annotated-exception

This library provides a special AnnotatedException type which allows you to decorate Haskell exceptions with additional information. This decoration is totally transparent, and even works with exceptions thrown outside of your application code.

To provide an annotation, you’d use the function checkpoint. This will attach the provided value to any exception that bubbles up through it.

import Control.Exception.Annotated

data MyException = MyException
    deriving (Show, Exception)

main :: IO ()
main = do
    checkpoint "Foo" $ do
        throw MyException

When this program crashes, it will crash with an AnnotatedException that contains the annotation "Foo".

λ> checkpoint "Foo" $ throw MyException
*** Exception: AnnotatedException {annotations = ["Foo"], exception = MyException}

These annotations survive, even if you catch and rethrow with a different exception.

data OtherException = OtherException
    deriving (Show, Exception)

woah :: IO ()
woah = do
    let
        checkpointed =
            checkpoint "Foo" (throw MyException)
        handler MyException =
            throw OtherException

    checkpointed
        `catch`
            handler

Notice how the checkpoint call doesn’t cover the throw OtherException - the exception [Annotation] lives on the thrown exception itself, and this library’s catch function ensures that we don’t lose that context.

λ> (checkpoint "Foo" (throw MyException)) `catch` \MyException -> throw OtherException
*** Exception: AnnotatedException {annotations = ["Foo"], exception = OtherException}

You can also attach a CallStack to any exception using throwWithCallStack.

Now, you’re about to report your exceptions, up near main. We can use try in this module to always get the annotations.

main = do
    eresult <- try $ myProgram
    case eresult of
        Left (AnnotatedException annotations exception) ->
            reportException annotations exception
        Right a ->
            pure a