July 2011 - Beginners

Re: [Haskell-beginners] Can i define a record without defining access method.
by yi huang 09 Jul '11

09 Jul '11

On Sat, Jul 9, 2011 at 6:45 PM, Tom Murphy <amindfv(a)gmail.com> wrote: > On 7/9/11, yi huang <yi.codeplayer(a)gmail.com> wrote: > > I'm trying to create a haskell implementation of json rpc, I try to > define > > protocol using record like this: > > > > data Request = Request { > > version :: Integer > > , id :: Integer > > , method :: String > > , args :: [Value] > > } deriving (Typeable, Data, Show) > > > > data Response = Response { > > version :: Integer > > , id :: Integer > > , code :: Integer > > , method :: String > > , result :: Value > > } deriving (Typeable, Data, Show) > > > > so i can use json library to encode/decode it. > > But this code fails, because haskell will define access function > > automaticlly, and function names conflicts. > > My question is, is there a way i can define record without access > function, > > so i can have same attribute name in multiple record. > > > If you don't want access functions defined, you can simply not name > >> your record fields: > >> > data Request = Request Integer Integer String [Value] > >> deriving (Typeable, Data, Show) > >> > > If you want it to be more readable, you can define type synonyms: > >> > type ID = Integer > >> type Version = Integer > >> [...] > >> > data Request = Request Version ID Method Args > >> deriving (Typeable, Data, Show) > >> > The two "instances" of ID won't conflict, then. > I have to define it as a record, so aeson can inspect the attribute names and encode it to a json object automatically, for example: "{\"args\":[],\"id\":1,\"method\":\"test\",\"version\":1}" But normal data constructor would be encoded to an array. Though i can define it as a normal data constructor, and implement ToJSON class manually, but the code would be more verbose. > > > Tom > -- http://www.yi-programmer.com/blog/

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Fwd: Can i define a record without defining access method.
by David McBride 09 Jul '11

09 Jul '11

I've never used the main aeson library, but I just tried the aeson-qq library and got this without any real problem. Try this: {-# LANGUAGE QuasiQuotes, TemplateHaskell, DeriveDataTypeable #-} import Data.Text import Data.Aeson import Data.Aeson.QQ import Data.Aeson.Generic import Data.Aeson.Types myReq = [aesonQQ| {age: <|age|>, version: <|version|>} |] where age = 34 :: Integer version = "1.2" myResp = [aesonQQ| {age: <|name|>, version: <|version|>} |] where name = "harold" version = "1.3" >encode $ Data.Aeson.Generic.toJSON myReq Chunk "{\"age\":34,\"version\":\"1.2\"}" Empty >encode $ Data.Aeson.Generic.toJSON myResp Chunk "{\"age\":\"harold\",\"version\":\"1.3\"}" Empty On Sat, Jul 9, 2011 at 12:41 PM, yi huang <yi.codeplayer(a)gmail.com> wrote: > On Sun, Jul 10, 2011 at 12:25 AM, Chaddaï Fouché <chaddai.fouche(a)gmail.com> > wrote: >> >> On Sat, Jul 9, 2011 at 5:59 PM, yi huang <yi.codeplayer(a)gmail.com> wrote: >> > On Sat, Jul 9, 2011 at 6:45 PM, Tom Murphy <amindfv(a)gmail.com> wrote: >> >> >> >> On 7/9/11, yi huang <yi.codeplayer(a)gmail.com> wrote: >> >> > I'm trying to create a haskell implementation of json rpc, I try to >> >> > define >> >> > protocol using record like this: >> >> > >> >> > data Request = Request { >> >> > version :: Integer >> >> > , id :: Integer >> >> > , method :: String >> >> > , args :: [Value] >> >> > } deriving (Typeable, Data, Show) >> >> >> >> > so i can use json library to encode/decode it. >> >> > But this code fails, because haskell will define access function >> >> > automaticlly, and function names conflicts. >> >> > My question is, is there a way i can define record without access >> >> > function, >> >> > so i can have same attribute name in multiple record. >> >> >> >> If you really want to keep those attribute names exactly, you'll have >> to check where the conflict is and find a way not to import the >> conflicting function name. Here I guess "id" is the big problem since >> it comes with the prelude you'll have to use the NoImplicitPrelude >> extension and explicitly "import Prelude hiding (id)", I suggest you >> make a separate module for your type definition so you don't have to >> worry about that in the rest of your code. > > Sorry i don't describe my problem well, actually i have two records, Request > and Response, some attributes have same names, e.g. version, id. > data Request = Request { > version :: Int > ... > } > data Response = Response { > version :: Int > ... > } > And yes, i really want to keep those names exactly, so aeson can > automatically encode them to right json object. > If there are no way to hide access function, then i guess i have to define > them in seperate module, or define them as normal data constructor with > encode/decode precedure defined manually. Both is not pleasant to me. >> >> -- >> Jedaï > > > > -- > http://www.yi-programmer.com/blog/ > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > >

