A Function represents a Lua function. It is like C#'s Func class, but reversed so that the result is first in its written type arguments (note how TResult is last in C# but first here). This is used explicitly for API documentation formatting and is not an actual type in FastCast (it's just here to make formatting nice).

For an example, if something is documented as Function<boolean, number, Instance> then in Lua the implementation might be:

function MyFunction(numValue, instanceValue)
    return true -- or return false, because this must return a boolean
end

If something is documented as Function<boolean> then that means the function must return a boolean and takes in no arguments.