[Haskell-beginners] Type error in sub-function

[양철웅]

Thank you, Brandon.
I changed the code like belows and it works.

                       ('x':'x':name2) -> do vv <- case value of
                                                      "client_ip_addr" ->
return $ clientIp request
                                                      "now" -> do utcTime
<- currentUTCTime
                                                                  return $
formatRFC1123 utcTime
                                                      _ -> return value
                                              let prev = clientReqHdr obj
                                              return obj { clientReqHdr =
((name2, vv) : prev) }



2014-07-23 23:22 GMT+09:00 Brandon Allbery <allbery.b at gmail.com>:

>
> On Wed, Jul 23, 2014 at 10:18 AM, 양철웅 <cwyang at 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 at gmail.com
> ballbery at sinenomine.net
> unix, openafs, kerberos, infrastructure, xmonad
> http://sinenomine.net
>
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