fun map_ (vec: [int]) (fn: |int| -> int) (current: int): [int] = do
    if current == @len(vec) then
        return []
    else
        do
            let new : [int] = []
            let x : int = @get(vec, current) |> fn(_)
            @insert(new, x)

            let y : [int] = map_(vec, fn, current + 1)
            return @concat(new, y)
        end
    end
end

fun map (vec: [int]) (fn: |int| -> int): [int] = return map_(vec, fn, 0)

fun mul10 (x: int): int = return x * 10

fun main: void = do
    let foo: [int] = [69, 420, 727, 1337, 42069, 69420]
    map(foo, mul10) |> @write(_)
end