Thank you all for kind replies. But I'm not sure whether I grasp the gist. Actual code is as follows. I want to scan the query argument of HTTP get method and build response object according to the query arguments. I use fold and the scanQuery function should return IO object for some reason. Since parseHeader function is fold function and uses query and request variable at the same time, I cannot write as a separate function. scanQuery :: Request -> IO Object scanQuery request = do let (p, query) = parseUrl (path request) foldM parseHeader defObject query where parseHeader obj (name, Just value) = case name of "len" -> return obj { contentLength = read value } "type" -> return obj { contentType = parseContentType value } "rate" -> return obj { rate = Just value } "status" -> return obj { httpStatus = read value } ('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? Lost in monad, Chul-Woong 2014-07-23 21:33 GMT+09:00 Kim-Ee Yeoh :

On Wed, Jul 23, 2014 at 7:08 PM, 양철웅 wrote:

...

Since the return type of foo Func is IO String and first case statement has "return timedVal", I think that ghc expects the type of timedVal as String. However, the error message shows that ghc expects timedVal should have type IO b0.

It's not really about timedVal nor the case 'statement'. (The scare quotes are because there are only expressions, not statements, in haskell.)

Consider the difference between

do { putStrLn "hello"; True; }

and

do { putStrLn "hello"; return True; }

Which one throws an error and why?

-- Kim-Ee

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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 :

On Wed, Jul 23, 2014 at 10:18 AM, 양철웅 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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Yes. I found that issue and change the function to pure form which receives time as input argument. When I tried to code in C++, I found myself coding in C only to feed the sources to g++. I found myself coding in do-block for majority of Haskell code now. :-( Thanks Yeoh! 2014-07-24 16:56 GMT+09:00 Kim-Ee Yeoh :

On Wed, Jul 23, 2014 at 9:45 PM, 양철웅 wrote:

...

I changed the code like belows and it works.

On a higher-level, you might want to reconsider the design of that function called parseHeader.

It has type: (a -> IO b) only because there's a special case "now" which apparently requires querying for the current time. Other than that, it's a pure function.

Is there a way to make it a pure function?

-- Kim-Ee

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