On Wed, Jul 23, 2014 at 10:18 AM, 양철웅 <cwyang@aranetworks.com> wrote:

                        ('x':'x':name2) -> return obj { clientReqHdr = ((name2, vv) : prev) }

                           where prev = clientReqHdr obj

                                 vv = case value of

                                        "client_ip_addr" -> clientIp request

                                        "now" -> do utcTime <- currentUTCTime

                                                    return $ formatRFC1123 utcTime

                                        _ -> value

                        _ -> return obj

However, above code does not compile also :-( 

*Main> :l test

[1 of 1] Compiling Main             ( test.hs, interpreted )

test.hs:101:64:

    Couldn't match expected type `[t0]' with actual type `IO UTCTime'

    In a stmt of a 'do' block: utcTime <- currentUTCTime

    In the expression:

      do { utcTime <- currentUTCTime;

           return $ formatRFC1123 utcTime }

    In a case alternative:

        "now"

          -> do { utcTime <- currentUTCTime;

                  return $ formatRFC1123 utcTime }

Failed, modules loaded: none.

Obviously outer do-block is inside IO monad, as the type of scanQuery is 

Request -> IO Object. But GHC puts inner do-block (in "now" case)  inside list monad,

doesn't it? Why does ghc look for List monad?

Because you're treating vv as a pure value when you use it, so ghc looks for a way to treat it as a monad and concludes that it is a List. If you want it to be in IO, you need to use <- on its result, not use it directly.

--

brandon s allbery kf8nh                               sine nomine associates

allbery.b@gmail.com                                  ballbery@sinenomine.net

unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net

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