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iterating over a range of dates
by Rolf Hanson 09 Jul '11

09 Jul '11

Hi, I'm trying to iterate over a range of dates and print them out. Here's how I am doing it in Ruby: 'require 'date' now = Date.parse('2011-07-11') end_date = Date.parse('2011-12-31') (now..end_date).each do |d| # print out a date that looks like: # Monday, July 11, 2011 puts "#{d.strftime('%A, %B %d, %Y')}" end I've been looking at the docs for Data.Time.Format and Data.Time.Calendar and am a bit puzzled at where to begin. Anyone have ideas? I've not found many (any?) examples of time and date manipulation in Haskell. RW

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Can i define a record without defining access method.
by yi huang 09 Jul '11

09 Jul '11

I'm trying to create a haskell implementation of json rpc, I try to define protocol using record like this: data Request = Request { version :: Integer , id :: Integer , method :: String , args :: [Value] } deriving (Typeable, Data, Show) data Response = Response { version :: Integer , id :: Integer , code :: Integer , method :: String , result :: Value } deriving (Typeable, Data, Show) so i can use json library to encode/decode it. But this code fails, because haskell will define access function automaticlly, and function names conflicts. My question is, is there a way i can define record without access function, so i can have same attribute name in multiple record. -- http://www.yi-programmer.com/blog/

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GLfloat cast... have I got it right?
by Sean Charles 08 Jul '11

08 Jul '11

As I chunder on through OpenGL-land with Haskell I find things that sometimes confuse me! Is there some kind of assumption being made here about the remaining two zeros? color $ Color3 (0::GLfloat) 0 0 I can see that is is typing the first zero to GLfloat but why don't I need to do it to the remaining two zeros? Color3 is "Color3 !a !a !a" The !a is, IIUIC, a strictness instruction that ensures that whatever expression I put here is evaluated immediately i.e. no thunk is generated and presumably no space leaks either: something rendering at 60 fps with 'interesting' calculations for RGB values for example could cripple the application! But back to the syntax: I am guessing (and hoping I've got it right for the *right* reasons) that it works because the definition says "a" for all three and that explicitly typing the first one automatically tells (infers!) the type inference system that the other two are to be treated as GLfloat types too. :)

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undefined symbol not allowed?
by Christopher Howard 08 Jul '11

08 Jul '11

I'm one of those evil people who loves to sprinkle Unicode throughout my code. I thought it was pretty cool when I learned about the UnicodeSyntax language extension, as well as the base-unicode-symbols package. However, I am a little perplexed that this symbol is not provided by either of those: ⊥ That is, the "bottom" symbol, Unicode #22A5 (\8869), which I have seen in some programming texts as representing the undefined expression. I tried just defining ⊥ = undefined But ghc won't parse that. Why is this? And is there a way to make it work? -- frigidcode.com theologia.indicium.us

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Does one need a ticket to attend ICFP?
by Sebastien Zany 07 Jul '11

07 Jul '11

There's a "Registration" item on the left menu at http://www.icfpconference.org/icfp2011/ but it doesn't link to anywhere. Can I just show up? Sebastien

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Throughly confused by this error message
by Michael Litchard 06 Jul '11

