GHC.Internal.Control.Monad.Fail

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

MonadFail ReadP

Since: base-4.9.0.0

Defined in GHC.Internal.Text.ParserCombinators.ReadP

Methods

fail :: String -> ReadP a

MonadFail ReadPrec

Since: base-4.9.0.0

Defined in GHC.Internal.Text.ParserCombinators.ReadPrec

Methods

fail :: String -> ReadPrec a

MonadFail IO

Since: base-4.9.0.0

Defined in GHC.Internal.Control.Monad.Fail

Methods

fail :: String -> IO a

MonadFail Maybe

Since: base-4.9.0.0

Defined in GHC.Internal.Control.Monad.Fail

Methods

fail :: String -> Maybe a

MonadFail []

Since: base-4.9.0.0

Defined in GHC.Internal.Control.Monad.Fail

Methods

fail :: String -> [a]

MonadFail f => MonadFail (Ap f)

Since: base-4.12.0.0

Defined in GHC.Internal.Data.Monoid

Methods

fail :: String -> Ap f a