Haskell Exceptions (only to be used with IO) #haskell

haskell_exceptions.hs

-- safe-exceptions Control.Exception.Safe > exceptions Control.Monad.Catch > base Control.Exception
import Control.Exception.Safe
import Control.Monad.IO.Class (MonadIO, liftIO)

data WhatException = ThisError
                   | ThatError
                   deriving (Show)

instance Exception WhatException

doThis :: IO String
doThis = do
  line <- getLine
  if null line then throw ThisError
  else if line == "f" then throw ThatError
  else return line

doThatWithCatch :: IO String
doThatWithCatch = do
  line <- catch
    doThis
    (\e -> case (e :: WhatException) of
        ThisError -> return "recovered this"
        ThatError -> return "recovered that")
  return $ line ++ "!"

doThatWithTry :: IO String
doThatWithTry = do
  line <- try doThis
  case line of
    (Left ThisError) -> return "recovered this!"
    (Left ThatError) -> return "recovered that!"
    (Right line') -> return $ line' ++ "!"

doThisGeneric :: (MonadIO m) => m String
doThisGeneric = do
  line <- liftIO getLine
  if null line then liftIO $ throw ThisError
  else if line == "f" then liftIO $ throw ThatError
  else return line

doThatGeneric :: (MonadIO m) => m String
doThatGeneric = do
  line <- liftIO $ catch doThisGeneric handle
  return $ line ++ "!"
  where
    handle :: (MonadIO m) => WhatException -> m String
    handle e = case e of
      ThisError -> return "recovered this"
      ThatError -> return "recovered that"