chainf.js

import {curry} from 'lodash/fp';

/**
 * Chain (flatMap) for Functions.
 *
 * chainf: (a -> x -> b) -> (x -> a) -> x -> b
 *
 * @param  {Function} g A function that takes two parameters of type a and x.
 * @param  {Function} h A function that takes one paramter of type x.
 * @param  {*}        x A value of type x.
 * @return {*}          g(h(x), x)
 */
const chainf = curry(function chainf(g, h, x) { // eslint-disable-line prefer-arrow-callback
    return g(h(x), x);
});

// This can also be written in the following way, which might be clearer.
// chainf:  (a -> x -> b) -> (x -> a) -> (x -> b)
// const chainf = curry(function chainf(g, h) {
//    return x => g(h(x), x);
// }