September 2010 - Haskell-Cafe

Do expression definition
by Alexander Kotelnikov 20 Sep '10

20 Sep '10

Hello. http://www.haskell.org/onlinereport/exps.html#sect3.14 a obscure (to me) note which says "As indicated by the translation of do, variables bound by let have fully polymorphic types while those defined by <- are lambda bound and are thus monomorphic." What actually does it mean? And, also, would it make any difference if do {p <- e; stmts} = let ok p = do {stmts} ok _ = fail "..." in e >>= ok is redefined as "e >>= (\p -> do {stmts})"? Thanks, Alexander

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OpenGL Speed!
by Eitan Goldshtrom 20 Sep '10

20 Sep '10

I'm having an unusual problem with OpenGL. To be honest I probably shouldn't be using OpenGL for this, as I'm just doing 2D and only drawing Points, but I don't know about any other display packages, so I'm making due. If this is a problem because of OpenGL however, then I'll have to learn another package. The problem is speed. I have a list of points representing the color of 800x600 pixels. All I'm trying to do is display the pixels on the screen. I use the following: renderPrimitive Points $ mapM_ display list flush where display [] = return () display ((x,y,i):n) = do color $ Color3 i i i vertex $ Vertex2 x y display n But, for some reason this takes FOREVER. I don't know how to use debugging hooks yet without an IDE -- and I don't use an IDE -- but I used a cleverly placed putStrLn to see that it was actually working, just really really slowly. Is there a solution to this speed problem or should I use a package that's more suited to 2D applications like this? Also, if I should use another package, are there any suggestions for which to use? Thanks for any help. -Eitan

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ANNOUNCE: text 0.9.0.0 and text-icu 0.4.0.0
by Bryan O'Sullivan 20 Sep '10

20 Sep '10

I've issued new releases of the text and text-icu packages, the fast, comprehensive Unicode text manipulation libraries. http://hackage.haskell.org/package/text http://hackage.haskell.org/package/text-icu Features of text: - Compact array-based data representation. - Library code based on stream fusion automatically reduces copying in your application, compared to a more naive library. - The lazy text type supports streaming of data much larger than RAM. Features of text-icu: - - Locale sensitive case mapping. - [NEW] Language sensitive text boundary analysis. - Text normalization. (What is normalization? http://unicode.org/faq/normalization.html) - [NEW] Access to the Unicode Character Database. - Locale sensitive string collation. - Conversion to and from a huge number of native encodings. - Efficient comparison of ByteString and Text.

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example in "All about Monads"
by ender 20 Sep '10

20 Sep '10

Hi all: I am a newbie of haskell and I'm reading "All about Monads<http://www.haskell.org/all_about_monads/html/>" right now, I have some question about the example shows in Chapter 2, it says that following code is ugly mothersPaternalGrandfather :: Sheep -> Maybe Sheep mothersPaternalGrandfather s = case (mother s) of Nothing -> Nothing Just m -> case (father m) of Nothing -> Nothing Just gf -> father gf and after intruduce the comb function, it become more cleaner: -- comb is a combinator for sequencing operations that return Maybe comb :: Maybe a -> (a -> Maybe b) -> Maybe b comb Nothing _ = Nothing comb (Just x) f = f x -- now we can use `comb` to build complicated sequences mothersPaternalGrandfather :: Sheep -> Maybe Sheep mothersPaternalGrandfather s = (Just s) `comb` mother `comb` father `comb` father my question is, why not define the function father and mother as type of father::Maybe Sheep -> Maybe Sheep? this can also clean the code and it avoid the additional function comb further more, why we need a function => with type of => m a -> ( a -> m b ) -> mb? define some function with type m a -> m b can solve these problems too. Thanks and BR

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Unified Haskell login
by Michael Snoyman 20 Sep '10

