Add Monad instance for Parser newtype

Author: evturn

…ic-parsing.hs/13.04-basic-definitions.hs …nadic-parsing/13.03-basic-definitions.hs13-monadic-parsing.hs/13.04-basic-definitions.hs renamed to 13-monadic-parsing/13.03-basic-definitions.hs 13-monadic-parsing.hs/13.04-basic-definitions.hs renamed to 13-monadic-parsing/13.03-basic-definitions.hs

@@ -0,0 +1,45 @@
+import           Control.Applicative
+import           Data.Char
+
+newtype Parser a = P (String -> [(a, String)])
+
+parse :: Parser a -> String -> [(a, String)]
+parse (P p) inp = p inp
+
+item :: Parser Char
+item = P $ \inp ->
+  case inp of
+    []     -> []
+    (x:xs) -> [(x, xs)]
+
+instance Functor Parser where
+  fmap f p = P $ \inp ->
+    case parse p inp of
+      []         -> []
+      [(v, out)] -> [(f v, out)]
+
+instance Applicative Parser where
+  pure v = P $ \inp -> [(v, inp)]
+
+  pf <*> px = P $ \inp ->
+    case parse pf inp of
+      []         -> []
+      [(f, out)] -> parse (fmap f px) out
+
+three :: Parser (Char, Char)
+three = pure f <*> item <*> item <*> item
+  where
+    f x y z = (x, z)
+
+instance Monad Parser where
+  p >>= f = P $ \inp ->
+    case parse p inp of
+      []         -> []
+      [(v, out)] -> parse (f v) out
+
+three' :: Parser (Char, Char)
+three' = do
+  x <- item
+  item
+  z <- item
+  return (x, z)