Defining Function Types

Functions are values, so functions have types. Luau lets you name a function's signature with a type alias, then require variables to hold only functions matching that shape:

type MathOp = (number, number) -> number

local add: MathOp = function(a, b)
    return a + b
end

local multiply: MathOp = function(a, b)
    return a * b
end

A function type lists the parameter types in parentheses, then an arrow ->, then the return type: (number, number) -> number is "takes two numbers, returns a number". Note the arrow is ->. This is type syntax, used only in annotations, never to define a function's body.

Notice add and multiply don't re-annotate their parameters: the variable's type is MathOp, so the checker already knows a and b are numbers and that the body must return one. One alias, many conforming functions: a blueprint your whole codebase can share.

Function types shine wherever functions travel: variables, table fields, and parameters (callbacks!) can all demand a precise signature instead of "some function, hopefully the right one".