Mutating with mut in Mojo

mut marks an argument the callee may change in place, edits landing on the caller's value.

@fieldwise_init
struct Basket(ImplicitlyCopyable, Movable):
    var count: Int

    def take(mut self, n: Int):
        self.count -= n


def refill(mut basket: Basket, n: Int):
    basket.count += n


def main():
    var basket = Basket(2)
    refill(basket, 5)
    basket.take(1)
    print(basket.count)

How it works

1. take(mut self, n: Int) lets a method write back to its own fields.
2. refill(mut basket: Basket, n: Int) mutates a struct owned by main.
3. Both edits stack up, so basket.count prints the running total.

Keywords and builtins used here

• Int
• Movable
• def
• main
• print
• refill
• self
• struct
• take
• var

The run, in numbers

Lines

17

Characters to type

269

Tokens

74

Three-star pace

80 tpm

At the three-star pace of 80 tokens a minute, this run takes about 56 seconds.

Type this snippet

Step 3 of 5 in Value semantics, step 8 of 22 in Structs & traits.

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