Control.Monad.Fail.Hoist

This module provides helpers for converting partiality types into MonadFail computations.

MonadFail's purpose is to handle pattern-match failures in do-expressions, and not to be a general-purpose error-handling mechanism. Despite this, some libraries use it as one, and this module can help you report errors via MonadFail.

The operator mnemonics are the same as in Control.Monad.Error.Hoist, but with # in place of ?. You can imagine a hastily-written F looking kinda-sorta like a #, if it helps.

Documentation

hoistFail :: (PluckError e t m, MonadFail m) => (e -> String) -> t a -> m a

Given a conversion from the error in t a to String, we can hoist the computation into m.

hoistFail :: MonadFail m => (() -> String) -> Maybe    a -> m a
hoistFail :: MonadFail m => (a  -> String) -> Either a b -> m b

hoistFail' :: (PluckError String t m, MonadFail m) => t a -> m a

Hoist computations whose error type is already String.

hoistFailM :: (PluckError e t m, MonadFail m) => (e -> String) -> m (t a) -> m a

A version of hoistFail that operates on values already in the monad.

hoistFailM :: MonadFail m => (() -> String) -> m (Maybe       a) ->           m a
hoistFailM :: MonadFail m => (a  -> String) -> m (Either  a   b) ->           m b
hoistFailM :: MonadFail m => (a  -> String) ->    ExceptT a m b  -> ExceptT a m b

hoistFailM' :: (PluckError String t m, MonadFail m) => m (t a) -> m a

A version of hoistFail' that operates on values already in the monad.

hoistFailM' :: MonadFail m => m (Maybe       a) ->           m a
hoistFailM' :: MonadFail m => m (Either  a   b) ->           m b
hoistFailM' :: MonadFail m =>    ExceptT a m b  -> ExceptT a m b

Operators

(<%#>) :: (PluckError e t m, MonadFail m) => t a -> (e -> String) -> m a infixl 8

A flipped synonym for hoistFail. Mnemonic: # looks a bit like F.

(<%#>) :: MonadFail m => Maybe    a -> (() -> e) -> m a
(<%#>) :: MonadFail m => Either a b -> (a  -> e) -> m b

(<%!#>) :: (PluckError e t m, MonadFail m) => m (t a) -> (e -> String) -> m a infixl 8

A flipped synonym for hoistFailM.

(<%!#>) :: MonadError e m => m (Maybe       a) -> (() -> e) ->           m a
(<%!#>) :: MonadError e m => m (Either  a   b) -> (a  -> e) ->           m b
(<%!#>) :: MonadError e m =>    ExceptT a m b  -> (a  -> e) -> ExceptT a m b

(<#>) :: (PluckError e t m, MonadFail m) => t a -> String -> m a infixl 8

A version of <%#> that ignores the error in t a and fails with a new one.

(<#>) :: MonadFail m => Maybe    a -> String -> m a
(<#>) :: MonadFail m => Either a b -> String -> m b

(<!#>) :: (PluckError e t m, MonadFail m) => m (t a) -> String -> m a infixl 8

A version of <#> that operates on values already in the monad.

(<!#>) :: 'MonadFail m => m (Maybe       a) -> String ->           m a
(<!#>) :: 'MonadFail m => m (Either  a   b) -> String ->           m b
(<!#>) :: 'MonadFail m =>    ExceptT a m b  -> String -> ExceptT a m b