Type Functions

Type functions are functions that run during analysis time and operate on types, instead of runtime values. They can use the types library to transform existing types or create new ones.

Here’s a simplified implementation of the builtin type function keyof. It takes a table type and returns its property names as a union of singletons.

type function simple_keyof(ty)
    -- Ignoring unions or intersections of tables for simplicity.
    if not ty:is("table") then
        error("Can only call keyof on tables.")
    end

    local union = nil

    for property in ty:properties() do
        union = if union then types.unionof(union, property) else property
    end

    return if union then union else types.singleton(nil)
end

type person = {
    name: string,
    age: number,
}
--- keys = "age" | "name"
type keys = simple_keyof<person>

Type function environment

In addition to the types library, type functions have access to:

• assert, error, print
• next, ipairs, pairs
• select, unpack
• getmetatable, setmetatable
• rawget, rawset, rawlen, raweq
• tonumber, tostring
• type, typeof
• math library
• table library
• string library
• bit32 library
• utf8 library
• buffer library