crocks/Maybe/safeAfter.js

A collection of well known Algebraic Datatypes for your utter enjoyment.

var ref = require('../core/Maybe');
var Just = ref.Just;
var Nothing = ref.Nothing;

var curry = require('../core/curry')
var isPredOrFunc = require('../core/isPredOrFunc')
var isFunction = require('../core/isFunction')
var predOrFunc = require('../core/predOrFunc')

// safeAfter :: ((b -> Boolean) | Pred) -> (a -> b) -> a -> Maybe b
function safeAfter(pred, fn) {
  if(!isPredOrFunc(pred)) {
    throw new TypeError('safeAfter: Pred or predicate function required for first argument')
  }
  if(!isFunction(fn)) {
    throw new TypeError('safeAfter: Function required for second argument')
  }

  return function (x) {
    var result = fn(x)
    return predOrFunc(pred, result)
      ? Just(result)
      : Nothing()
  }
}

module.exports = curry(safeAfter)