- Haskell-Cafe - Haskell.org

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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alex documentation
by j.romildo＠gmail.com 19 Mar '09

19 Mar '09

Hello. I need the documentation of alex (a lexical analyser generator in Haskell) in PDF format, so I used the Makefile in the doc directory of the distribution of alex to get it. But unfortunatly, the documentation includes some code listings that are too wide and does not fit in the default page width used by the tools generating the PDF file. So, it would be nice if someone could rearrange the listings int the documentation source, so that it fits better in a printed page. José Romildo PS: I am going to use alex in the Compiler Construction classes I am teaching.

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Re[2]: [Haskell] ANNOUNCE: jhc 0.6.0 Haskell Compiler
by Bulat Ziganshin 19 Mar '09

19 Mar '09

Hello sylvain, Thursday, March 19, 2009, 2:16:06 AM, you wrote: > Something is wrong: how can Haskell be faster than C? 1. you measure efficiency of libs, not compilers 2. don't forget about -O2/-O3 -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Announce: language-python
by Bernie Pope 19 Mar '09

19 Mar '09

Language-python provides a parser (and lexer) for Python written in Haskell. Currently it only supports version 3 of Python (the most recent version), but it will support version 2 in the future. The parser is implemented using Happy, and the lexer is implemented using Alex. Web page: http://projects.haskell.org/language-python/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/language-python Darcs: darcs get http://code.haskell.org/language-python/ Example: $ ghci Prelude> :m Language.Python.Version3.Parser Prelude Language.Python.Version3.Parser> parseModule "def inc(x): return x + 1\n" [] Right (Module [Fun {fun_name = Ident "inc", fun_args = [Param {param_name = Ident "x", param_annotation = Nothing, param_default = Nothing}], fun_result_annotation = Nothing, fun_body = [Return {return_expr = Just (BinaryOp {operator = Plus, left_op_arg = Var (Ident "x"), right_op_arg = Int 1})}]}]) Prelude Language.Python.Version3.Parser> :m Language.Python.Version3.Lexer Prelude Language.Python.Version3.Lexer> lexOneToken "def inc(x): return x + 1\n" [] Right (Def (Sloc {sloc_filename = "", sloc_row = 1, sloc_column = 1})," inc(x): return x + 1\n") This is the first release of the package (version 0.1.1) and there is still much to be improved, in particular: - Unicode support is incomplete. - Source locations are sub-optimal, and will eventually become source spans. - The pretty printer needs polish. - The parser only supports version 3 of Python. Support for Version 2 is a major goal, and should be straightforward. - Only minimal performance tuning and correctness testing has been performed. Version 0.1.1 is not intended for production use. Cheers, Bernie.

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What *not* to use Haskell for
by Dave Tapley 19 Mar '09

19 Mar '09

Hi everyone So I should clarify I'm not a troll and do "see the Haskell light". But one thing I can never answer when preaching to others is "what does Haskell not do well?" Usually I'll avoid then question and explain that it is a 'complete' language and we do have more than enough libraries to make it useful and productive. But I'd be keen to know if people have any anecdotes, ideally ones which can subsequently be twisted into an argument for Haskell ;) Cheers, Dave

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