Control.Exception.Safe
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import Safe (Exception, MonadThrow, SomeException, throwM)
import Control.Monad (join)
import Control.Monad.IO.Class (MonadIO, liftIO)
import Data.Typeable (TypeRep, Typeable, typeRep)
import Text.Read (readMaybe)
import Control.Exception.Base (try)
data ReadException = ReadException String TypeRep
deriving (Typeable)
instance Show ReadException where
show (ReadException s typ) = concat [ "Unable to parse as " , show typ , ": " , show s ]
instance Exception ReadException
readM :: (MonadThrow m, Read a, Typeable a) => String -> m a
readM s = res
where
res =
case readMaybe s of
Just x -> return x
Nothing -> throwM $ ReadException s (typeRep res)
readLine1 :: (MonadIO m, MonadThrow n, Read a, Typeable a) => m (n a)
readLine1 = fmap readM (liftIO getLine)
-- Without the usage of liftIO here, we'd need both MonadIO and
-- MonadThrow constraints.
readLine2 :: (MonadIO m, Read a, Typeable a) => m a
readLine2 = liftIO (join readLine1)
main :: IO ()
main = do
putStrLn "Enter an Int (non-runtime exception)"
res1 <- readLine1
print (res1 :: Either SomeException Int)
putStrLn "Enter an Int (runtime exception)"
res2 <- try readLine2
print (res2 :: Either SomeException Int)
Haskell
INFO