Add withException, similar to safe-exceptions and unliftio. Closes #304

effectful-core/CHANGELOG.md

@@ -1,4 +1,5 @@
 # effectful-core-2.6.0.0 (????-??-??)
+* Add `withException` to `Effectful.Exception`, similar to safe-exceptions and unliftio.
 * **Breaking changes**:
   - Change the order of type parameters in `raise` for better usability.
   - `Effectful.Error.Static.ErrorWrapper` is no longer caught by `catchSync`.

effectful-core/src/Effectful/Exception.hs

@@ -59,6 +59,7 @@ module Effectful.Exception
   , C.ExitCase(..)
   , finally
   , onException
+  , withException
 
     -- * Utils
 
@@ -512,6 +513,21 @@ onException
 onException action handler = reallyUnsafeUnliftIO $ \unlift -> do
   E.onException (unlift action) (unlift handler)
 
+-- | Like 'onException', but provides the handler the thrown exception.
+withException :: C.Exception e => Eff es a -> (e -> Eff es b) -> Eff es a
+withException thing after = uninterruptibleMask $ \restore -> do
+    res1 <- try $ restore thing
+    case res1 of
+        Left e1 -> do
+            -- explicitly ignore exceptions from after. We know that
+            -- no async exceptions were thrown there, so therefore
+            -- the stronger exception must come from thing
+            --
+            -- https://github.com/fpco/safe-exceptions/issues/2
+            _ :: Either C.SomeException b <- try $ after e1
+            throwIO e1
+        Right x -> pure x
+
 ----------------------------------------
 -- Utils