22 / Types and patterns

Trait Bounds

Trait bounds constrain generic type parameters so a function can call trait methods on values of unknown type. The syntax <T: TraitName> means "this function works for any type T that implements TraitName".

22-trait-bounds.nxSource →

// Trait bounds: función genérica que acepta cualquier tipo con Display

trait Display {
    fn to_string(self) -> String
}

struct Point {
    x: int,
    y: int
}

impl Display for Point {
    fn to_string(self) -> String {
        return "(" + int_to_string(self.x) + ", " + int_to_string(self.y) + ")"
    }
}

struct Circle {
    radius: int
}

impl Display for Circle {
    fn to_string(self) -> String {
        return "Circulo{radio=" + int_to_string(self.radius) + "}"
    }
}

fn print_it<T: Display>(x: T) -> String {
    return x.to_string()
}

fn show_twice<T: Display>(x: T) {
    let s: String = x.to_string()
    print(s)
    print(s)
}

fn main() -> int {
    let p: Point = Point { x: 7, y: 2 }
    print(print_it<Point>(p))

    let c: Circle = Circle { radius: 10 }
    print(print_it<Circle>(c))

    show_twice<Point>(p)

    return 0
}

Outputstdout

(7, 2)
Circulo{radio=10}
(7, 2)
(7, 2)

How it works

print_it<T: Display> is a generic function that accepts any type T as long as it implements Display. The bound T: Display tells the compiler two things: it may generate a specialized version for each concrete type used at the call site (monomorphization), and it guarantees that x.to_string() is always a valid call inside the function body.

show_twice demonstrates that trait-bounded generics are ordinary functions — they can hold local variables, call the trait method multiple times, and produce any side effects. The compiler produces separate machine-code versions for Point and any other type you pass, so there is zero virtual dispatch overhead.

At the call site, the type argument is written explicitly as print_it<Point>(p). Nyx can often infer the type argument, but being explicit is always valid and sometimes clearer when reading code.

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