Parametric structs in Mojo

A struct parameter fixes a size at compile time, so no capacity check survives.

struct Ring[size: Int](Copyable, Movable):
    var slots: List[Int]

    def __init__(out self):
        self.slots = List[Int](capacity=Self.size)
        for _ in range(Self.size):
            self.slots.append(0)

    def put(mut self, index: Int, value: Int):
        self.slots[index % Self.size] = value

    def room(self) -> Int:
        return Self.size


def main():
    var ring = Ring[4]()
    ring.put(6, 99)
    print(ring.room(), ring.slots[2])

How it works

1. Ring[size: Int] declares the parameter in the struct header.
2. Inside the body it is always written Self.size; bare size is an error.
3. Ring[4]() stamps out a four-slot ring, and put wraps the index with %.

Keywords and builtins used here

• Copyable
• Int
• List
• Movable
• def
• for
• main
• print
• put
• range
• return
• room
• self
• struct
• var

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Step 3 of 5 in Compile-time Mojo, step 3 of 22 in Parameters & SIMD.

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