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Added liftMaybe for lifting Maybe into MonadError #137

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Added liftMaybe for lifting Maybe into MonadError #137

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eceeb6a
Add 'liftMaybe'
emlautarom1 Dec 15, 2022
dbc850f
Update docs
emlautarom1 Dec 15, 2022
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13 changes: 12 additions & 1 deletion Control/Monad/Error/Class.hs
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Expand Up @@ -45,6 +45,7 @@ The Error monad (also called the Exception monad).
module Control.Monad.Error.Class (
MonadError(..),
liftEither,
liftMaybe,
tryError,
withError,
handleError,
Expand Down Expand Up @@ -74,7 +75,7 @@ import Control.Monad.Trans.Class (lift)
import Control.Exception (IOException, catch, ioError)
import Control.Monad (Monad)
import Data.Monoid (Monoid)
import Prelude (Either (Left, Right), Maybe (Nothing), either, flip, (.), IO, pure, (<$>), (>>=))
import Prelude (Either (Left, Right), Maybe (Nothing), either, maybe, flip, (.), IO, pure, (<$>), (>>=))

{- |
The strategy of combining computations that can throw exceptions
Expand Down Expand Up @@ -122,6 +123,16 @@ where @action1@ returns an 'Either' to represent errors.
liftEither :: MonadError e m => Either e a -> m a
liftEither = either throwError pure

{- |
Lifts a @'Maybe'@ into any @'MonadError' e@, using a supplied @e@ as the error if the 'Maybe' is 'Nothing'.

> do { val <- liftMaybe e =<< action1; action2 }

where @action1@ returns a 'Maybe'.
-}
liftMaybe :: MonadError e m => e -> Maybe a -> m a
liftMaybe e = maybe (throwError e) pure

instance MonadError IOException IO where
throwError = ioError
catchError = catch
Expand Down