//@ 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
}
// 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<i32>` to get back our `NumberOrNothing`.
type NumberOrNothing = SomethingOrNothing<i32>;
-//@ However, 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.)
+//@ 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.
+//@ 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>`").
+//@ 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>`.
}
// ## 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*.
//@ 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 to arguments of the same type, but I made the
+//@ 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;
}
//@ 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
//@ 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.
}
// 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.
+//@ 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 {
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<f32>` (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<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) | [next](part03.html)
+//@ [index](main.html) | [previous](part01.html) | [raw source](workspace/src/part02.rs) |
+//@ [next](part03.html)
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