module GHC.Data.Maybe (
module Data.Maybe,
MaybeErr(..),
failME, isSuccess,
orElse,
firstJust, firstJusts,
whenIsJust,
expectJust,
rightToMaybe,
MaybeT(..), liftMaybeT, tryMaybeT
) where
import GHC.Prelude
import Control.Monad
import Control.Monad.Trans.Maybe
import Control.Exception (catch, SomeException(..))
import Data.Maybe
import GHC.Utils.Misc (HasCallStack)
infixr 4 `orElse`
firstJust :: Maybe a -> Maybe a -> Maybe a
firstJust a b = firstJusts [a, b]
firstJusts :: [Maybe a] -> Maybe a
firstJusts = msum
expectJust :: HasCallStack => String -> Maybe a -> a
expectJust _ (Just x) = x
expectJust err Nothing = error ("expectJust " ++ err)
whenIsJust :: Monad m => Maybe a -> (a -> m ()) -> m ()
whenIsJust (Just x) f = f x
whenIsJust Nothing _ = return ()
orElse :: Maybe a -> a -> a
orElse = flip fromMaybe
rightToMaybe :: Either a b -> Maybe b
rightToMaybe (Left _) = Nothing
rightToMaybe (Right x) = Just x
liftMaybeT :: Monad m => m a -> MaybeT m a
liftMaybeT act = MaybeT $ Just `liftM` act
tryMaybeT :: IO a -> MaybeT IO a
tryMaybeT action = MaybeT $ catch (Just `fmap` action) handler
where
handler (SomeException _) = return Nothing
data MaybeErr err val = Succeeded val | Failed err
deriving (Functor)
instance Applicative (MaybeErr err) where
pure = Succeeded
(<*>) = ap
instance Monad (MaybeErr err) where
Succeeded v >>= k = k v
Failed e >>= _ = Failed e
isSuccess :: MaybeErr err val -> Bool
isSuccess (Succeeded {}) = True
isSuccess (Failed {}) = False
failME :: err -> MaybeErr err val
failME e = Failed e