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Today's Topics:
1. Re: Manage the type of a variable in the show implementation
(Kim-Ee Yeoh)
2. Re: Manage the type of a variable in the show implementation
(Christian Sperandio)
3. Re: Type error in sub-function (???)
4. Re: Type error in sub-function (Brandon Allbery)
----------------------------------------------------------------------
Message: 1
Date: Wed, 23 Jul 2014 19:42:01 +0700
From: Kim-Ee Yeoh <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Manage the type of a variable in the
show implementation
Message-ID:
<capy+zdtcmui2jbv-pqqd6re1r9otk8sc6jqk_khdm1u3ml1...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Wed, Jul 23, 2014 at 7:40 PM, Chadda? Fouch? <[email protected]>
wrote:
> Anyway, using Show for this is wrong, Show is supposed to translate
> datatypes in a form you could copy in your code to get the value back, it
> isn't supposed to be for presentation. You have libraries that do pretty
> formatting better.
Very insightful, thanks!
-- Kim-Ee
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Message: 2
Date: Wed, 23 Jul 2014 15:04:29 +0200
From: Christian Sperandio <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Manage the type of a variable in the
show implementation
Message-ID:
<cakc7jjjmurf2ptmo05g8zh2h1wnd9cpjx-knnq0u8c3oqvq...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
I chose the newtype solution.
Thanks :)
2014-07-23 14:40 GMT+02:00 Chadda? Fouch? <[email protected]>:
> On Wed, Jul 23, 2014 at 2:20 PM, Christian Sperandio <
> [email protected]> wrote:
>
>> I know the quote problem comes from the show function on String value.
>> How could I do a show for no-string values and return directly the value
>> for strings?
>>
>
> You can't ! (ok you can but you'll need to use OverlappingInstances)
>
> Anyway, using Show for this is wrong, Show is supposed to translate
> datatypes in a form you could copy in your code to get the value back, it
> isn't supposed to be for presentation. You have libraries that do pretty
> formatting better.
>
> If you absolutely want this, I would suggest writing another class for it
> and use OverlappingInstances to allow for one "DataLog String" instance and
> one "(Show a) => YourClass (DataLog a)" instance.
>
> Another possibility is to do a newtype for strings that have a Show
> instance that doesn't put quotes around. That might even be a better idea,
> maybe your Strings really represent variables or something in this case
> anyway ?
> --
> Jeda?
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
>
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Message: 3
Date: Wed, 23 Jul 2014 23:18:11 +0900
From: ??? <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Type error in sub-function
Message-ID:
<calmycjp6ozdgufy_w96_mj8gh3hmzyupw2g-mb+fg1_brc3...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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 <[email protected]>:
>
> On Wed, Jul 23, 2014 at 7:08 PM, ??? <[email protected]> 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
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
>
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Message: 4
Date: Wed, 23 Jul 2014 10:22:57 -0400
From: Brandon Allbery <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Type error in sub-function
Message-ID:
<CAKFCL4W3XeMai=q-gdcrg3pzkoolxdqafpdz9sjldgjvpuk...@mail.gmail.com>
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On Wed, Jul 23, 2014 at 10:18 AM, ??? <[email protected]> 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
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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