19_b_1: Add solution

1 files changed, 73 insertions, 0 deletions

diff --git a/ch19/Module_19_b_1.hs b/ch19/Module_19_b_1.hs
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+-- Write a many parser, with type Parser a -> Parser [a]. It should apply a
+-- parser until it fails.
+
+-- Control.Monad.Error is deprecated. Control.Monad.Except should be used
+-- instead. I modified the code from examples here to use it.
+
+
+{-- From examples/examples/ch19/ParseInt.hs and modified --}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+
+module Module_19_b_1 where
+
+import Control.Monad.Except
+import Control.Monad
+import Control.Monad.State
+import qualified Data.ByteString.Char8 as B
+
+data ParseError = NumericOverflow
+                | EndOfInput
+                | Chatty String
+                  deriving (Eq, Ord, Show)
+
+newtype Parser a = P {
+      runP :: ExceptT ParseError (State B.ByteString) a
+    } deriving (Functor, Applicative, Monad, MonadError ParseError)
+
+liftP :: State B.ByteString a -> Parser a
+liftP m = P (lift m)
+
+satisfy :: (Char -> Bool) -> Parser Char
+satisfy p = do
+  s <- liftP get
+  case B.uncons s of
+    Nothing         -> throwError EndOfInput
+    Just (c, s')
+        | p c       -> liftP (put s') >> return c
+        | otherwise -> throwError (Chatty "satisfy failed")
+
+runParser :: Parser a -> B.ByteString
+          -> Either ParseError (a, B.ByteString)
+runParser p bs = case runState (runExceptT (runP p)) bs of
+                   (Left err, _) -> Left err
+                   (Right r, bs) -> Right (r, bs)
+
+optional :: Parser a -> Parser (Maybe a)
+optional p = (Just `liftM` p) `catchError` \_ -> return Nothing
+{-- End of code from examples --}
+
+
+many :: Parser a -> Parser [a]
+many p = ((:) <$> p <*> many p) `catchError` \_ -> return []
+
+
+-- ghci> :l Module_19_b_1.hs
+-- [1 of 1] Compiling Module_19_b_1    ( Module_19_b_1.hs, interpreted )
+-- Ok, one module loaded.
+
+-- ghci> :m +Data.Char
+
+-- ghci> runParser (many (satisfy isDigit)) (B.pack "")
+-- Right ("","")
+
+-- ghci> runParser (many (satisfy isDigit)) (B.pack "abc")
+-- Right ("","abc")
+
+-- ghci> runParser (many (satisfy isDigit)) (B.pack "9abc")
+-- Right ("9","abc")
+
+-- ghci> runParser (many (satisfy isDigit)) (B.pack "987abc")
+-- Right ("987","abc")
+
+-- ghci> runParser (many (satisfy isDigit)) (B.pack "x987abc")
+-- Right ("","x987abc")