[Haskell-cafe] [ANNOUNCE] First Public Release of the Snap Framework

[Gregory Collins]

Michael Snoyman <michael at snoyman.com> writes:

> Congratulations on the release. I was interested in seeing how this
> would work as a WAI handler, and came across some questions:
>
> * I noticed that the Method datatype is restricted to a set of
> specific methods. Seeing as the list of methods can be expanded[1],
> why was this chosen?

The answer is "no particular reason" -- nobody really uses this. I think
you're right that this makes us technically out of spec, if it becomes
an issue for anyone we'll add a custom constructor later.


> * The CIByteString datatype provides no way of accessing directly the
> lower-case version of the bytestring, or of setting it. Seeing as WAI
> already lower-cases the headers (following your suggestion btw) it
> would be more efficient to only do this once. Would you consider
> exposing the constructor?

Would you accept:

    ciToLower :: CIByteString -> ByteString

instead? I prefer opaque datatypes in general. We didn't see a need for
that use-case, the idea was that the string representation would be the
same but we would do a case-insensitive compare.


> * For simplicity at the moment, I decided to use the getRequestBody
> function, but it seems to be returning an empty result. Is there a
> known gotcha here?

If the POST body has content-type "application/x-www-form-urlencoded" we
parse it for you and put the fields in the parameter mapping. If this
isn't your case I'd appreciate a bug report. This is a place where we
made a different design decision than you did -- a WAI application which
expects to parse the POST body itself won't work. I'll think about
adding a knob to make this behaviour optional.


> Overall, writing the WAI wrapper is pretty straight-forward. The main
> problem is that the WAI request body does not require an inversion of
> control approach, while the Snap version does; some usage of lazy I/O
> here could solve the problem, though that's obviously sub-optimal.

A Chan & a forkIO could work here also (we've discussed that one before
I think.)


> Also, it seems a little unclear whether the writeBS et al functions
> store the body in memory before returning the result, though the
> documentation implies it. Could you provide some clarifications?

The Snap monad carries a "Response" in its state, with "rspBody" being
an output Enumerator. The "writeBS" function composes "enumBS foo" with
the Enumerator from that state; so when you call writeBS you're really
building up a *program* to send the response body out later. So yes, we
hang onto that data until the request is served.

If you want to stream in O(1) space you need to provide an Enumerator to
do so; the Enumerator has access to the IO monad though. We provide
writeBS/writeLBS for those situations in which it's
convenient/appropriate to build up the entire response in memory.

G.
-- 
Gregory Collins <greg at gregorycollins.net>