The Easiest Way to Build a Type Checker(jimmyhmiller.com)

    function f<T>(x: T) { return x; }
    function g(x: number) { return { a: f(x), b: f(x.toString()) }; }

    function append<T>(xs: list<T>, ys: list<T>) {
      return match xs {
        Nil() -> ys,
        Cons(hd, tl) -> Cons(hd, append(tl, ys)),
      };
    }
    function flatten<T>(xs: list<list<T>>) {
      return match xs {
        Nil() -> Nil(),
        Cons(hd, tl) -> append(hd, flatten(xs)),
      };
    }
    function map<T, U>(f: (T) => U, xs: list<T>) {
      return match xs {
        Nil() -> Nil(),
        Cons(hd, tl) -> Cons(f(hd), tl),
      };
    }
    function sum(xs: list<number>) {
      return match xs {
        Nil() -> 0,
        Cons(hd, tl) -> hd + length(tl),
      };
    }
    function length<T>(xs: list<T>) { return sum(map((_) -> 1, xs)); }
    function f<T>(xs: list<list<T>>) {
      return length(flatten(xs)) === sum(map(length, xs));
    }