fallible

An interface for fallible data types like Maybe and Either.

Example

Dealing with Maybe and Either gets a bit annoying when monadic actions are involved, often resulting in deep nests like this:

run :: String -> Token -> Bool -> IO ()
run targetName token verbose = do
  users <- getUsers token
  case users of
    Left err -> logDebug' err
    Right us -> do
      case userId <$> L.find isTarget us of
        Nothing  -> logDebug' emsg
        Just tid -> do
          channels <- getChannels token
          case channels of
            Left err  -> logDebug' err
            Right chs -> do
              let chs' = filter (elem tid . channelMembers) chs
              mapM_ (logDebug' . channelName) chs'

This package offers you several combinators to tidy up this eyesore. See example/Main.hs:

import           Data.Fallible
import qualified Data.List     as L

run :: String -> Token -> Bool -> IO ()
run targetName token verbose = evalContT $ do
  users    <- lift (getUsers token) !?= exit . logDebug'
  targetId <- userId <$> L.find isTarget users ??? exit (logDebug' emsg)
  channels <- lift (getChannels token) !?= exit . logDebug'
  lift $ mapM_ (logDebug' . channelName) $
    filter (elem targetId . channelMembers) channels
  where
    logDebug' = logDebug verbose
    emsg = "user not found: " ++ targetName
    isTarget user = userName user == targetName

logDebug :: Bool -> String -> IO ()
logDebug verbose msg = if verbose then putStrLn msg else pure ()

Notice the couple of operators, (!?=) and (???):

(!?=) :: Monad m => m (Either e a) -> (e -> m a) -> m a
(!??) :: Monad m => m (Maybe a) -> m a -> m a
(??=) :: Applicative f => Either e a -> (e -> f a) -> f a
(???) :: Applicative f => Maybe a -> f a -> f a

As you can guess from their type signatures, they run the right side when the first arguments returns Left or Nothing. Then you can use whatever you like to handle failures -- exception, MaybeT, ExceptT, ContT etc.

exec using ghci:

$ stack ghci
>>> :l example/Main.hs
*Main> run "Alice" "dummy" True
general
random
secret
*Main> run "Curry" "dummy" True
general
random
owners
*Main> run "Haskell" "dummy" True
user not found: Haskell
*Main> run "Haskell" "" True
invalid token

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