Create parser.hs

2016-04-18/parser.hs

@@ -0,0 +1,39 @@
+module Parser where
+
+import qualified Data.Char as Char
+
+newtype Parser i o = Parser (i -> Maybe (o, i))
+
+instance Functor (Parser i) where
+  fmap f (Parser p) = Parser $ \i -> fmap (\(o, i') -> (f o, i')) $ p i
+
+instance Applicative (Parser i) where
+  pure x = Parser $ \i -> Just (x, i)
+
+  Parser ff <*> Parser fv = Parser $ \i -> case ff i of
+    Just (f, i') -> case fv i' of
+      Just (v, i'') -> Just (f v, i'')
+      Nothing -> Nothing
+    Nothing -> Nothing
+
+charParser :: Char -> Parser String Char
+charParser c = Parser $ \i -> case i of
+  (x : xs) | x == c -> Just (c, xs)
+  _                 -> Nothing
+
+newtype Digit = Digit { getIntOfDigit :: Int } deriving (Show, Eq, Ord)
+
+charToDigit :: Char -> Maybe Digit
+charToDigit c =
+  if Char.isDigit c then Just $ Digit (Char.digitToInt c) else Nothing
+
+digitParser :: Parser String Digit
+digitParser = Parser $ \i -> case i of
+  (c : cs) -> fmap (\d -> (d, cs)) . charToDigit $ c
+  _        -> Nothing
+
+numberParser :: Parser String Int
+numberParser = undefined
+
+runParser :: i -> Parser i o -> Maybe o
+runParser i (Parser p) = fmap fst (p i)