simple impl of shortcut fusion(also called: deflorestation, stream fusion) using javascript generators

short-cut-fusion.js

function* map(morphism, itr) {
  let value;
  while(value = itr.next().value) {
    yield morphism(value);
  }
}


function* filter(predicate, itr) {
  let value;
  while(value = itr.next().value) {
    if(predicate(value)) {
      yield value;
    }
  }
}

function toItr(list){
  return list[Symbol.iterator]()
}

function first(itr) {
  return itr.next().value
}

function toList(itr) {
  let value;
  let all = []
  while(value = itr.next().value) {
    all.push(value);
  }

  return all
}


const itrList = toItr([1,2,3,4,5,6,7,8,9])

// Will not run map or filter for 4,5,6,7,8,9
// because of the consumer (first) only asks for the first
// item
first(
 map(
  x => { 
    console.log('map', x); // 3
    return x + 1;
  },
  filter(x => {
    console.log('filter', x); // 1, 2, 3
    return x >= 3
  }, itrList)
))) // 4