##   Copyright (c) Eduardo Julian. All rights reserved.
##   The use and distribution terms for this software are covered by the
##   Eclipse Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
##   which can be found in the file epl-v10.html at the root of this distribution.
##   By using this software in any fashion, you are agreeing to be bound by
##   the terms of this license.
##   You must not remove this notice, or any other, from this software.

(;import lux
         (lux/control (monoid #as m #refer #all)
                      (functor #as F #refer #all)
                      (monad #as M #refer #all)))

## [Types]
(deftype #export (Writer l a)
  (, l a))

## [Structures]
(defstruct #export Writer/Functor (All [l]
                                    (Functor (Writer l)))
  (def (F;map f fa)
    (let [[log datum] fa]
      [log (f datum)])))

(defstruct #export (Writer/Monad mon) (All [l]
                                        (-> (Monoid l) (Monad (Writer l))))
  (def M;_functor Writer/Functor)

  (def (M;wrap x)
    [(:: mon m;unit) x])

  (def (M;join mma)
    (let [[log1 [log2 a]] mma]
      [(:: mon (m;++ log1 log2)) a])))