[Haskell-cafe] How to handle exceptions in conduit?

Michael Snoyman michael at snoyman.com
Mon Nov 5 20:51:15 CET 2012

On Nov 5, 2012 2:42 PM, "Hiromi ISHII" <konn.jinro at gmail.com> wrote:
> Hi, there
> On 2012/11/01, at 21:23, Michael Snoyman wrote:
> > Due to various technical reasons regarding the nature of conduit, you
can't currently catch exceptions within the Pipe monad. You have two
> >
> > * Catch exceptions before `lift`ing.
> > * Catch exceptions thrown from the entire Pipe.
> >
> > Since the exceptions are always originating in the underlying monad,
the first choice is certainly possible in theory, though may require
reworking the library you're using a bit.
> Thanks. In my case, used library is relatively small so I can rewrite it
to ignore exception before lifting.
> But I think it is more convenient doing the same thing without modifying
existing code.
> The second choice does not match my case because it cannot resume the
process from the place just after an exception occurred.

I agree that it would be great if conduit could meet your use case better.
I haven't spent enough cycles looking at this yet to determine if the
reason we don't have this support is a limitation in the conduit approach
itself, or just a limitation in what I was able to implement so far. If you
can think of a way to implement more fine-grained exception handling (or
anyone else for that matter), I'd love to hear about it.

> > One other possibility that I haven't actually tried would be to use
transPipe[1] to catch all of the exceptions, though I'm not sure how well
that would work in practice.
> The type of the first argument of `transPipe` should be general, so I
think we can't compose it with `catch` function.

That makes sense.

> -- Hiromi ISHII
> konn.jinro at gmail.com
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