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@branneman branneman/curry.js
Last active Sep 14, 2019

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Higher order auto curry function
'use strict'
const curryN = (() => {
const isPlaceholder = x => x['@@functional/placeholder'] === true
const filterPlaceholders = xs => xs.filter(x => isPlaceholder(x))
const filterValues = xs => xs.filter(x => !isPlaceholder(x))
return (arity, fn, accIn = []) => (...args) => {
const accOut = accIn.slice()
if (filterPlaceholders(accIn).length > 0) {
filterValues(args).forEach(val => {
const i = accOut.findIndex(isPlaceholder)
const idx = i === -1 ? accOut.length : i
accOut[idx] = val
})
} else {
accOut.push(...args)
}
if (filterValues(accOut).length >= arity) {
return fn.apply(null, accOut)
}
return curryN(arity, fn, accOut)
}
})()
const curry = fn => curryN(fn.length, fn)
const _ = { '@@functional/placeholder': true }
module.exports = { curry, curryN, _ }
const { curryN, curry, _ } = require('./curry')
describe('curryN()', () => {
it('should correctly curry to given arity', () => {
const threeArg = (a, b, c = 42) => c
const fn = curryN(2, threeArg)
const res1 = fn() //=> fn with arity of 2
const res2 = fn(1) //=> fn with arity of 1
const res3 = fn(1, 2) //=> run function
const res4 = fn(1, 2, 3) //=> run function
const res5 = fn(1, 2, 3, 4) //=> run function
expect(typeof res1).toStrictEqual('function')
expect(typeof res2).toStrictEqual('function')
expect(res3).toStrictEqual(42)
expect(res4).toStrictEqual(3)
expect(res5).toStrictEqual(3)
})
})
describe('curry()', () => {
it('returns a new function on subsequent calls', () => {
const sum = (a, b) => a + b
const fn0 = curry(sum)
const fn1 = fn0()
const fn2 = fn1()
const fn3 = fn2()
expect(fn3(10, 5)).toStrictEqual(15)
})
it('handles more arguments than given arity', () => {
// sumAll arity = 2
const sumAll = curry(function(a, b) {
const xs = Array.from(arguments)
return xs.reduce((acc, curr) => acc + curr, 0)
})
const result = [
sumAll()(40)(20),
sumAll(20)(40),
sumAll(30, 30),
sumAll(10, 20, 30),
sumAll(10)(20, 30),
sumAll(10, 20, 30),
sumAll(10)(20, 20, 10),
sumAll(10)(10, 10, 10, 20)
]
expect(result.every(x => x === 60)).toBe(true)
})
it('accepts an arity of 0', () => {
const give15 = () => 10 + 5
const fn = curry(give15)
expect(fn()).toStrictEqual(15)
})
it('accepts an arity of 1', () => {
const add5 = n => n + 5
const fn = curry(add5)
expect(fn(10)).toStrictEqual(15)
})
it('accepts an arity of 2', () => {
const sum = (a, b) => a + b
const fn = curry(sum)
const result = [fn()(10)(5), fn(10)(5), fn(10, 5)]
expect(result).toStrictEqual([15, 15, 15])
})
it('accepts an arity of 3', () => {
const sum3 = (a, b, c) => a + b + c
const fn = curry(sum3)
const result = [
fn(10, 20, 30),
fn(10)(20, 30),
fn(10, 20)(30),
fn(10)(20)(30)
]
expect(result).toStrictEqual([60, 60, 60, 60])
})
it('accepts an arity of 12', () => {
const sum12 = (a, b, c, d, e, f, g, h, i, j, k, l) =>
a + b + c + d + e + f + g + h + i + j + k + l
const fn = curry(sum12)
const result = [
fn(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12),
fn(1, 2)(3, 4, 5, 6, 7, 8, 9, 10, 11, 12),
fn(1, 2, 3, 4, 5, 6)(7)(8)(9, 10, 11, 12),
fn(1, 2)(3, 4)(5, 6, 7, 8)(9, 10, 11, 12),
fn(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)
]
expect(result).toStrictEqual([78, 78, 78, 78, 78])
})
})
describe('placeholder', () => {
it('it allows gaps via placeholder value', () => {
const sum3 = (a, b, c) => a + b + c
const fn = curry(sum3)
const result = [
fn(10, 20, 30),
fn(_, 20, 30)(10),
fn(_, _, 30)(10)(20),
fn(_, _, 30)(10, 20),
fn(_, 20)(10)(30),
fn(_, 20)(10, 30),
fn(_, 20)(_, 30)(10)
]
expect(result.every(x => x === 60)).toBe(true)
})
})
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