X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/4f61be32dd480f23a7fef05ee66c42ae27c980c6..d29f51c818176f4249179ceba33de1ff41db2c90:/src/part02.rs diff --git a/src/part02.rs b/src/part02.rs index 41bb3bc..2bf4d97 100644 --- a/src/part02.rs +++ b/src/part02.rs @@ -3,7 +3,8 @@ //@ 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. +//@ later a `FloatOrNothing`? Certainly we don't want to re-write the type and all its inherent +//@ methods. // ## Generic datatypes @@ -17,21 +18,23 @@ 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 +//@ What this does is define an entire family of types: We can now write //@ `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.) +//@ However, we can also write `SomethingOrNothing` or even +//@ `SomethingOrNothing>`. 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`. 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` -//@ from an `Option`, and vice versa. +//@ The types are so similar, that we can provide a generic function to construct a +//@ `SomethingOrNothing` from an `Option`, and vice versa. //@ //@ 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`"). +//@ 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`. @@ -55,18 +58,18 @@ fn call_constructor(x: i32) -> SomethingOrNothing { } // ## Traits -//@ Now that we have a generic `SomethingOrNothing`, wouldn't it be nice to also gave a generic +//@ 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.
-//@ The function `min` takes to arguments of the same type, but I made the +//@ you want to have implemented, and their argument and return types.
+//@ 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 function wherever possible. +//@ However, in Rust one typically prefers methods over static functions wherever possible. pub trait Minimum : Copy { fn min(self, b: Self) -> Self; } @@ -74,12 +77,12 @@ pub trait Minimum : Copy { //@ 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 +//@ 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!
+//@ 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 { @@ -98,13 +101,14 @@ pub fn vec_min(v: Vec) -> SomethingOrNothing { //@ 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. +//@ 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. +//@ 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. @@ -117,9 +121,9 @@ impl Minimum for i32 { } // 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. +//@ 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 { @@ -141,9 +145,11 @@ pub fn main() { min.print(); } -//@ If this printed `3`, then you generic `vec_min` is working! So get ready for the next part. +//@ 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` (where `f32` is the type -// of 32-bit floating-point numbers). You should not change `vec_min` in any way, obviously! +// **Exercise 02.1**: 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) +//@ [index](main.html) | [previous](part01.html) | [raw source](workspace/src/part02.rs) | +//@ [next](part03.html)