import { curry } from './curry.js'

function mergeWithFn(
  mergeFn, a, b
){
  const willReturn = {}

  Object.keys(a).forEach(key => {
    if (b[ key ] === undefined){
      willReturn[ key ] = a[ key ]
    } else {
      willReturn[ key ] = mergeFn(a[ key ], b[ key ])
    }
  })

  Object.keys(b).forEach(key => {
    if (willReturn[ key ] !== undefined) return

    if (a[ key ] === undefined){
      willReturn[ key ] = b[ key ]
    } else {
      willReturn[ key ] = mergeFn(a[ key ], b[ key ])
    }
  })

  return willReturn
}

export const mergeWith = curry(mergeWithFn)