March 2009 - Haskell-Cafe

Why do functions not inherit class when renamed?
by Duane Johnson 19 Mar '09

19 Mar '09

I'm not sure I understand what is going on here, but it seems to me that the following assignment of "mult = (*)" causes "mult" to lose out on some of the polymorphic behavior of (*). Is that right? If so, why? > Prelude Data.Function Data.List Data.Char> let sizeMult = (*) `on` > length > Prelude Data.Function Data.List Data.Char> sizeMult "Duane" "Johnson" > 35 > Prelude Data.Function Data.List Data.Char> let mult = (*) > Prelude Data.Function Data.List Data.Char> let sizeMult2 = mult `on` > length > > <interactive>:1:26: > Couldn't match expected type `Integer' against inferred type `Int' > In the second argument of `on', namely `length' > In the expression: mult `on` length > In the definition of `sizeMult2': sizeMult2 = mult `on` length > Prelude Data.Function Data.List Data.Char> Thanks, Duane Johnson http://blog.inquirylabs.com/

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curl w/ ghc-win32
by Sigbjorn Finne 19 Mar '09

19 Mar '09

Hi, there's a steady stream of people reporting that getting started with curl on Windows is cumbersome and tricky to get right. A legitimate complaint! :-) To hopefully help out a bit in that regard, I've put together some notes covering the steps reqd + made available a pre-packaged curl binary dist -- http://haskell.forkio.com/Home/curl-win32 enjoy.. :) --sigbjorn

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[ANN] random-stream package
by Manlio Perillo 19 Mar '09

19 Mar '09

Hi. I'm pleased to announce the availability of my random-stream package. The cabalized package is available at: http://haskell.mperillo.ath.cx/random-stream-0.0.1.tar.gz Note that I have not uploaded it on Hackage, and I do not plan to upload it in the near future, at least until I will repute the package mature enough. From package description: Portable interface for the operating system source of pseudo random data. Supported sources are Unix /dev/urandom, Win32 CryptGenRandom and OpenSSL pseudo random numbers generator. This package is based on idea from os.urandom implementation, in CPython. The idea is to view the system pseudo random generator as a stream of infinite random bytes. So, I have used a lazy bytestring, and unsafePerformIO/unsafeInterleaveIO. The underlying data is: newtype Stream = Stream L.ByteString and the constructor *is exposed*. The package configuration *must* be done using a flag to specify the "source" to use. As an example: runghc Setup.hs configure -O2 -fHAVE_URANDOM runghc Setup.hs configure -O2 -fHAVE_SSL runghc Setup.hs configure -O2 -fHAVE_WIN32_CRYPT If the flag is not specified, the compilation will fail. Note that Windows it not yet supported. The stream generator implements tha RandomGen interface. Some notes: 1) The system *pseudo* random generator is used. As an example, on Linux, /dev/random produces "true" random numbers, but it may block if there is not enough entropy in the system. More details on http://en.wikipedia.org/wiki//dev/random 2) When reading data, the lazy bytestring chunk size is used. The default chunk size, however, is fine when reading from a regular file, but may not be the best choice when reading pseudo random numbers. For SSL (and Windows) support, the chunk size can be easily changed; but for Unix support this seems to not be possible, unless I reimplement hGetContents. The Data.ByteString.Lazy module have an hGetContentsN function. Unfortunately it is not exposed. I find this annoying; is it possible to export the *N functions from Data.ByteString.Lazy.Internal? 3) I have tested the package on Debian Linux Etch, using GHC 6.8.2. Feedback will be appreciate. I not sure about some details. Usage example: module Main where import System.Random import System.Random.Stream gen = mkStream l :: [Int] l = randoms gen :: [Int] main = do print l Manlio Perillo

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Hoogling to no avail ...
by Galchin, Vasili 19 Mar '09

19 Mar '09

Hello, I need sleep briefly(1/2 seconds) in code that I have written. I tried searching for "sleep" and "wait" in Hoogle and either didn't get a hit or got something inappropriate. ?? Regards, Vasili

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Summer Of Code 2009 project idea
by Csaba Hruska 19 Mar '09

19 Mar '09

Hi! I'd like to participate in gsoc2009. My project idea is to complete my initial render backend ( http://www.haskell.org/haskellwiki/LambdaCubeEngine) what can be useful for (possibe more) haskell projects. I'd like to extend it to support physics (reusing hpysics source code), etc. Current features: + easy (from user view) model loading and rendering + definig materials (texture layers, blending setup, shaders, etc) via readable material scripts + fast rendering via vertex buffers Some planned features: + skeletal animation (in progress) + physics (hpysics) integration + efficient terrain rendering + some mesh modifier combinators etc.. Do you think is this useful? Or should i think out a more relevant idea? Cheers, Csaba Hruska

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Re: [Haskell-cafe] Re: Haskell Logo Voting has started!
by Wolfgang Jeltsch 19 Mar '09

19 Mar '09

Am Mittwoch, 18. März 2009 13:31 schrieben Sie: > On Wed, Mar 18, 2009 at 04:36, Wolfgang Jeltsch wrote: > > Am Mittwoch, 18. März 2009 10:03 schrieb Benjamin L.Russell: > >> Just go through the list, choose your top favorite, and assign rank 1 > >> to it; > > > > Is rank 1 the best or the worst? > > The condorcet info page makes it clear that higher is better. > > http://www.cs.cornell.edu/w8/~andru/civs/rp.html So assigning rank 1 to my favorite, as Benjamin suggested, might not be the best idea. :-( I hope, everyone who voted did it the right way. Best wishes, Wolfgang

