# The Problem

A `trait` that is generic over its container type has type specification
requirements - users of the `trait` *must* specify all of its generic types.

In the example below, the `Contains` `trait` allows the use of the generic
types `A` and `B`. The trait is then implemented for the `Container` type,
specifying `i32` for `A` and `B` so that it can be used with `fn difference()`.

Because `Contains` is generic, we are forced to explicitly state *all* of the
generic types for `fn difference()`. In practice, we want a way to express that
`A` and `B` are determined by the *input* `C`. As you will see in the next
section, associated types provide exactly that capability.

```rust,editable
struct Container(i32, i32);

// A trait which checks if 2 items are stored inside of container.
// Also retrieves first or last value.
trait Contains<A, B> {
    fn contains(&self, _: &A, _: &B) -> bool; // Explicitly requires `A` and `B`.
    fn first(&self) -> i32; // Doesn't explicitly require `A` or `B`.
    fn last(&self) -> i32;  // Doesn't explicitly require `A` or `B`.
}

impl Contains<i32, i32> for Container {
    // True if the numbers stored are equal.
    fn contains(&self, number_1: &i32, number_2: &i32) -> bool {
        (&self.0 == number_1) && (&self.1 == number_2)
    }

    // Grab the first number.
    fn first(&self) -> i32 { self.0 }

    // Grab the last number.
    fn last(&self) -> i32 { self.1 }
}

// `C` contains `A` and `B`. In light of that, having to express `A` and
// `B` again is a nuisance.
fn difference<A, B, C>(container: &C) -> i32 where
    C: Contains<A, B> {
    container.last() - container.first()
}

fn main() {
    let number_1 = 3;
    let number_2 = 10;

    let container = Container(number_1, number_2);

    println!("Does container contain {} and {}: {}",
        &number_1, &number_2,
        container.contains(&number_1, &number_2));
    println!("First number: {}", container.first());
    println!("Last number: {}", container.last());

    println!("The difference is: {}", difference(&container));
}
```

### See also:

[`struct`s][structs], and [`trait`s][traits]

[structs]: ../../custom_types/structs.md
[traits]: ../../trait.md
