Add example using Alternative type class

Author: evturn

13-monadic-parsing/13.05-making-choices.hs

@@ -0,0 +1,42 @@
+import           Control.Applicative
+
+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 $ \str ->
+    case parse p str of
+      []        -> []
+      [(x, xs)] -> [(f x, xs)]
+
+instance Applicative Parser where
+  pure x = P $ \str -> [(x, str)]
+
+  pf <*> px = P $ \str ->
+    case parse pf str of
+      []        -> []
+      [(f, xs)] -> parse (fmap f px) xs
+
+instance Monad Parser where
+  return = pure
+
+  p >>= f = P $ \str ->
+    case parse p str of
+      []        -> []
+      [(x, xs)] -> parse (f x) xs
+
+instance Alternative Parser where
+  empty = P $ \x -> []
+
+  p <|> q = P $ \x ->
+    case parse p x of
+      []       -> parse q x
+      [(v, y)] -> [(v, y)]