Add parsers code

parsers.hs

@@ -0,0 +1,30 @@
+import Control.Monad
+
+newtype Parser a = Parser (String -> [(a,String)])
+
+parse :: Parser a -> String -> [(a,String)]
+parse (Parser p) = p
+
+instance Functor Parser where
+  fmap f p = Parser (\s -> [(f x, s') | (x,s') <- parse p s])
+
+instance Applicative Parser where 
+    pure = return
+    (<*>) = ap
+
+instance Monad Parser where
+  return x = Parser (\s -> [(x,s)])
+  p >>= q  = Parser (\s -> concat [parse (q x) s' | (x,s') <- parse p s ])
+
+item :: Parser Char
+item = Parser (\s -> case s of
+                       "" -> []
+                       (c:s') -> [(c,s')])
+
+many :: Parser a -> Parser [a]
+many p = do
+  a  <- p
+  as <- many p
+  return (a:as)
+
+main = return ()