hw11: starting files

[yorgey.git] / hw11 / AParser.hs

diff --git a/hw11/AParser.hs b/hw11/AParser.hs
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+module AParser (Parser, runParser, satisfy, char, posInt) where
+
+import           Control.Applicative
+import           Data.Char
+
+newtype Parser a = Parser { runParser :: String -> Maybe (a, String) }
+
+satisfy :: (Char -> Bool) -> Parser Char
+satisfy p = Parser f
+  where
+    f [] = Nothing
+    f (x:xs)
+        | p x       = Just (x, xs)
+        | otherwise = Nothing
+
+char :: Char -> Parser Char
+char c = satisfy (== c)
+
+posInt :: Parser Integer
+posInt = Parser f
+  where
+    f xs
+      | null ns   = Nothing
+      | otherwise = Just (read ns, rest)
+      where (ns, rest) = span isDigit xs
+
+inParser f = Parser . f . runParser
+
+first :: (a -> b) -> (a,c) -> (b,c)
+first f (x,y) = (f x, y)
+
+instance Functor Parser where
+  fmap = inParser . fmap . fmap . first
+
+instance Applicative Parser where
+  pure a = Parser (\s -> Just (a, s))
+  (Parser fp) <*> xp = Parser $ \s ->
+    case fp s of
+      Nothing     -> Nothing
+      Just (f,s') -> runParser (f <$> xp) s'
+
+instance Alternative Parser where
+  empty = Parser (const Nothing)
+  Parser p1 <|> Parser p2 = Parser $ liftA2 (<|>) p1 p2