- Haskell-Cafe - Haskell.org

Re: [Haskell] Does GHC support the standard CPP functionalities?
by Malcolm Wallace 22 Nov '08

22 Nov '08

[moved to haskell-cafe] > {-# LANGUAGE CPP #-} > > module Packer where > > #define FLASH_APP_START 1 > #define FLASH_APP_END 2 > #define INSERT_SECTION(x) (#x, (FLASH_##x##_START, FLASH_##x##_END)) The CPP stringization operator # and the token-catenation operator ## are ANSI additions over the traditional K+R CPP. Ghc uses the - traditional flag to exclude them, because both # and ## are valid Haskell operators, and most people would not want them to be removed by the CPP preprocessor. There are also some other lexical differences between Haskell and C which make it a good ides for ghc to use cpp -traditional. However, if you really want to use ANSI additions, you can switch to cpphs, which does know about the lexical syntax of Haskell. http://haskell.org/cpphs To use it with ghc, you will need to pass the following extra options: ghc -cpp -pgmPcpphs -optP--cpp -optP-ansi Unfortunately, I'm not certain I can guarantee that the -ansi flag will take effect - it depends on whether ghc internally adds the - traditional argument before or after it. Regards, Malcolm

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Operator cheat sheet, and monadic style q
by Owen Smith 22 Nov '08

22 Nov '08

Greetings, I'm a longtime Haskell-curious programmer who, after a few aborted attempts at getting started and long nights staring at academic papers, finally managed to get the bug. I've been pleased with my progress so far, but a couple of things have bugged me enough to seek advice from the rest of y'all. 1. Contending with the use of frequently unfamiliar non-alphanumeric operators has been an uphill battle for me. I think the main reason for this is that I've had no luck in Googling up their definitions (my primary approach for dealing with every other unknown in the Haskell universe) due to their very non-alphanumeric nature. I'm curious if 1) anyone has compiled a cheat sheet of common operators and their meanings/modules, in some sort of text-search-friendly format (e.g. spelling out the operator characters, listing alternate names), or 2) a better way for searching for these things? 2. There's a lot I need to learn about good Haskell style, especially coming from a C++ background. Even my experience in Lisp seems to result in way more parentheses than Haskell coders are comfortable with. :-) In particular, I'm curious about how people actually use monadic operators. The following simple forms with the Maybe monad, for example, appear to be equivalent (hope I and QuickCheck are right about that!): foo :: Int -> Maybe Int bar :: Int -> Maybe Int baz :: Int -> Maybe Int baz n = (foo n) >>= bar baz n = bar =<< (foo n) baz n = (foo >=> bar) n baz n = (foo <=< bar) n and I'm thinking the latter two are more idiomatically written as: baz = foo >=> bar -- I think this one is my fave, naively speaking baz = bar <=< foo Yeah, so "there's more than one way to do it"--though I would never have known about =<<, >=>, and <=< from looking at the introductory material I've seen on the subject. What do people here prefer using in what circumstances? Or is everyone off using do notation or arrows instead? :-) Thanks for the help! -- O

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Counting beta reductions for a Haskell program...
by kk08 22 Nov '08

22 Nov '08

Does GHC supports/has a command for counting total beta reductions taken by a program? Thanks. -- View this message in context: http://www.nabble.com/Counting-beta-reductions-for-a-Haskell-program...-tp2… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell-cafe] varargs zip
by Jason Dusek 21 Nov '08

21 Nov '08

Sean Leather <sean.leather(a)gmail.com> wrote: > > example = zip [1,2::Int] ['a','b'] ["1","b"] :: > > [(Int,Char,String)] > > BTW, I realized that this example also works in yours. You > weren't giving enough type annotations. Yes, exactly. Or in other words, the class definition does not correctly specify the dependencies. That is what I am writing in about -- I don't want clients to 'zip' to be required to put all those type annotations in. -- _jsn

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Begginer question about alignment in Storable
by Maurício 21 Nov '08

21 Nov '08

Hi, I need to make the following structure into an instance of Storable: struct { double w[2]; } Can I assume that its function 'alignment' should return the same as "alignment (1::CDouble)"? Or should it be "2 * alignment (1::CDouble)"? (I've read the wikipedia article about memory alignment, and what I understand is that there's a better alignment for hardware efficiency but compilers and programmers can do almost anything about it. What can I do when writing a library wrapper if I don't know what kind of compiler pragmas were used in all platforms that library was compiled?) Thanks, Maurício

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System Fc in GHC
by Hugo Pacheco 21 Nov '08

21 Nov '08

Since System Fc is implemented in the latest releases of GHC, is it implemented in Haskell so that I can play with the translator? Can I find it in the ghc sources? Thanks, hugo -- www.di.uminho.pt/~hpacheco

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portable arrow instances
by Wolfgang Jeltsch 21 Nov '08

21 Nov '08

Hello, how do I make a library work with both GHC 6.8 and GHC 6.10? According to <http://www.haskell.org/haskellwiki/Upgrading_packages#Arrow_instances>, I should change all my Arrow instances but then they don’t work with GHC 6.8 anymore, do they? Or is the solution to force GHC 6.8 users to use base-4.0.0.0? Best wishes, Wolfgang

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Hackage policy question
by Iavor Diatchki 20 Nov '08

20 Nov '08

Hi, I just noticed that hackage has introduced a new policy to disallow changes to a package without bumping the version. I understand that this is probably a good idea for changes to the source code, but it really would be nice to have a backdoor that allows for other changes. For example, I just uploaded a package, and realized that I forgot to add a home-page entry in the cabal file. I do not plan to increase the version number of my application, only so that I can upload a new version (the source code has not changed after all!). I can imagine similar problems related to fixing typos in the description, and other fixes to the meta-data. So, could we please revert to the old policy? (if we really want to be fancy, the hackage upload script could check that the source code, and other fields, such as LICENSE have not changed, as these should really bump the version... in the mean time though, I think just being responsible members of the community would work just as well!). -Iavor

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Problem with GTK on OS X -- installed from source
by Jeff Heard 20 Nov '08

20 Nov '08

Building my app with gtk2hs, I get the following error when I try to compile a file that uses Template Haskell (I'm not using TH to do anythign with GTK, but it loads the package anyway) Loading package gtk-0.9.13 ... <command line>: can't load .so/.DLL for: gthread-2.0 (dlopen(libgthread-2.0.dylib, 10): image not found)

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Haskell Reddit
by Don Stewart 20 Nov '08

20 Nov '08

I'm not sure if the cafe@ knows this, but there's a bit of a Haskell community (1200+ subscribers) built up around the Haskell subreddit on reddit.com, http://www.reddit.com/r/haskell/ A good source for news other than what appears on Planet Haskell. -- Don P.S. Feel free to submit things you like, so I don't have to :-)

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