Understanding transducers

1. the-basic-problem.js

function add1(v) { return v + 1};
function isOdd(v) {return v % 2 === 1};
function sum (total, v) {return total + v};

var list = [2, 5, 8, 11, 14, 17, 20];

list
.map(add1)
.filter(isOdd)
.reduce(sum)

2. express-list-operations-as-reducers.js

// as `map`, `filter` has different shapes, we will express them
// such that they have the same shape as `reducer`

function mapReducer(mappingFn) {
  return function reducer(list, v) {
    list.push(mappingFn(v));
    return list;
  }
}

function filterReducer(predicateFn) {
  return function reducer(list, v) {
    if (predicateFn(v)) list.push(v) 
    return list;
  }
}

var list = [2, 5, 8, 11, 14, 17, 20];

list
.reduce(mapReducer(add1), [])
.reduce(filterReducer(isOdd), [])
.reduce(sum)

3. extract-common-functionality.js

function listCombination(list, v) {
  list.push(v);
  return list;
}


function mapReducer(mappingFn) {
  return function reducer(list, v) {
    return listCombination(list, mappingFn(v));
  }
}

function filterReducer(predicateFn) {
  return function reducer(list, v) {
    if (predicateFn(v))
      listCombination(list, v)
  }
}

var list = [2, 5, 8, 11, 14, 17, 20];

list
.reduce(mapReducer(add1), [])
.reduce(filterReducer(isOdd), [])
.reduce(sum)

4. introducing-transducer.js

function listCombination(list, v) {
  list.push(v);
  return list;
}

var mapReducer = curry(function mapReducer(mappingFn, combineFn) {
  return function reducer (list, v) {
    return combineFn(list, mappingFn(v))
  }
})

var filterReducer = curry(function filterReducer(predicateFn, combineFn) {
  return function reducer (list, v) {
    if (predicateFn(v)) {
        return combineFn(list, v)
    }
    
    return list;
  }

})


var transducer = compose(mapReducer(add1), filterReducer(isOdd))
var list = [2, 5, 8, 11, 14, 17, 20];

list
  .reduce(transducer(listCombination), [])
  .reduce(sum)