fn identity<T>(x: T): T = x

type Pair<A, B> =
    | MkPair(A, B)

fn first<A, B>(p: Pair<A, B>): A =
    match p {
    MkPair(a, _) => a,
}

fn second<A, B>(p: Pair<A, B>): B =
    match p {
    MkPair(_, b) => b,
}

fn mapOption<T, U>(opt: Option<T>, f: fn(T): U): Option<U> =
    match opt {
    None => None,
    Some(x) => Some(f(x)),
}

fn double(x: Int): Int = x * 2

let id_int = identity(42)

let id_str = identity("hello")

let pair = MkPair(1, "one")

let fst = first(pair)

let snd = second(pair)

let doubled = mapOption(Some(21), double)

fn showOption(opt: Option<Int>): String =
    match opt {
    None => "None",
    Some(x) => "Some(" + toString(x) + ")",
}

fn printResults(): Unit with {Console} = {
    Console.print("identity(42) = " + toString(id_int))
    Console.print("identity(\"hello\") = " + id_str)
    Console.print("first(MkPair(1, \"one\")) = " + toString(fst))
    Console.print("second(MkPair(1, \"one\")) = " + snd)
    Console.print("map(Some(21), double) = " + showOption(doubled))
}

let output = run printResults() with {}
