Safe Haskell	Safe
Language	Haskell2010

Monad.Fail

Synopsis

class Monad m => MonadFail (m :: * -> *) where

Documentation

class Monad m => MonadFail (m :: * -> *) where #

When a value is bound in do-notation, the pattern on the left hand side of <- might not match. In this case, this class provides a function to recover.

A Monad without a MonadFail instance may only be used in conjunction with pattern that always match, such as newtypes, tuples, data types with only a single data constructor, and irrefutable patterns (~pat).

Instances of MonadFail should satisfy the following law: fail s should be a left zero for >>=,

fail s >>= f  =  fail s

If your Monad is also MonadPlus, a popular definition is

fail _ = mzero

Since: base-4.9.0.0

Minimal complete definition

fail

Methods

fail :: String -> m a #

Instances

MonadFail []	Since: base-4.9.0.0
Instance details Defined in Control.Monad.Fail Methods fail :: String -> [a] #
MonadFail Maybe	Since: base-4.9.0.0
Instance details Defined in Control.Monad.Fail Methods fail :: String -> Maybe a #
MonadFail IO	Since: base-4.9.0.0
Instance details Defined in Control.Monad.Fail Methods fail :: String -> IO a #
MonadFail Q
Instance details Defined in Language.Haskell.TH.Syntax Methods fail :: String -> Q a #
MonadFail IResult
Instance details Defined in Data.Aeson.Types.Internal Methods fail :: String -> IResult a #
MonadFail Result
Instance details Defined in Data.Aeson.Types.Internal Methods fail :: String -> Result a #
MonadFail Parser
Instance details Defined in Data.Aeson.Types.Internal Methods fail :: String -> Parser a #
MonadFail ReadPrec	Since: base-4.9.0.0
Instance details Defined in Text.ParserCombinators.ReadPrec Methods fail :: String -> ReadPrec a #
MonadFail ReadP	Since: base-4.9.0.0
Instance details Defined in Text.ParserCombinators.ReadP Methods fail :: String -> ReadP a #
MonadFail ReadM
Instance details Defined in Options.Applicative.Types Methods fail :: String -> ReadM a #
MonadFail P	Since: base-4.9.0.0
Instance details Defined in Text.ParserCombinators.ReadP Methods fail :: String -> P a #
MonadFail (ST s)	Since: base-4.11.0.0
Instance details Defined in GHC.ST Methods fail :: String -> ST s a #
Monad m => MonadFail (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods fail :: String -> MaybeT m a #
Monad m => MonadFail (ListT m)
Instance details Defined in List.Transformer Methods fail :: String -> ListT m a #
MonadFail m => MonadFail (IdentityT m)
Instance details Defined in Control.Monad.Trans.Identity Methods fail :: String -> IdentityT m a #
MonadFail m => MonadFail (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods fail :: String -> ExceptT e m a #
(Functor f, Monad m) => MonadFail (FreeT f m)
Instance details Defined in Control.Monad.Trans.Free Methods fail :: String -> FreeT f m a #
(Monad m, Error e) => MonadFail (ErrorT e m)
Instance details Defined in Control.Monad.Trans.Error Methods fail :: String -> ErrorT e m a #
MonadFail m => MonadFail (StateT s m)
Instance details Defined in Control.Monad.Trans.State.Lazy Methods fail :: String -> StateT s m a #
MonadFail m => MonadFail (StateT s m)
Instance details Defined in Control.Monad.Trans.State.Strict Methods fail :: String -> StateT s m a #
MonadFail m => MonadFail (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS.Internal Methods fail :: String -> WriterT w m a #
MonadFail m => MonadFail (ReaderT r m)
Instance details Defined in Control.Monad.Trans.Reader Methods fail :: String -> ReaderT r m a #
Stream s => MonadFail (ParsecT e s m)
Instance details Defined in Text.Megaparsec.Internal Methods fail :: String -> ParsecT e s m a #
MonadFail m => MonadFail (ContT r m)
Instance details Defined in Control.Monad.Trans.Cont Methods fail :: String -> ContT r m a #