06 Jul '11

I'd like someone to help me learn what questions to ask about this error. I'm not sure how to contextualize. I will include code segments that I think are important, and trust your questions will reveal what I am leaving out. HtmlParsing.lhs:81:25: The first argument of ($) takes one argument, but its type `URLString' has none In the second argument of `($)', namely `initial $ method_GET' In a stmt of a 'do' expression: curlResp curl $ initial $ method_GET In the expression: do { let initial = urlInitial urlSequence login = urlLogin urlSequence ....; curlResp curl $ initial $ method_GET; curlResp curl $ urlLogin urlSequence $ loginOpts user pass; curlResp curl $ urlFlash1 urlSequence resourceOpts; .... } > generateResourceHtml :: Curl -> String -> String -> FilePath -> IO (Either String String) > generateResourceHtml curl user pass ipFile = do > urlSequence <- popURLrec ipFile > let makeIDPage = do > let initial = urlInitial urlSequence > login = urlLogin urlSequence > flash1 = urlFlash1 urlSequence > flash2 = urlFlash2 urlSequence > showWebForwards = urlShowWebForwards urlSequence > quickCreate = urlQuickCreate urlSequence > getResource = urlGetResource urlSequence > curlResp curl $ initial $ method_GET > curlResp curl $ urlLogin urlSequence $ loginOpts user pass > curlResp curl $ urlFlash1 urlSequence resourceOpts > curlResp curl $ urlFlash2 urlSequence resourceOpts > curlResp curl $ urlShowWebForwards urlSequence resourceOpts > curlResp curl $ urlQuickCreate urlSequence resourceOpts > curlResp curl $ urlGetResource urlSequence resourceOpts > runErrorT makeIDPage > data URLSequence = URLSequence { urlInitial :: URLString > , urlLogin :: URLString > , urlFlash1 :: URLString > , urlFlash2 :: URLString > , urlShowWebForwards :: URLString > , urlQuickCreate :: URLString > , urlGetResource :: URLString > } deriving Show > curlResp :: (Error e, MonadError e m, MonadIO m) => > Curl -> URLString -> [CurlOption] -> m String --CurlResponse > curlResp curl url opts = do > resp <- liftIO $ (do_curl_ curl url opts :: IO CurlResponse) > let code = respCurlCode resp > status = respStatus resp > if code /= CurlOK || status /= 200 > then throwError $ strMsg $ "Error: " ++ show code ++ " -- " ++ show status > else return $ respBody resp

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I get no speedup with RWH examples of parallel sorting
by Kees Bleijenberg 06 Jul '11

06 Jul '11

I've carefully followed the instructions in Real World Haskell for parallel sorting: In SortMain.hs 1. Set testfunction=sort Then del *.o (force recompilation) ghc --make -O2 sortMain.hs run SortMain and I get 500000 numbers => 3.9735 sec. 2. Set testFunction = parSort2 2 del *.o ghc -threaded --make -O2 sortMain.hs run SortMain +RTS -N2 -RTS, I get => 5.3 sec run SortMain +RTS -N1 -RTS, I get => 4.3 sec With the +RTS -s option I get 3 SPARK3 (3 converted, 0 pruned) All other testFunctions are worse. I have a dual core computer that runs Windows XP. I've experimented a lot with par, pseq ... in other programs. I almost always get a few hundred sparks. Half of them are converted and the other half gets pruned. But I never got a faster program (then sequential). What am I doing wrong, any ideas? Kees

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Understandig types
by moowoo9＠fastmail.fm 05 Jul '11

05 Jul '11

Hi everyone. I've got a few questions about the Haskell programming language. 1) what's the difference between these two code statements? I'm convinced they should be the same type T = [Char] type CurrentValue = Char My concern is that in the second one there are no brackets Anyway they are actually declarations arent'they? 2) What is Maybe String? For instance: type P = (Char, Maybe String) Is that a function which has two arguments ? 3) What is Maybe Char ? For instance : type D = ((Maybe Char) , Char) It's another function having three arguments..am I right? Thanks a lot. 777P -- moowoo9(a)fastmail.fm -- http://www.fastmail.fm - The professional email service

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