+// ## Traits
+//@ 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. <br/>
+//@ 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 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
+//@ 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! <br/>
+//@ This is in strong contrast to C++, where the compiler only checks such details when the
+//@ function is actually used.
+pub fn vec_min<T: Minimum>(v: Vec<T>) -> SomethingOrNothing<T> {
+ let mut min = Nothing;
+ for e in v {
+ min = Something(match min {
+ Nothing => e,
+ // Here, we can now call the `min` function of the trait.
+ 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 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.
+//@ 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 `vec_min` usable with a `Vec<i32>`, we implement the `Minimum` trait for `i32`.
+impl Minimum for i32 {
+ fn min(self, b: Self) -> Self {
+ if self < b { self } else { b } /*@*/
+ }
+}
+
+// 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.
+impl NumberOrNothing {
+ pub fn print(self) {
+ match self {
+ Nothing => println!("The number is: <nothing>"),
+ Something(n) => println!("The number is: {}", n),
+ };
+ }
+}
+
+// Now we are ready to run our new 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.
+fn read_vec() -> Vec<i32> {
+ vec![18,5,7,3,9,27]
+}
+pub fn main() {
+ let vec = read_vec();
+ let min = vec_min(vec);
+ min.print();
+}
+
+//@ 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!
+
+//@ [index](main.html) | [previous](part01.html) | [raw source](workspace/src/part02.rs) |
+//@ [next](part03.html)
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