Transducers

transducers.js

const reduce = reducer => z => as => as.reduce((z, a) => reducer(z)(a), z);

// reducer :: (z -> a -> z)

// transducer :: (z -> a -> z) -> (z -> b -> z)

// contramapT :: (b -> a) -> (z -> a -> z) -> (z -> b -> z)
const contramapT = fn => reducer => (z => b => reducer(z)(fn(b)));

// filterT :: (a -> Bool) -> (z -> a -> z) -> (z -> a -> z)
const filterT = pr => reducer => (z => a => pr(a) ? reducer(z)(a) : z);

// takeT :: Int -> (z -> a -> z) -> (z -> a -> z)
const takeT = n => reducer => (c => (z => a => c-- > 0? reducer(z)(a) : z))(n);

const C = (...fns) => fns.reduceRight((cfn, fn) => x => cfn(fn(x)), x => x);

const cons = as => a => ([...as, a]);

const triple = contramapT(x => x * 3);
const odds = filterT(x => x % 2);
const takeTwo = takeT(2);

const transduce = transducer => base => z => as => reduce(transducer(base))(z)(as);

transduce(C(takeTwo, odds, triple))(cons)([])([1,2,3,4,5,6,7,8,9,10]);
// [3, 9]

const sum = z => a => z + a;

transduce(C(takeTwo, odds, triple))(sum)(0)([1,2,3,4,5,6,7,8,9,10]);
// 12