module ParserCombinators(
    Parse
  , none
  , suceed
  , token
  , spot
  , (>*>)
  , use
  , use2
  , (*>)
  , (>*)
  , alt
  , list
  , some
  ) where

type Parse a b = [a]-> [(b, [a])] 

none :: Parse a b
none = const []

suceed :: b-> Parse a b
suceed b inp = [(b, inp)]

spot  :: (a-> Bool)-> Parse a a
spot p []     = []
spot p (x:xs) = if p x then [(x, xs)] else []

token :: Eq a=> a-> Parse a a
token t = spot (\c-> t == c)

infixl 3 `alt`
alt :: Parse a b-> Parse a b-> Parse a b
alt p1 p2 i = p1 i ++ p2 i

infixl 5 >*>
(>*>) :: Parse a b-> Parse a c-> Parse a (b, c)
(>*>) p1 p2 i =
  concatMap (\(b, r)-> 
     map (\(c, s)-> ((b, c), s)) (p2 r)) (p1 i)

infix 4 `use`, `use2`
use :: Parse a b-> (b-> c)-> Parse a c
use p f i = map (\(o, r)-> (f o, r)) (p i)

use2 :: Parse a (b, c)-> (b-> c-> d)-> Parse a d
use2 p f = use p (uncurry f)

infixl 5 *>, >*
(*>) :: Parse a b-> Parse a c-> Parse a c
(>*) :: Parse a b-> Parse a c-> Parse a b
p1 *> p2 = p1 >*> p2 `use` snd
p1 >* p2 = p1 >*> p2 `use` fst

list :: Parse a b-> Parse a [b]
list p = p >*> list p `use2` (:)
         `alt` suceed []

some :: Parse a b-> Parse a [b]
some p = p >*> list p `use2` (:)