[Haskell-beginners] AMQP and nested exception handlers

[Kostiantyn Rybnikov]

Hi.

I would suggest to separate data-extraction from request stage and
data-sending one. Create a data-structure that will represent a thing you
will want to send into RabbitMQ, and build it before you send anything.
Then catch exceptions in IO-based layer to handle exception case.

This way you won't need any evaluation tricks and will get all exceptions
during that phase.

Cheers.
 12 трав. 2015 05:11 "Alex" <alex323 at gmail.com> пише:

> Hi:
>
> I am writing a small application which receives HTTP requests,
> translates them to JSON, and queues the requests using RabbitMQ.
>
> I am using exceptions to handle extreme cases, such as when a client's
> HTTP request lacks a critical header:
>
> lookupHeader :: RequestHeaders -> HeaderName -> Text
> lookupHeader hs h = decodeUtf8 $ fromMaybe notFound
>                                            (lookup h hs)
>   where
>     notFound = throw $ MissingHeader $ decodeUtf8 $ original h
>
> The problem I am running in to is that the header isn't actually looked
> up until I call the AMQP library's publishMsg function. If I purposely
> do not supply a critical header, I get the following error printed to
> the screen:
>
> ConnectionClosedException "ConnectionClosedException \"UNEXPECTED_FRAME
> - expected content header for class 60, got non content header frame
> instead\""
>
> If I add the following line just above the publishMsg function (to
> force evaluation), my exception handler is called successfully:
>
> print $ encode req
>
> As a result, I suspect that this is due to the fact that the "throw
> MissingHeader" is getting captured by the AMQP library. What's the best
> way to deal with this situation?
>
> --
> Alex
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