Stateless transducer setup by injecting a new transducer each step (no init or completion functions)

stateless_transducers.js

function map(f) {
  return function(rf) {
    return function(result, item) {
      const [new_rf, new_result] = rf(result, f(item))
      return [map(f)(new_rf), new_result]
    }
  }
}

function takeNth(n, state = n) {
  return function(rf) {
    return function(result, item) {
      if(n === state) {
        const [new_rf, new_result] = rf(result, item)
        return [takeNth(n, 1)(new_rf), new_result]
      } else {
        return [takeNth(n, state+1)(rf), result];
      }
    }
  }
}

function comp(...fs) {
  return function(arg) {
    return fs.reverse().reduce((result, f) => f(result), arg)
  }
}

function concat(result = [], item) {
  return [...result, item]
}

function stateless_reducer(f) {
  return function(result, item) {
    return [stateless_reducer(f), f(result, item)]
  }
}

function stateless_reduce(stateless_rf, init, coll) {
  return coll.reduce(
    ([rf, result], item) => rf(result, item),
    [stateless_rf, init]
  )[1];
}

const xf = comp(
  map(i => i + 1),
  takeNth(5),
  map(i => i % 7)
)
const pipe = xf(stateless_reducer(concat));

const countTo1000 = Array.from({ length: 1000 }).map((_, i) => i);
const result = stateless_reduce(pipe, [], countTo1000)
console.log(result)