Generic functions over traits in Mojo

A trait bound lets one function serve every conforming type, compiled fresh for each.

trait Priced:
    def price(self) -> Float64:
        ...


@fieldwise_init
struct Book(ImplicitlyCopyable, Movable, Priced):
    var cost: Float64

    def price(self) -> Float64:
        return self.cost


def with_tax[T: Priced](item: T) -> Float64:
    return item.price() * 1.0825


def main():
    print(with_tax(Book(20.0)))

How it works

1. Priced requires a price method returning a Float64.
2. with_tax[T: Priced](item: T) accepts any type that conforms.
3. Passing a Book stamps out a specialized version with no dynamic dispatch.

Keywords and builtins used here

• Float64
• Movable
• def
• main
• price
• print
• return
• self
• struct
• trait
• var
• with_tax

The run, in numbers

Lines

19

Characters to type

295

Tokens

75

Three-star pace

75 tpm

At the three-star pace of 75 tokens a minute, this run takes about 60 seconds.

Type this snippet

Step 3 of 5 in Traits, step 18 of 22 in Structs & traits.

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