Declaring a trait in Mojo

A trait names the methods a type must supply, without saying how.

trait Greeter:
    def greeting(self) -> String:
        ...


@fieldwise_init
struct Robot(ImplicitlyCopyable, Movable, Greeter):
    var model: String

    def greeting(self) -> String:
        return "unit " + self.model + " online"


def main():
    var arm = Robot("K9")
    print(arm.greeting())

How it works

1. trait Greeter: lists greeting with ... standing in for the body.
2. Robot claims the trait in its header and then has to implement it.
3. The compiler checks conformance up front, so the call cannot miss.

Keywords and builtins used here

• Movable
• String
• def
• greeting
• main
• print
• return
• self
• struct
• trait
• var

The run, in numbers

Lines

16

Characters to type

265

Tokens

68

Three-star pace

80 tpm

At the three-star pace of 80 tokens a minute, this run takes about 51 seconds.

Type this snippet

Step 1 of 5 in Traits, step 16 of 22 in Structs & traits.

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