(;module:
  lux
  ["F" //functor]
  (lux/data/coll [list "list/" Fold<List>]))

## [Signatures]
(sig: #export (CoMonad w)
  {#;doc "CoMonads are the opposite/complement to monads.

          CoMonadic structures are often infinite in size and built upon lazily-evaluated functions."}
  (: (F;Functor w)
     functor)
  (: (All [a]
       (-> (w a) a))
     unwrap)
  (: (All [a]
       (-> (w a) (w (w a))))
     split))

## [Types]
(type: #export (CoFree F a)
  {#;doc "The CoFree CoMonad."}
  [a (F (CoFree F a))])

## [Syntax]
(def: _cursor Cursor ["" +0 +0])

(macro: #export (be tokens state)
  {#;doc (doc "A co-monadic parallel to the \"do\" macro."
              (let [square (function [n] (i.* n n))]
                (be CoMonad<Stream>
                  [inputs (iterate i.inc 2)]
                  (square (head inputs)))))}
  (case tokens
    (#;Cons comonad (#;Cons [_ (#;Tuple bindings)] (#;Cons body #;Nil)))
    (if (|> bindings list;size (n.% +2) (n.= +0))
      (let [g!map (: Code [_cursor (#;Symbol ["" " map "])])
            g!split (: Code [_cursor (#;Symbol ["" " split "])])
            body' (list/fold (: (-> [Code Code] Code Code)
                                (function [binding body']
                                  (let [[var value] binding]
                                    (case var
                                      [_ (#;Tag ["" "let"])]
                                      (` (let (~ value) (~ body')))

                                      _
                                      (` (|> (~ value) (~ g!split) ((~ g!map) (function [(~ var)] (~ body')))))
                                      ))))
                             body
                             (list;reverse (list;as-pairs bindings)))]
        (#;Right [state (#;Cons (` ("lux case" (~ comonad)
                                    {(~' @)
                                     ("lux case" (~' @)
                                      {{#functor {#F;map (~ g!map)} #unwrap (~' unwrap) #split (~ g!split)}
                                       (~ body')})}))
                                #;Nil)]))
      (#;Left "'be' bindings must have an even number of parts."))

    _
    (#;Left "Wrong syntax for 'be'")))