TypeScript State Monad

state_monad.ts

type StateM<S, T> = {
    runState: (state: S) => T,
    then: <U>(func: (t: T, pure: <U>(u: U) => StateM<S, U>) => StateM<S, U>) => StateM<S, U>,
};

const stateM = function <S, T>(runState: (state: S) => [S, T]): StateM<S, T> {
    return {
        runState: function (s: S): T {
            const [, t] = runState(s);
            return t;
        },
        then: function <U>(func: (t: T, pure: <U>(u: U) => StateM<S, U>) => StateM<S, U>) {
            return stateM<S, U>((s: S) => {
                const [sUpd, t] = runState(s);
                const pure = function <U>(u: U) { return stateM<S, U>(_ => [sUpd, u]); }
                return [sUpd, func(t, pure).runState(sUpd)];
            });
        },
    };
};

const createCounterMonad = (testFunc: ((n: number) => boolean)) => stateM<number[], number>((nums) => {
    const newState = [...nums].filter(n => !testFunc(n)); //filter out all that dont test
    const countFiltered = nums.length - newState.length; //count number of filtered out; 
    return [newState, countFiltered];
});

const evenNumbers = createCounterMonad(n => n % 2 === 0);
const div3Numbers = createCounterMonad(n => n % 3 === 0);

const results = evenNumbers // even: X, div: Y
    .then((evens, _) => div3Numbers
        .then((div3s, pure) => pure<string>(`even: ${evens}, div3: ${div3s}`)))
    .runState([1, 2, 3, 4, 5, 6, 7, 8, 9, 0]);