Control.Monad.Fail

Transitional module providing the MonadFail class and primitive instances.

This module can be imported for defining forward compatible MonadFail instances:

import qualified Control.Monad.Fail as Fail

instance Monad Foo where
  (>>=) = {- ...bind impl... -}

  -- Provide legacy fail implementation for when
  -- new-style MonadFail desugaring is not enabled.
  fail = Fail.fail

instance Fail.MonadFail Foo where
  fail = {- ...fail implementation... -}

See https://gitlab.haskell.org/haskell/prime/-/wikis/libraries/proposals/monad-fail for more details.

1 class

classclass 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.

Methods

fail :: String -> m a

Instances7MonadFail, …

MonadFail MaybeDefined in ghc-internal-9.1003.0 · GHC.Internal.Control.Monad.Fail
MonadFail PDefined in ghc-internal-9.1003.0 · GHC.Internal.Text.ParserCombinators.ReadP
MonadFail ReadPDefined in ghc-internal-9.1003.0 · GHC.Internal.Text.ParserCombinators.ReadP
MonadFail ReadPrecDefined in ghc-internal-9.1003.0 · GHC.Internal.Text.ParserCombinators.ReadPrec
MonadFail IODefined in ghc-internal-9.1003.0 · GHC.Internal.Control.Monad.Fail
MonadFail []Defined in ghc-internal-9.1003.0 · GHC.Internal.Control.Monad.Fail
MonadFail f => MonadFail (Ap f)Defined in ghc-internal-9.1003.0 · GHC.Internal.Data.Monoid