X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/55d7df9f941d9979b568d094930c5009731737cf..8592458fd3494ebce80727341663c7b0036e3ede:/src/part02.rs diff --git a/src/part02.rs b/src/part02.rs index 44a20d4..a24a9d0 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 @@ -20,18 +21,20 @@ pub use self::SomethingOrNothing::*; //@ 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, 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.) +//@ 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`. @@ -74,7 +77,7 @@ 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 @@ -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 also inherit from our abstract base class after the fact. +//@ 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 { @@ -143,7 +147,9 @@ pub fn main() { //@ 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) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part02.rs) | [next](part03.html) +//@ [index](main.html) | [previous](part01.html) | [raw source](workspace/src/part02.rs) | +//@ [next](part03.html)