- Haskell-Cafe - Haskell.org

Proving stuff about IORefs
by Ben Franksen 18 Oct '10

18 Oct '10

I have a formal proof where I am stuck at a certain point. Suppose we have a function f :: IORef a -> IO b I want to prove that f r == do s1 <- readIORef r r' <- newIORef s1 x <- f r' s3 <- readIORef r' writeIORef r s3 return x What happens here is that the temporary IORef r' takes the place of the argument r, and after we apply f to it we take its content and store it in the original r. This should be the same as using r as argument to f in the first place. How can I prove this formally? Cheers Ben

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Text: find
by Antoine Latter 17 Oct '10

17 Oct '10

On Sun, Oct 17, 2010 at 3:25 PM, Bryan O'Sullivan <bos(a)serpentine.com> wrote: > On Mon, Oct 11, 2010 at 1:12 PM, Malcolm Wallace <malcolm.wallace(a)me.com> > wrote: >> >> find find find (Text -> Bool) -> Text -> Maybe Text >> >> Proposal: rename Text.findBy to Text.find. > > I agree with the change of type, not so much with the naming. That's roughly > my stance in the other cases, too. > What would the definition of a function of the form: > find :: (Text -> Bool) -> Text -> Maybe Text look like? What sort of requirements would be placed on the input predicate? Maybe I have a failure of imagination, but that sounds like a tricky thing to get correct for all predicates. Antoine

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Haskell Front Page Ideas
by caseyh＠istar.ca 17 Oct '10

17 Oct '10

How about a bullet list of Haskell's features (maybe pros & cons) might be better; followed by a list of a few other languages and their trade-offs. As Jon Bentley has said, "design" is usually trade-offs but when you can come up with something that is a mutual benefit; so much the better. :)

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Strict Core?
by Gregory Crosswhite 17 Oct '10

17 Oct '10

Hey everyone, Out of curiosity, are there any plans for GHC to eventually use the Strict Core language described in http://www.cl.cam.ac.uk/~mb566/papers/tacc-hs09.pdf? Cheers, Greg

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Heterogenous network stream
by Antoine Latter 17 Oct '10

17 Oct '10

Forwarding to list. ---------- Forwarded message ---------- From: Antoine Latter <aslatter(a)gmail.com> Date: Sun, Oct 17, 2010 at 10:48 AM Subject: Re: [Haskell-cafe] Heterogenous network stream To: Charles-Pierre Astolfi <cpa(a)crans.org> On Sun, Oct 17, 2010 at 7:31 AM, Charles-Pierre Astolfi <cpa(a)crans.org> wrote: > Hi cafe, > > I'd like to implement a toy-protocol and I'm looking for the sanest > way (for me) to do so. > > Here's what it looks like: > The server receives, in that order: > - Ascii string (4 bytes) > - 16 bits int (2 bytes) > - 8 bits int (1 byte) > - UTF8 string (128 bytes) > > I tried to use hGetBuf, but I don't know if it's the way to go and I > did not understood what to do with the buf::Ptr a argument. I'm not > very concerned about efficiency. > > I don't know how I should convert my 16 and 8 bits int (read as Chars, > I guess) to an Int. Is there something for that in the standard > library? > I recommend reading the data in as one bytestring[1] (since you have a fixed length) and then parsing it using one of the many nice bytestring parsing libraries: * binary[2] * cereal[3] Others may have other suggestions. For the UTF8 parsing, I think the text[4] package has support for decoding UTF8 from a bytestring. Antoine [1] http://hackage.haskell.org/packages/archive/bytestring/0.9.1.7/doc/html/Dat… [2] http://hackage.haskell.org/package/binary [3] http://hackage.haskell.org/package/cereal [4] http://hackage.haskell.org/package/text

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Re: [Haskell-cafe] UTF-8 problems when decoding JSON data coming from Network.HTTP
by Ionut G. Stan 17 Oct '10

