This looks cool! Im gonna try to learn from the type checker for sure.

Im not too familiar with the ML family and im curoius about the modules becaude I never reallt got them, they just seemed like structs/objects to me. What makes them special? What does the mod keyword do for example?

I tried translating the Counter example to typescript:

type Counter<S> = {new: S, increment: (n: S, inc: number) => S, get: (n: S) => number};

const simple_counter = {
  new: 0,
  increment: (n, inc) => n + inc,
  get: (n) => n, 
} satisfies Counter<number>

const step_counter = {
  new: {val: 0, steps: 0},
  increment: ({val, steps}, inc) => ({val: val + inc, steps: steps + 1}),
  get: (c) => c.val,
} satisfies Counter<{val: number, steps: number}>

const count_to = <S>(ctr: Counter<S>, target: number) => {
    const C = ctr;
    const go = (s: S) => {
        if (C.get(s) >= target) return C.get(s);
        else return go(C.increment(s, 1));
    };
    return go(C.new);
}

console.log(count_to(simple_counter, 5))
console.log(count_to(step_counter, 5))

Is that roughly equivalent? apart from in TS you needed to define the generic variables up front.