Add tryError

haskell · Mar 4, 2019 · 5ae7255 · 5ae7255
1 parent 6eee658
commit 5ae7255
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diff --git a/Control/Monad/Error/Class.hs b/Control/Monad/Error/Class.hs
@@ -42,8 +42,10 @@ module Control.Monad.Error.Class (
     Error(..),
     MonadError(..),
     liftEither,
+    tryError,
   ) where
 
+import Control.Applicative (Applicative(pure))
 import Control.Monad.Trans.Except (Except, ExceptT)
 import Control.Monad.Trans.Error (Error(..), ErrorT)
 import qualified Control.Monad.Trans.Except as ExceptT (throwE, catchE)
@@ -118,6 +120,11 @@ where @action1@ returns an 'Either' to represent errors.
 liftEither :: MonadError e m => Either e a -> m a
 liftEither = either throwError return
 
+-- | Catch an exception and return it in 'Left', or return a successful result
+-- in 'Right'.
+tryError :: (MonadError e m, Applicative m) => m a -> m (Either e a)
+tryError m = catchError (fmap Right m) (pure . Left)
+
 instance MonadError IOException IO where
     throwError = ioError
     catchError = catch

diff --git a/Control/Monad/Except.hs b/Control/Monad/Except.hs
@@ -38,6 +38,8 @@ module Control.Monad.Except
     -- * Monads with error handling
     MonadError(..),
     liftEither,
+    tryError,
+
     -- * The ExceptT monad transformer
     ExceptT(ExceptT),
     Except,