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Hoist between ExceptT error types #10

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Hoist between ExceptT error types #10

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5 changes: 4 additions & 1 deletion src/Control/Monad/Error/Hoist.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -52,7 +52,7 @@ import Control.Monad.Error.Class (MonadError (..))

import Data.Either (Either, either)

import Control.Monad.Except (Except, ExceptT, runExcept,
import Control.Monad.Except (Except, ExceptT(..), runExcept,
runExceptT)

-- | Given a conversion from the error in @t a@ to @e'@, we can hoist the
Expand Down Expand Up @@ -194,3 +194,6 @@ instance Applicative m => PluckError e (Either e) m where
instance Monad m => PluckError e (ExceptT e m) m where
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Suggested change
instance Monad m => PluckError e (ExceptT e m) m where
instance (m ~ n, Monad m) => PluckError e (ExceptT e m) n where

I think this is what we want

pluckError = runExceptT
foldError f g = either f g <=< runExceptT

instance Monad m => PluckError e (ExceptT e m) (ExceptT e' m) where
pluckError = ExceptT . fmap Right . runExceptT
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