Fieldwise initialization in Mojo

@fieldwise_init writes the constructor for you, one argument per field in order.

@fieldwise_init
struct Color(ImplicitlyCopyable, Movable):
    var red: Int
    var green: Int
    var blue: Int

    def brightness(self) -> Int:
        return (self.red + self.green + self.blue) // 3


def main():
    var teal = Color(0, 128, 128)
    print(teal.red, teal.green, teal.blue)
    print(teal.brightness())

How it works

1. The decorator generates __init__ taking red, green, and blue.
2. brightness averages the three channels with floor division.
3. Color(0, 128, 128) calls the generated constructor positionally.

Keywords and builtins used here

• Int
• Movable
• brightness
• def
• main
• print
• return
• self
• struct
• var

The run, in numbers

Lines

14

Characters to type

286

Tokens

80

Three-star pace

80 tpm

At the three-star pace of 80 tokens a minute, this run takes about 60 seconds.

Type this snippet

Step 3 of 5 in A first struct, step 3 of 22 in Structs & traits.

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