Keep calm and curry on

Andreas Møller

CTO @ Sentia

twitter.com/cullophid

github.com/cullophid

Sometimes, the elegant implementation is just a function. Not a method. Not a class. Not a framework. Just a function.

- John Fucking Carmack


// A Function



const add = (x, y) => x + y


const three = add(1, 2)


// A Function


// add :: (Number, Number) -> Number
const add = (x, y) => x + y

// three :: Number
const three = add(1, 2)

// merge :: ({String: a}, {String: a}) -> {String: a}
const merge = (x, y) => Object.assign({}, x, y)

// fromPairs :: [[String,a]] -> {String: a}
const fromPairs = pairs => 
    pairs.reduce((obj, [k, v]) => merge(obj, {[k]: v}), {})

// Compose

// compose :: (y -> z, ..., a -> b) -> (a -> z)
const compose = (...fs) => 
    fs.reduce(compose2, x => x)

// compose2 :: ((y -> z), (x -> y)) -> (x -> z)
const compose2 = (g, f) => x => g(f(x))

// Compose

// getUrlQuery :: String -> String
const getUrlQuery = compose(last, x => x.split('?'))

// splitPairs :: [String] -> [[String, String]]
const splitPairs = xs => 
    xs.map(x => x.split('='))

// parseQuery :: String -> {String: String}
const parseQuery = compose(fromPairs, splitPairs, x => x.split('&'))

// Curry

const _curried = init => f => 
    (...args) => {
      const acc = [...init, ...args]
      return acc.length >= f.length ? f(...acc) : _curried(acc)(f)
    }

const curry = _curried([])

// Currying

// split :: String -> String -> [String]
const split = curry((separator, str) => str.split(separator)

// splitByAmp :: String -> [String]
const splitByAmp = split('&')

// meAndYou :: [String]
const meAndYou = splitByAmp("me&you") // ["me", "you"]

// Currying

// split :: String -> String -> [String]
const split = curry((separator, str) => str.split(separator)

// map :: (a -> b) -> [a] -> [b]
const map = curry((f, xs) => xs.map(f))

// parseQuery :: String -> {String: String}
const parseQuery = compose(fromPairs, map(split('=')), split('&'))

// mapObj :: (a -> b) -> {String: a} -> {String: b}
const mapObj = curry((f, obj) => 
    Object.keys(obj)
        .reduce((acc, k) => merge(acc, {[k]: f(obj[k])}), {})

// parseQuery :: String -> {String: String}
const parseQuery = 
    compose(mapObj(decodeURIComponent), fromPairs, map(split('=')), split('&'))

// buildQuery :: {String: String} -> String
const buildQuery = 
    compose(join('&'), map(join('=')), toPairs, mapObj(encodeURIComponent))

// Async


// getJSON :: String -> Promise JSON
const getJSON => url =>
    fetch(url)
        .then(r => r.json())


// getUserPermissions :: String -> [Permission]
const getUserPermissions => id =>
    getJSON(`/api/users/${id}`)
        .then(prop('permissions'))

// Async

// mapP :: (a -> b) -> Promise a -> Promise b
const mapP = curry((f, p) => p.then(f))

// getUserPermissions :: String -> Promise [Permission]
const getUserPermissions = 
    compose(mapP(prop('permissions')), getJSON, concat('/api/users/'))

// Async

// getUsers :: {String: String} -> Promise [User]
const getUsers = 
    compose(getJSON, concat('/api/users?'), buildQuery)

// getAdminUsers :: {String: String} -> Promise [User]
const getAdminUsers = 
    compose(mapP(filter(prop('admin')), getUsers)


getAdminUsers({age: '32', city: 'copenhagen'})
   .then(users => {
        // ...
    })

Keep calm and curry on

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Questions?

Keep calm and curry on (CopenhagenJS)

Keep calm and curry on (CopenhagenJS)

Andreas Møller

Keep calm and curry on

We Are Hiring

Questions?

Keep calm and curry on (CopenhagenJS)

More from Andreas Møller