練習實作一支 Map type 與其身上多支指令，例如 map(), reduce(), foldMap() 等...

Map.js

const Mapp = val => ({

	val,

	// {a: 'aa', b: 'bb'}
	// f :: v -> value
	map: f => {
		const o = {}
		// let resultArr = Object.keys(val).map((key, idx) => ({ [key]: f(val[key]) })) // 錯的，會變成[{}, {}]
		Object.keys(val).map((key, idx) => o[key] = f(val[key]) )
		// return o // 錯的，要包回 Mapp
		return Mapp(o)
	},

	// f :: (acc, itm) -> value
	reduce: (f, acc) => {
		return Object.keys(val).reduce( (acc, key) => f(acc, val[key]), acc )
	},

	concat: ({val: y}) => {
		let arr = Object.keys(val)
		let brr = arr.map( key => val[key].concat(y[key]) )
		return brr
	},

	fold: (empty) => {
		return this.foldMap(x => x, empty)
	},

	foldMap: (f, empty) => {
		return empty != null
			? this.reduce((acc, x) => acc.concat(f(x)), empty)
			: this.reduce((acc, x) => acc.concat(f(x)))
	},

	sequence: point => {
		return this.traverse(point, x => x)
	},

	traverse: (point, f) => {
		return this.reduce((acc, v, k) =>
		    f(v, k).map(x => y => y.merge({[k]: x})).ap(acc), point(this.empty))
	},

})

console.log( Mapp.of({a: 1, b: 2}).map( v => v*2 ) ) // [ { a: 2 }, { b: 4 } ]
console.log( Mapp.of({a: 1, b: 2}).reduce( (acc, itm) => acc+itm, 100 ) )  // 103

我也來練習一下：

const Map = (val) => ({
  fmap: f => Map(Object.keys(val)
    .map(k => ({k, v: f(val[k])}))
    .reduce((acc, {k, v}) => ({...acc, [k]: v}), {})),
  extract: () => val
})

const v = Map({a: 1, b: 2})
console.log(
  v.fmap(a => a + 1)
   .fmap(a => a * a)
   .extract()
) // {a: 4, b: 9}

tested in babel try it out