transducers.js

const even = R.converge(R.equals, [R.always(0), R.modulo(R.__,2)]);
const odd = R.converge(R.equals, [R.always(1), R.modulo(R.__,2)]);


// 1. map/filter implemented with reduce

// map with reduce

const map = (f, functor) =>
  R.reduce((acc, next) => R.concat(acc, f(next)), [], functor)

expect(map(R.inc, [1, 2, 3])).toEqual([2, 3, 4]);
console.log('#map passed')

// filter with reduce

const filter = (pred, functor) =>
  R.reduce((acc, next) => pred(next)? R.concat(acc, next): acc, [], functor)

expect(filter(even, [1, 2, 3, 4, 5])).toEqual([2, 4]);
console.log('#filter passed')


// 2. decouple map/filter from reduce
const mapper = (f, functor) =>
  (acc, next) => R.concat(acc, f(next))

expect(R.reduce(mapper(R.inc), [], [1, 2, 3])).toEqual([2, 3, 4]);
console.log('#mapper passed')

const filterer = (pred, functor) =>
  (acc, next) => pred(next)? R.concat(acc, next): acc

expect(R.reduce(filterer(even), [], [1, 2, 3, 4, 5])).toEqual([2, 4]);
console.log('#filterer passed')


// 3. decouple map/filter from concat

const mapping = transform => step => (acc, next) => step(acc, transform(next));

expect(R.reduce(mapping(R.inc)(R.concat), [], [1, 2, 3])).toEqual([2, 3, 4]);
console.log('#mapping passed')

const filtering = pred => step => (acc, next) => pred(next)? step(acc, next): acc;

expect(R.reduce(filtering(odd)(R.concat), [], [1, 2, 3])).toEqual([1, 3]);
console.log('#filtering passed')

const transformation = R.compose( mapping( R.multiply(3) )
                                , filtering( odd )
                                );

expect(R.reduce(transformation(R.concat), [], [1, 2, 3])).toEqual([3, 9]);
console.log('#compose passed')

expect(R.reduce(transformation((acc, next) => acc + next + ''), '', [1, 2, 3])).toEqual('39');
console.log('#join passed')

const transduce = R.compose( R.reduce
                           , mapping( R.multiply(3) )
                           , filtering( odd )
                           );

expect(transduce(R.concat)([], [1, 2, 3])).toEqual([3, 9]);
console.log('#transduce passed')


// 4. add reduced (to stop transformation)