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Proof of duality theorem of fold?
by R J 19 Mar '09

19 Mar '09

I'm trying to prove the following duality theorem of fold for finite lists: foldr f e xs = foldl (flip f) e (reverse xs) where reverse :: [a] -> [a] reverse [] = [] reverse (x:xs) = reverse xs ++ [x] flip :: (a -> b -> c) -> b -> a -> c flip f y x = f x y foldr :: (a -> b -> b) -> b -> [a] -> b foldr f e [] = e foldr f e (x : xs) = f x (foldr f e xs) foldl :: (b -> a -> b) -> b -> [a] -> b foldl f e [] = e foldl f e (x : xs) = foldl f (f e x) xs I'm stuck on the induction case. Can someone help? Here's what I've got so far: Proof: Case _|_. Left-side reduction: foldr f e _|_ = {case exhaustion of "foldr"} _|_ Right-side reduction: foldl (flip f) e (reverse _|_) = {case exhaustion of "reverse"} foldl (flip f) e _|_ = {case exhaustion of "foldl"} _|_ Case []. Left-side reduction: foldr f e [] = {first equation of "foldr"} e Right-side reduction: foldl (flip f) e (reverse []) = {first equation of "reverse"} foldl (flip f) e [] = {first equation of "foldl"} e Case (x : xs). Left-side reduction: foldr f e (x : xs) = {second equation of "foldr"} f x (foldr f e xs) = {induction hypothesis: foldr f e xs = foldl (flip f) e (reverse xs)} f x (foldl (flip f) e (reverse xs)) Right-side reduction: foldl (flip f) e (reverse (x : xs)) = {second equation of "reverse"} foldl (flip f) e (reverse xs ++ [x]) _________________________________________________________________ Hotmail® is up to 70% faster. Now good news travels really fast. http://windowslive.com/online/hotmail?ocid=TXT_TAGLM_WL_HM_70faster_032009

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Text bug in Hieroglyph 1.2
by Danny Chan 19 Mar '09

19 Mar '09

Since I haven't found out how to report bugs for this package, I am posting this here. Changing the color of Text elements in a Hieroglyph image like this: text {attributes = plain {strokeRGBA = opaque blue}} does not work correctly. Instead, the color used for the text is the one of the previously drawn element. With the little knowledge I have about cairo, it seems that the bug is in the renderPrimitive function in Cairo.hs where the order of Cairo.setSourceRGBA and Cairo.showText is wrong. Something like this seems to work: renderPrimitive _ s0 (Text str (Point ox oy) state) = do applyAttributeDelta s0 state Cairo.moveTo ox oy let (sr,sg,sb,sa) = colourToTuple . strokeRGBA $ state when (outlined state) $ Cairo.setSourceRGBA sr sg sb sa Cairo.showText str when (clipped state) $ Cairo.clip return state By the way, for the complete newbie that I am, is there a standard way to report bugs for a package from Hackage? Danny

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Proposal for associated type synonyms in Template Haskell
by Thomas van Noort 19 Mar '09

19 Mar '09

Hello, Recently, we released a library on Hackage for generic rewriting (package "rewriting" if you are curious). The user of the library is expected to define type class instances to enable rewriting on his or her own datatypes. As these instances follow the datatype declarations closely, we tried to generate the instances using Template Haskell. Unfortunately, associated type synonyms are not yet supported by TH. After a presentation at the WGP'08, Simon encouraged us to write a proposal about adding associated type synonyms to TH, so that it can be added to GHC. So, here is our proposal. The TH AST must allow 1) kind declarations of associated type synonyms in class declarations and 2) their definitions in instance declarations. For example, class Foo a where type Bar a :: * instance Foo Int where type Bar Int = String The TH library defines a datatype Dec which contains a constructor for class declarations and instance declarations: data Dec = ... | ClassD Cxt Name [Name] [FunDep] [Dec] | InstanceD Cxt Type [Dec] ... 1) Associated type synonym kind declarations We suggest to add a constructor to the Dec type: ... | AssocTySynKindD Name [Name] (Maybe Kind) ... assocTySynKindD :: Name -> [Name] -> Maybe KindQ -> DecQ The first field is the name of the associated type synonym, the second field is a list of type variables, and the third field is an optional kind. Since kinds are not yet defined in TH, we have to add some kind of kind definition (pun intended): data Kind = StarK | ArrowK Kind Kind type KindQ = Q Kind starK :: KindQ arrowK :: KindQ -> KindQ -> KindQ We explicitly choose not to reuse the Type type to define kinds (i.e., type Kind = Type as in GHC) since we think a separation between the two worlds is much clearer to the users of TH. 2) Associated type synonym definitions We suggest to add another constructor to the Dec type: ... | AssocTySynD Name [Type] Type ... assocTySynD :: Name -> [TypeQ] -> TypeQ -> DecQ The first field is the name of the type synonym, the second field is a list of type arguments, and the third field is the body of the type synonym. We would like to hear your comments to this proposal. Regards, Thomas

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doubts about runGetState in the binary package
by Manlio Perillo 19 Mar '09

19 Mar '09

Hi. I have some doubts about the runGetState function in the binary package. The signature is: runGetState :: Get a -> LBS -> Int64 -> (a, LBS, Int64) however the Int64 "input parameter" is not documented. What value should I pass? How will be used? Thanks Manlio Perillo

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