X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/3dc9d065dfa259c88b3109717e2c6f6e65a45cc4..35c4d2161ea07cfbb4085d7e5242ab9939889afa:/src/part02.rs?ds=inline diff --git a/src/part02.rs b/src/part02.rs index dc885c0..e4f50be 100644 --- a/src/part02.rs +++ b/src/part02.rs @@ -1,12 +1,11 @@ // 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 @@ -19,44 +18,46 @@ pub enum SomethingOrNothing { // 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` to get back our `NumberOrNothing`, but we -// can also write `SomethingOrNothing` or even `SomethingOrNothing>`. -// 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`. -// Go check out its [documentation](http://doc.rust-lang.org/stable/std/option/index.html)! +// `SomethingOrNothing` to get back our `NumberOrNothing`. +type NumberOrNothing = SomethingOrNothing; +// However, we can also write `SomethingOrNothing` or even `SomethingOrNothing>`. +// 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`. 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). +// ## Generic `impl`, Static functions +// The types are so similar, that we can provide a generic function to construct a `SomethingOrNothing` +// from an `Option`, 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 `` // 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`"). +// +// 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`. impl SomethingOrNothing { fn new(o: Option) -> Self { - panic!("Not yet implemented.") + unimplemented!() } fn to_option(self) -> Option { - panic!("Not yet implemented.") + unimplemented!() } } -// 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: +// +// You can call static functions, and in particular constructors, as demonstrated in `call_constructor`. fn call_constructor(x: i32) -> SomethingOrNothing { 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*. @@ -64,19 +65,24 @@ fn call_constructor(x: i32) -> SomethingOrNothing { // 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. +// you want to have implemented, and their argument and return types.
+// 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(a: Self, b: Self) -> Self; + fn min(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`. +// 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. // -// Notice a crucial difference to templates in C++: We actually have to declare which traits +// 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! There is no reason to believe that `T` provides such an operation. +// we cannot call `min`. Just try it!
// This is in strong contrast to C++, where the compiler only checks such details when the // function is actually used. pub fn vec_min(v: Vec) -> SomethingOrNothing { @@ -84,23 +90,36 @@ pub fn vec_min(v: Vec) -> SomethingOrNothing { for e in v { min = Something(match min { Nothing => e, - Something(n) => T::min(n, 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`, we implement the `Minimum` trait for `i32`. impl Minimum for i32 { - fn min(a: Self, b: Self) -> Self { - std::cmp::min(a, b) + fn min(self, b: Self) -> Self { + if self < b { self } else { 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 { +// 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`), +// 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: "), @@ -112,14 +131,6 @@ impl SomethingOrNothing { // 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] } @@ -129,30 +140,9 @@ pub fn main() { 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. +// If this printed `3`, then you generic `vec_min` is working! So get ready for the next part. -// **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.") - } -} +// **Exercise 02.2**: Change your program such that it computes the minimum of a `Vec` (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) | [next](part03.html)