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Free(er) monads in JS (pt 1)
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const daggy = require('daggy') | |
const compose = (f, g) => x => f(g(x)) | |
const id = x => x | |
const kleisli_comp = (f, g) => x => f(x).chain(g) | |
//=============FREE============= | |
const Free = daggy.taggedSum({Impure: ['x', 'f'], Pure: ['x']}) | |
const {Impure, Pure} = Free | |
Free.of = Pure | |
Free.prototype.chain = function(f) { | |
return this.cata({ | |
Impure: (x, g) => Impure(x, kleisli_comp(g, f)), | |
Pure: x => f(x) | |
}) | |
} | |
Free.prototype.map = function(f) { | |
return this.chain(a => Free.of(f(a))) | |
} | |
const liftF = command => Impure(command, Pure) | |
//=============IO via FREE============= | |
const IO = daggy.taggedSum({IO: ['f']}) | |
const io = compose(liftF, IO.IO) | |
const unsafePerformIO = free => | |
free.cata({ | |
Pure: id, | |
Impure: (m, q) => | |
unsafePerformIO(q(m.f())) | |
}) | |
//=============Maybe via FREE============= | |
const Maybe = daggy.taggedSum({ Just: ['x'], Nothing: [] }) | |
const {Just, Nothing} = Maybe | |
const just = compose(liftF, Just) | |
const nothing = liftF(Nothing) | |
const runMaybe = free => | |
free.cata({ | |
Pure: x => x, | |
Impure: (m, q) => | |
m.cata({ | |
Just: x => runMaybe(q(x)), | |
Nothing: () => Nothing | |
}) | |
}) | |
//=============Examples============= | |
const safeProp = (k, o) => o[k] ? just(o[k]) : nothing | |
const localStorage = io(() => ({preferences: 'blue'})) | |
const printLn = x => io(() => console.log(x)) | |
//maybe_name :: Free (Maybe String) | |
const maybe_name = safeProp('user', {user: {name: 'jerry'}}) | |
.chain(u => safeProp('name', u)) | |
.map(n => n.toUpperCase()) | |
runMaybe(maybe_name) | |
// JERRY | |
//i :: Free (IO ()) | |
const i = localStorage | |
.map(l => l.preferences) | |
.chain(printLn) | |
unsafePerformIO(i) | |
// blue |
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