parser.hs

{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE RebindableSyntax #-}

module Parser where

import Prelude (Char, String)


type Parser a = String -> [(a,String)]


parse :: Parser a -> String -> [(a, String)]
parse parser input = parser input


-- The following four functions are necessary for the do notation.

return :: a -> Parser a
return v = \input -> [(v,input)]

(>>=) :: Parser a -> (a -> Parser b) -> Parser b
p >>= f = \input -> case parse p input of
                     [] -> []
                     [(v,out)] -> parse (f v) out

(>>) :: Parser a ->  Parser b -> Parser b
p >> f = p >>= \_ -> f

fail :: Parser a
fail = \_ -> []


item :: Parser Char
item = \input -> case input of
  [] -> []
  (x:xs) -> [(x,xs)]


tupleParse :: Parser (Char, Char)
tupleParse = item >>= \x ->
             item >>= \_ ->
             item >>= \y ->
             return (x, y)


-- This is the syntactic sugar version that uses do notation.
tupleParseSugar :: Parser (Char, Char)
tupleParseSugar = do x <- item
                     item
                     y <- item
                     return (x, y)