// a generic type `SomethingOrNothing`. enum SomethingOrNothing { Something(T), Nothing, @@ -25,4 +25,134 @@ use self::SomethingOrNothing::{Something,Nothing}; // 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` and `Option`. 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 `` +// as *declaring* a type variable ("I am doing something for all types `T`"), and the second `` as +// *using* that variable ("The thing I do, is implement `SomethingOrNothing`"). +impl SomethingOrNothing { + fn new(o: Option) -> Self { + panic!("Not yet implemented.") + } + + fn to_option(self) -> Option { + panic!("Not yet implemented.") + } +} +// Inside an `impl`, `Self` refers to the type we are implementing things for. Here, it is +// an alias for `SomethingOrNothing`. +// 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: +fn call_constructor(x: i32) -> SomethingOrNothing { + SomethingOrNothing::new(Some(x)) +} + +// 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. +trait Minimum : Copy { + fn min(a: Self, b: Self) -> Self; +} + +// Now we can write `vec_min`, generic over a type `T` that we demand to satisfy the `Minimum` trait. +// This is called a *trait bound*. +// The only difference to the version from the previous part is that we call `T::min` (the `min` +// function provided for type `T`) instead of `std::cmp::min`. +// +// Notice 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! There is no reason to believe that `T` provides such an operation. +// This is in strong contrast to C++, where the compiler only checks such details when the +// function is actually used. +fn vec_min(v: &Vec) -> SomethingOrNothing { + let mut min = Nothing; + for e in v { + let e = *e; + min = Something(match min { + Nothing => e, + Something(n) => T::min(n, e) + }); + } + min +} + +// To make the function usable with a `Vec`, we implement the `Minimum` trait for `i32`. +impl Minimum for i32 { + fn min(a: Self, b: Self) -> Self { + std::cmp::min(a, b) + } +} + +// In order to run our code and see the result, we again provide a `print` function. +// This also shows that we can have multiple `impl` blocks for the same type, and we +// can provide some methods only for certain instances of a generic type. +impl SomethingOrNothing { + fn print(self) { + match self { + Nothing => println!("The number is: "), + 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. +// +// 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. +fn read_vec() -> Vec { + vec![18,5,7,3,9,27] +} +pub fn part_main() { + let vec = read_vec(); + let min = vec_min(&vec); + min.print(); +} + +// If this printed `3`, then you generic `vec_min` is working! +// +// 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. + +// **Exercise**: Define a trait "Print" to write a generic version of `SomethingOrNothing::print`. +// Implement that trait for `i32`, and change the code above to use it. +// I will again provide a skeleton for this solution. It also shows how to attach bounds to generic +// implementations (just compare it to the `impl` block from the previous exercise). +// You can read this as "For all types `T` satisfying the `Print` trait, I provide an implementation +// for `SomethingOrNothing`". +// +// Notice that I called the function on `SomethingOrNothing` `print2` to disambiguate from the `print` defined above. +// +// *Hint*: There is a macro `print!` for printing without appending a newline. +trait Print { + /* Add things here */ +} +impl SomethingOrNothing { + fn print2(self) { + panic!("Not yet implemented.") + } +} + // [index](main.html) | [previous](part01.html) | [next](part03.html)