Catching Exceptions during a stream?

[jerbaroo]

Stream.FileSystem.File.read path
  & Stream.onException (throwError NoInfoError)

Consider I am using Streamly.FileSystem.File.readFile to return a Stream m Word8, where m has some Error ReadError :> es constraint (from effectful) allowing for the use of throwError.

How can I catch IOException that might be produced during the stream, and then use throwError from effectful to propagate that exception info? The above code compiles, but unfortunately onException does not make any information from the exception available to the handler.

Thank you

[adithyaov]

1. You can use Stream.handle
2. You can handle the exception later when consuming the effect (after folding the stream)

[jerbaroo]

What I am looking for is a variant of onException that is not hardcoded to SomeException. Would that be a reasonable approach? Here is an example of what I am thinking: #3119 This would allow me to write the following:

Stream.FileSystem.File.read path
  & Stream.onException \(e :: IOException -> throwError $ ReadFileIOException e)
  -- Note that in my case:
  --   throwError :: (Error e :> es) e -> Eff es a

1. I would prefer not to use Stream.handle if the monad m in which the stream is operating already has the capability to manage errors.
2. If this is referring to a solution whereby the caller needs to remember to write exception handling logic, without a compilation error otherwise, then I would not go this route.

[adithyaov]

I would prefer not to use Stream.handle if the monad m in which the stream is operating already has the capability to manage errors.

I'm not sure I understand this properly.

onExceptionE f = handle (\e -> f e >> Stream.nil)

[jerbaroo]

Thank you, with a slight change from >> to $> this works.

Previously I had been looking at the definition of handle from streamly rather than handle from streamly-core, which if I'm not mistaken, the former would not suffice here.

onExceptionE
  :: (Exception e, MonadCatch m) => (e -> m a) -> Stream m b -> Stream m b
onExceptionE f = Stream.handle (\e -> f e $> Stream.nil)

[adithyaov]

handle from streamly is deprecated, I believe. Yes, it would not have worked.

I was meaning to write this:

onExceptionE f = handle (\e -> f e >> pure Stream.nil)