[Haskell-cafe] How to use http-enumerator with hoauth?

15 Feb 2011

      Hi,

I'm trying to use http-enumerator with Twitter's streaming API, which
requires OAuth authentication.

I was hoping to use the hoauth package for this, but it seems that
combining with with http-enumerator is pretty awkward.

In principle, it should be straightforward since hoauth defines a
HttpClient typeclass, and so I would just need to write an instance for
http-enumerator.

But in practice it is pretty awkward for a few reasons.  One is that
both packages define their own Request and Response types - however they
are semantically identical, so writing conversion functions is tedious
but possible.

However, I can't work out how to implement error handling;
http-enumerator uses the Control.Failure exception mechanism for
returning errors, but I can't work out how to fit that into HttpClient's
runClient function.

If I use "http" rather than "httpRedirect", I can avoid this to some
extent (since http doesn't use Failure HttpException m), but I still
can't get it to typecheck.

I can get a simple non-streaming instance to typecheck using httpLbs
(but again, not httpLbsRedirect):

data HttpOAuth = HttpOAuth { }

instance O.HttpClient HttpOAuth where
    runClient c r = (HE.httpLbs . http_cvt_request) r >>= return . cvt
        where 
          cvt :: HE.Response -> Either String O.Response
          cvt r@(HE.Response st _ b) | 200 <= st && st < 300 = Right $ http_cvt_response r
                                     | otherwise             = Left $ "HTTP status" ++ show st

However, I can't get the streaming version to type check at all:

newtype HttpOAuthStream a m b = HttpOAuthStream { iter :: W.Status -> W.ResponseHeaders -> DE.Iteratee a m b }

instance O.HttpClient (HttpOAuthStream a m b) where
    --   runClient :: (MonadIO m) => c -> Request -> m (Either String Response)
    runClient c r = liftM cvt $ DE.run $ HE.http (http_cvt_request r) (iter c)

cvt :: Show a => Either a HE.Response -> Either String O.Response
cvt (Left a) = Left $ show a
cvt (Right r) = Right $ http_cvt_response r

(Full code below)

And since I'm still trying to get my head around enumerators, I may have
that aspect of things completely wrong.  I haven't even tried running
any of this yet, so I don't know if I've made any non-type errors.

Am I even barking up the right tree at all?  Should I try to be using
hoauth this way at all, or should I just hack it up to work within
http-enumerator?  That seems counter-productive.

A general comment:

There are many partially functional http packages in hackage.  It seems
to me that rather than requiring one package be a complete http library,
we would get further by allowing different packages to implement
different aspects of http, so long as they can all be composed in a
reasonable way.  At the very least, is it really necessary for hoauth to
define its own Request/Response types?  Would it be worthwhile trying to
define general types which all http packages could use?  Is that the
goal of Network.Wai?  What about the HTTP package?  Should I just use
that instead?  What about Network.Curl?

Thanks,
    J

import qualified Data.Enumerator as DE

import qualified Data.ByteString.Char8 as C8

import Control.Monad (liftM)
import Control.Applicative ((<$>), (<*>), (<|>), empty, pure)
import Control.Arrow (first, second, (***))

import qualified Network.OAuth.Consumer as O
import qualified Network.OAuth.Http.HttpClient as O
import qualified Network.OAuth.Http.Request as O
import qualified Network.OAuth.Http.Response as O

import qualified Network.HTTP.Enumerator as HE
import qualified Network.Wai as W

import Data.List (intercalate)

import Control.Failure
import Control.Exception (SomeException)

-- Convert a Network.OAuth.Http.Request into a Network.HTTP.Enumerator.Request
-- What. A. Pain.
http_cvt_request :: O.Request -> HE.Request
http_cvt_request oar = HE.Request method secure host port path query headers body
    where method = C8.pack . show . O.method $ oar
          secure = O.ssl oar
          host = C8.pack . O.host $ oar
          port = O.port oar
          path = C8.pack . intercalate "/" $ O.pathComps oar
          query = packpair <$> (O.toList . O.qString $ oar)
          headers = (first W.mkCIByteString) . packpair <$> (O.toList . O.reqHeaders $ oar)
          body = O.reqPayload oar

-- Convert a Network.HTTP.Enumerator.Response into a Network.OAuth.Http.Response
-- See above.
http_cvt_response :: HE.Response -> O.Response
http_cvt_response her = O.RspHttp status reason headers payload
    where status = HE.statusCode her
          reason = "" -- ?
          headers = O.fromList $ (unpackpair . first W.ciOriginal) <$> HE.responseHeaders her
          payload = HE.responseBody her

mappair f (a,b) = (f a, f b)
packpair = mappair C8.pack
unpackpair = mappair C8.unpack

newtype HttpOAuthStream a m b = HttpOAuthStream { iter :: W.Status -> W.ResponseHeaders -> DE.Iteratee a m b }

instance O.HttpClient (HttpOAuthStream a m b) where
    --   runClient :: (MonadIO m) => c -> Request -> m (Either String Response)
    runClient c r = liftM cvt $ DE.run $ HE.http (http_cvt_request r) (iter c)

cvt :: Show a => Either a HE.Response -> Either String O.Response
cvt (Left a) = Left $ show a
cvt (Right r) = Right $ http_cvt_response r

data HttpOAuth = HttpOAuth { }

instance O.HttpClient HttpOAuth where
    runClient c r = (HE.httpLbs . http_cvt_request) r >>= return . cvt
        where 
          cvt :: HE.Response -> Either String O.Response
          cvt r@(HE.Response st _ b) | 200 <= st && st < 300 = Right $ http_cvt_response r
                                     | otherwise             = Left $ "HTTP status" ++ show st

[Haskell-cafe] How to use http-enumerator with hoauth?

Jeremy Fitzhardinge