20 Sep '10

Hi cafe, Let me preface this by stating that this is purposely a half-baked idea, a straw man if you will. I'd like to hear what the community thinks about this. I mentioned yesterday that I was planning on building haskellers.com. The first technicality I considered was how login should work. There are a few basic ideas: * Username/password on the site. But who wants to deal with *another* password? * OpenID. Fixes the extra password problem, but doesn't give us any extra information about the user (email address, etc). * Facebook/Twitter/Google: We get the users email address, but do we *really* want to force users to have one of those accounts? I then started thinking about the Yesod documentation site[1], and realized in the not-too-distant future I'm going to want to provide a feature tracker. Once again, I'll need to face the exact same problem. And then I realized something: I already have *two* Haskell-centric logins: one for Hackage, and one for the Haskell wiki. Consolidating our logins as a community could be a huge plus. If we keep the same kind of system as we have now with Hackage and the wiki, we can verify each new user to keep things "clean". Or even better: we could have a built-in permissions system: permissions for uploading to Hackage, modifying the wiki, feature requests, etc. Users get simplification of only needing to apply for an account once and only need to remember one password. (In fact, if we wanted to, we could bypass the password some of the time by allowing OpenID authentication.) But perhaps the biggest advantage would be the community building advantage. Imagine if you go to Hackage and the upload by field is a link to someone's Haskellers profile. Imagine going to Haskellers and seeing a list of all the users uploaded packages and wiki contributions. We could even start with some clever things like badges per user. I'm sure there are lots of possibilities out there I haven't considered. Obviously there are some technical hurdles to overcome. We would probably need to do some significant work on the wiki to get this to happen. But given that we seem to have had trouble with mediawiki in the past (I remember hearing about some migration issues), maybe it's time to eat our own dog food and switch to a Haskell-based wiki[2] that could be more easily modified to suit our needs. We would also need some kind of protocol for the cross-site authentication; OAuth 2.0 might be worth considering for this. All of this may just be the ramblings of a mad-man (I haven't had breakfast yet), but I do think that *some* form of unified login could really push Haskell forward. Michael [1] http://docs.yesodweb.com/ [2] http://hackage.haskell.org/package/gitit

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Haskell/JDK/tail-calls etc. (please vote on bug No. 6804517)
by Karel Gardas 20 Sep '10

20 Sep '10

Hello, from time to time request for Haskell running on top of Java's VM pops on the haskell related mailing list and then usually dies off when someone mentions that JDK does not have proper support for tail-calls. I think haskell community might do something with this fact or at least attempt to do something with it. I think the proper request for enhancement is here: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6804517 if you agree with it (as it's proposed on the rfe's mentioned blog article[1]), then please either join or if you are already a member of SDN then just login and vote for this bug. That's something we can do for free and will take just few minutes yet perhaps push a little bit more weight behind this bug (err, just 4 votes now, why's that?). Thanks, Karel [1]: http://blogs.sun.com/jrose/entry/tail_calls_in_the_vm

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"Introducing The Monads" Presentation Slides
by aditya siram 20 Sep '10

20 Sep '10

Hi folks, I just did a presentation on monads and how to use the basic ones (IO,Reader,Writer,State,ReaderT,WriterT,StateT) in practice. I thought the community might want to look at it. Is there some place I can upload the slides (it's just PDF)? -deech

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Memoization/call-by-need
by Alex Rozenshteyn 20 Sep '10

20 Sep '10

I feel that there is something that I don't understand completely: I have been told that Haskell does not memoize function call, e.g. > slowFib 50 will run just as slowly each time it is called. However, I have read that Haskell has call-by-need semantics, which were described as "lazy evaluation with memoization" I understand that > fib50 = slowFib 50 will take a while to run the first time but be instant each subsequent call; does this count as memoization? (I'm trying to understand "Purely Functional Data Structures", hence this question) -- Alex R

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searching haskell-cafe ?
by briand＠aracnet.com 19 Sep '10

19 Sep '10

is there a way to search the haskell-cafe list. I found a potential link on the haskell wiki, but the link is busted. Didn't see anything on the haskell-cafe mailing list page. Thanks, Brian

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cabal problem on OS X
by Bill Atkins 19 Sep '10

19 Sep '10

When I run "cabal update" on my Mac (Snow Leopard, Intel), I get: % cabal update Downloading the latest package list from hackage.haskell.org cabal: Codec.Compression.Zlib: incompatible zlib version Anyone else seeing this? Reinstalling the Haskell Platform hasn't helped. Thanks!

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