Safe Haskell	Safe-Inferred
Language	Haskell2010

Distribution.Compat.MonadFail

Description

Compatibility layer for Control.Monad.Fail

Synopsis

class Monad m => MonadFail (m :: Type -> Type) where
- fail :: String -> m a

Documentation

class Monad m => MonadFail (m :: Type -> Type) 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

fail s should be an action that runs in the monad itself, not an exception (except in instances of MonadIO). In particular, fail should not be implemented in terms of error.

Since: base-4.9.0.0

Methods

fail :: String -> m a #

Instances

Instances details

MonadFail ParsecParser Source #
Instance details Defined in Distribution.Parsec Methods fail :: String -> ParsecParser a #
MonadFail P	Since: base-4.9.0.0
Instance details Defined in Text.ParserCombinators.ReadP Methods fail :: String -> P a #
MonadFail ReadP	Since: base-4.9.0.0
Instance details Defined in Text.ParserCombinators.ReadP Methods fail :: String -> ReadP a #
MonadFail ReadPrec	Since: base-4.9.0.0
Instance details Defined in Text.ParserCombinators.ReadPrec Methods fail :: String -> ReadPrec a #
MonadFail Get
Instance details Defined in Data.Binary.Get.Internal Methods fail :: String -> Get 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 Maybe	Since: base-4.9.0.0
Instance details Defined in Control.Monad.Fail Methods fail :: String -> Maybe a #
MonadFail []	Since: base-4.9.0.0
Instance details Defined in Control.Monad.Fail Methods fail :: String -> [a] #
Monad m => MonadFail (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods fail :: String -> MaybeT m a #
MonadFail f => MonadFail (Ap f)	Since: base-4.12.0.0
Instance details Defined in Data.Monoid Methods fail :: String -> Ap f a #
(Monoid w, MonadFail m) => MonadFail (AccumT w m)
Instance details Defined in Control.Monad.Trans.Accum Methods fail :: String -> AccumT w m a #
MonadFail m => MonadFail (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods fail :: String -> ExceptT e m a #
MonadFail m => MonadFail (IdentityT m)
Instance details Defined in Control.Monad.Trans.Identity Methods fail :: String -> IdentityT m a #
MonadFail m => MonadFail (ReaderT r m)
Instance details Defined in Control.Monad.Trans.Reader Methods fail :: String -> ReaderT r m a #
MonadFail m => MonadFail (SelectT r m)
Instance details Defined in Control.Monad.Trans.Select Methods fail :: String -> SelectT r 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 Methods fail :: String -> WriterT w m a #
(Monoid w, MonadFail m) => MonadFail (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods fail :: String -> WriterT w m a #
(Monoid w, MonadFail m) => MonadFail (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods fail :: String -> WriterT w m a #
MonadFail m => MonadFail (Reverse m)
Instance details Defined in Data.Functor.Reverse Methods fail :: String -> Reverse m a #
MonadFail (ParsecT s u m)	Since: parsec-3.1.12.0
Instance details Defined in Text.Parsec.Prim Methods fail :: String -> ParsecT s u m a #
MonadFail m => MonadFail (ContT r m)
Instance details Defined in Control.Monad.Trans.Cont Methods fail :: String -> ContT r m a #
MonadFail m => MonadFail (RWST r w s m)
Instance details Defined in Control.Monad.Trans.RWS.CPS Methods fail :: String -> RWST r w s m a #
(Monoid w, MonadFail m) => MonadFail (RWST r w s m)
Instance details Defined in Control.Monad.Trans.RWS.Lazy Methods fail :: String -> RWST r w s m a #
(Monoid w, MonadFail m) => MonadFail (RWST r w s m)
Instance details Defined in Control.Monad.Trans.RWS.Strict Methods fail :: String -> RWST r w s m a #