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Functional Lenses (Van Laarhoven)
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// Combinators | |
const K = a => b => a | |
const I = a => a | |
// Operators | |
const eq = a => b => a === b | |
// Types | |
const typeOf = a => typeof a | |
const isArray = a => Array.isArray(a) | |
// Lists | |
const reverse = xs => xs.slice().reverse() | |
const reduce = f => a => xs => xs.reduce(f, a) | |
// Functions | |
const compose = (...fns) => x => reduce((acc, fn) => fn(acc))(x)(reverse(fns)) | |
// Objects | |
const assign = Object.assign | |
const prop = key => x => x[key] | |
const assoc = key => value => x => assign(isArray(x) ? [] : {}, x, {[key]: value}) | |
// Functors | |
const Identity = value => ({ join: K(value), map: fn => Identity(fn(value))}) | |
const Constant = value => ({ join: K(value), map: () => Constant(value)}) | |
// Lenses | |
const Lens = (get, set) => fn => x => fn(get(x || {})).map((value) => set(value)(x)) | |
const Prop = name => Lens(prop(name), assoc(name)) | |
const Get = lens => x => lens(Constant)(x).join() | |
const Over = lens => f => x => lens(compose(Identity, f))(x).join() | |
const Set = lens => v => x => Over(lens)(K(v))(x) | |
// Example | |
const o = {a: {b: {c: 0}}} | |
const a = Prop('a') | |
const b = Prop('b') | |
const c = Prop('c') | |
const abc = compose(a, b, c) | |
console.log(Get(abc)(o)) | |
console.log(Get(abc)({})) | |
console.log(Set(abc)(1)(o)) |
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