17 Oct '10

On 17/Oct/10 8:02 AM, Michael Snoyman wrote: > In the gist you sent, the problem is that you are reading the HTTP > response as a String. The HTTP library doesn't deal well with > non-Latin characters when doing String requests; you should be using > ByteString and then converting. It's a little tedious using the HTTP > library with ByteStrings, which is one of the reasons I wrote > http-enumerator. Here's some working code. The main point is to > convert the UTF8 octets to a String. > > You could also consider using one of the JSON libraries that support > bytestrings directly instead of strings, which will likely result in > much better performance. Contenders include JSONb[1] and > yajl-enumerator[2]. > > import Network.HTTP.Enumerator > import qualified Text.JSON as JSON > import qualified Data.ByteString.Lazy.UTF8 as BSLU > > data GithubUser = GithubUser { > name :: String, > location :: String > } deriving (Eq, Show) > > > instance JSON.JSON GithubUser where > readJSON (JSON.JSObject object) = > let (Just a) = lookupM "user" $ JSON.fromJSObject object > (JSON.JSObject b) = a > user = JSON.fromJSObject b > in do name<- lookupM "name" user>>= JSON.readJSON > location<- lookupM "location" user>>= JSON.readJSON > return $ GithubUser { > name = name, > location = location > } > > showJSON user = JSON.makeObj [ > ("name", JSON.showJSON $ name user), > ("location", JSON.showJSON $ location user) > ] > > > lookupM :: (Monad m) => String -> [(String, a)] -> m a > lookupM x xs = maybe (fail $ "No such element: " ++ x) return (lookup x xs) > > main = do jsonLbs<- simpleHttp "http://github.com/api/v2/json/user/show/igstan" > let jsonText = BSLU.toString jsonLbs > let result = JSON.decode jsonText :: JSON.Result GithubUser > showResult result > where showResult (JSON.Ok json) = putStrLn $ name json > showResult (JSON.Error e) = putStrLn e > > Michael > > [1] http://hackage.haskell.org/package/JSONb-1.0.2 > [2] http://hackage.haskell.org/package/yajl-enumerator Thanks Michael, now it works indeed. But I don't understand, is there any inherent problem with Haskell's built-in String? Should one choose ByteString when dealing with Unicode stuff? Or, is there any resource that describes in one place all the problems Haskell has with Unicode? -- Ionuț G. Stan | http://igstan.ro

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Heterogenous network stream
by Charles-Pierre Astolfi 17 Oct '10

17 Oct '10

Hi cafe, I'd like to implement a toy-protocol and I'm looking for the sanest way (for me) to do so. Here's what it looks like: The server receives, in that order: - Ascii string (4 bytes) - 16 bits int (2 bytes) - 8 bits int (1 byte) - UTF8 string (128 bytes) I tried to use hGetBuf, but I don't know if it's the way to go and I did not understood what to do with the buf::Ptr a argument. I'm not very concerned about efficiency. I don't know how I should convert my 16 and 8 bits int (read as Chars, I guess) to an Int. Is there something for that in the standard library? As I also write the client, I'm looking for a conversion in both ways. Thanks, -- Charles-Pierre

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Opening a gtk2hs link in default browser
by Dmitry V'yal 17 Oct '10

17 Oct '10

Hello list, I'm trying to make a clickable link in gtk2hs program. Clicking it should open an url in the default browser. Can I hope to find some portable solution in gtk2hs (it should work in linux and windows) or should I start searching platform specific ones, like /usr/bin/xdg-open or something? Can any examples be found in existing haskell software? Dmitry

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Systematic treatment of static arguments
by Stephen Tetley 17 Oct '10

17 Oct '10

Hello list The Monad and Applicative instances for functions are "equivalent" to the respective Reader vesions (I use "equivalent" along the lines of - operationally the same but without the type distinction / newtype). There is also the Monoid instance for functions which is pretty slick. Has anyone looked at variations with two or more static arguments though? For example mappend with two static arguments needs to be defined either as a new function: > mappendR2 :: Monoid a => (r1 -> r2 -> a) -> (r1 -> r2 -> a) -> r1 -> r2 -> a > mappendR2 f g = \x y -> f x y `mappend` g x y or an overlapping instance: > instance Monoid a => OPlus (r1 -> r2 -> a) where > f `mappend` g = \x y -> f x y `mappend` g x y Working in graphics I've found two static arguments comes up quite often - preliminarily most of my functions are functions from the DrawingContext to something (drawing context is an environment that tracks line width, stroke colour, fill colour, etc.): fn1 :: DrawingCtx -> a Many of my functions statically use a 'start' point as the only coordinate reference, so they are in a "coordinate free" style: fn2 :: Point -> DrawingCtx -> a Some functions even have have a third static argument, for example drawing arrowheads for lines seems nice if the angle of the line is parametric and not used explicitly: fn3 :: Radian -> Point -> DrawingCtx -> a The help from Applicative, Monad and Monoid is "used up" by the one static argument version, so I find that I have to introduce points (in the pointed / point-free sense not in the graphic sense) in the fn2 and fn3 versions where a point-free version might still be nice. Is there any prior work looking at sets of combinators for these higher arity functions - papers or code? I'd prefer not to introduce a whole new lexicon of combinator names if I can help it. Thanks Stephen

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Text.Regex Segfault
by Duane Johnson 17 Oct '10

17 Oct '10

I bumped into a segmentation fault in the regex library today and thought I'd warn others in case similar behavior is observed: Prelude> :m Text.Regex > Prelude Text.Regex> map read (splitRegex (mkRegex "\\|") "0|1|2|4") :: > [Int] > Loading package syb ... linking ... done. > Loading package array-0.2.0.0 ... linking ... done. > Loading package bytestring-0.9.1.4 ... linking ... done. > Loading package regex-base-0.72.0.2 ... linking ... done. > Loading package regex-posix-0.72.0.3 ... linking ... done. > Loading package regex-compat-0.71.0.1 ... linking ... done. > [0,1,2,4] > Prelude Text.Regex> Segmentation fault > Regards, Duane

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