crocks/Pair/writerToPair.js

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

/** @license ISC License (c) copyright 2017 original and current authors */
/** @author Ian Hofmann-Hicks (evil) */

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

var isWriter =
  function (x) { return !!x && isFunction(x.read); }

var applyTransform = function (w) { return w.read(); }

// writerToPair : Monoid m => Writer m a -> Pair m a
// writerToPair : Monoid m => (a -> Writer m a) -> Pair m b
function writerToPair(writer) {
  if(isFunction(writer)) {
    return function(x) {
      var m = writer(x)

      if(!isWriter(m)) {
        throw new TypeError('writerToPair: Writer returning function required')
      }

      return applyTransform(m)
    }
  }

  if(isWriter(writer)) {
    return applyTransform(writer)
  }

  throw new TypeError('writerToPair: Writer or Writer returning function required')
}

module.exports = curry(writerToPair)