@ephetic/functional

functional

Installation

yarn add @ephetic/functional

Erlang-inspired pattern matching

Patterns are checked in order, stopping with the first match. If no patterns match, the generated function will return undefined.

The pattern wildcard is the function itself (and can be aliased to any symbol you prefer).

Examples

import { matcher as m } from 'functional'

Match on constants or predicate functions

const isHi = m(['hi', true])

console.log(isHi('hi'))     // true
console.log(isHi('hello'))  // undefined

const fact = m(
  [1, 1],
  [m, n => n * fact(n-1)]
)

const fibo = m(
  [n => n <= 2, 1],
  [m, n => fibo(n-1) + fibo(n-2)]
)

Type/contract checking example

const t = (types, fn) => 
  process.env.NODE_ENV == 'production' 
  ? fn 
  : m([types, fn], 
      [m, (...args) => {throw `Type Check error: [${args}] doesn't match [${types}].`}])

const Integer = n => n === (n|0)
const addi = t([Integer, Integer], (a,b) => a + b)

const NonnegativeInteger = n => Integer(n) && n >= 0
const powish = t([Number, NonnegativeInteger], (b, e) => e > 0 ? b * pow(b, e - 1) : 1)
console.log(powish(2.2,3)) // works
console.log(powish(2,-3))  // throws

Match with RegExp

const isLikeHi = m([/.*hi.*/, s => `${s} is close enough`])

Match on argument types, values, and don't-cares

const secondIsNumber = m(
  [ [m, Number], (a,b) => console.log(`${b} is a Number`)],
  [ m,           (a,b) => console.log(`${b} is not a Number`)]
)

secondIsNumber('asdf', 2)   // 2 is a Number
secondIsNumber(2, 'asdf')   // asdf is not a Number

Note: since an array literal condition is used to represent the arguments list, to test against an actual array wrap it in a dummy object.

Match on given object shape

const isDuckman = m([{name: 'Duckman', age:40}, () => console.log('Duuuuckmaaaaan!')])
isDuckman({name: 'Duckman', age:40})  // Duuuuckmaaaaan!
isDuckman({name: 'Duckman'})          // undefined

const isDuckPerson = m([{name: m, age: m}, () => console.log('wubba lubba dub dub')])
isDuckPerson({name: 'Rick', age: 55}) // wubba lubba dub dub

Partial function application

Uses itself as wildcard to leave arbitrary parameters unbound.

import { partial as p } from 'functional'
const add = (a,b) => a + b
const div = (a,b) => a / b

const add1 = p(add, 1)
const halve = p(div, p, 2)