- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] How to write elegant Haskell programms?
by John Hughes 30 Jan '07

30 Jan '07

> >I think that whole "program flow" thing is something you get used to. In > > >>true, pure functional programming (i.e. Haskell) "program flow" is a >>meaningless term, basically. Haskell is a declarative language, not an >>imperative one. You have to learn to give up that control and trust the >>runtime to Do The Right Thing. (In this regard it's similar to logic >>programming languages.) >> >> >> > >I think it's important/useful to point out that "program flow" in a pure >functional language is really a matter of data dependency. The compiler is >only free to arbitrarily order computations if there are no data >dependencies. Furthermore, monads are not special in any way (they are after >all just a useful set of combinators; e.g. >http://sigfpe.blogspot.com/2006/08/you-could-have-invented-monads-and.html) >they only wind up sequencing computations because they set up a data >dependency between the two arguments of the bind operator. > >-Jeff > And actually they don't even do that, always. A useful example in practice: then Gen monad in QuickCheck does *not* necessarily set up any data dependencies, so do in the Gen monad does not force sequencing. The fact that it's non-strict is what enables us to generate infinite random data-structures with it. John Hughes

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GHC-Cygwin
by Daniil Elovkov 30 Jan '07

30 Jan '07

Hello Haskellers My desire is simple. I want GHC-Cygiwn. I spent quite a lot of time struggling, trying to build ghc-6.6-Cygwin with ghc-6.4.1-mingw32. After having patched a lot of #ifdefs and the like I finally did it. Threaded RTS doesn't seem to work, but that's ok. And all that only to eventually realise that it crashes compiling non-trivial programs. The message is 3 [main] ghc-6.6 3648 _cygtls::handle_exceptions: Error while dumping state (probably corrupted stack) I will probably fight it again some time, but now while I'm taking a rest, whould anybody be so kind, who has a working GHC-Cygwin, package it and put somewhere so that I can just take it. I suppose that many people will appreciate it. Thank you.

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Simple HTTP lib for Windows?
by Greg Fitzgerald 29 Jan '07

29 Jan '07

I'd like to write a very simple Haskell script that when given a URL, looks up the page, and returns a string of HTML. I don't see an HTTP library in the standard libs, and the one in Hackage requires Windows machines have GHC and MinGW to be installed and in the PATH. Is there a simple way to get the contents of a webpage using Haskell on a Windows box? Thanks, Greg

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How to write elegant Haskell programms? (long posting)
by Michael Roth 29 Jan '07

29 Jan '07

Hello list, I'm new to Haskell and I'm trying to learn how to write elegant code using Haskell. I decided to convert the following small tool, written in ruby: =========================================================== #! /usr/bin/env ruby require 'pathname' BASENAMES = %w{ mail.log thttpd.log } ARCHIVEDIR = Pathname.new '/var/log/archive' LOGDIR = Pathname.new '/var/log' class Pathname def glob glob_pattern Pathname.glob self.join(glob_pattern) end def timestamp stat.mtime.strftime '%Y%m%d' end end for basename in BASENAMES for oldname in LOGDIR.glob "#{basename}.*.gz" newname = ARCHIVEDIR.join "#{basename}.#{oldname.timestamp}.gz" puts "mv #{oldname} #{newname}" File.rename oldname, newname end end =========================================================== My solution in Haskell is: =========================================================== import System.Directory (getDirectoryContents, getModificationTime, renameFile) import System.Locale (defaultTimeLocale) import System.Time (ClockTime, toUTCTime, formatCalendarTime) import Text.Regex (mkRegex, matchRegex) import Maybe import Control.Monad logdir, archivedir :: String logfiles :: [String] logfiles = [ "mail.log", "thttpd.log" ] logdir = "/var/log" archivedir = "/var/log/archive" basename :: String -> String basename filename = head . fromMaybe [""] $ matchRegex rx filename where rx = mkRegex "^(.+)(\\.[0-9]+\\.gz)$" isLogfile :: String -> Bool isLogfile filename = basename filename `elem` logfiles timestamp :: ClockTime -> String timestamp time = formatCalendarTime defaultTimeLocale "%Y%m%d" (toUTCTime time) makeOldname :: String -> String makeOldname fn = logdir ++ '/' : fn makeNewname :: String -> String -> String makeNewname bn ts = archivedir ++ '/' : bn ++ '.' : ts ++ ".gz" move :: String -> String -> IO () move oldname newname = do putStrLn $ "mv " ++ oldname ++ ' ' : newname renameFile oldname newname main :: IO () main = do files <- liftM (filter isLogfile) (getDirectoryContents logdir) let oldnames = map makeOldname files times <- mapM getModificationTime oldnames let newnames = zipWith makeNewname (map basename files) (map timestamp times) zipWithM_ move oldnames newnames =========================================================== Ok, the tool written in Haskell works. But, to me, the source doesn't look very nice and even it is larger than the ruby solution, and more imporant, the programm flow feels (at least to me) not very clear. Are there any libraries available to make writing such tools easier? How can I made the haskell source looking more beautiful? Michael Roth

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RE: [Haskell-cafe] How did you stumble on Haskell?
by Bob Davison 29 Jan '07

29 Jan '07

>From: "Alexy Khrabrov" <deliverable(a)gmail.com> >To: haskell-cafe(a)haskell.org >Subject: [Haskell-cafe] How did you stumble on Haskell? >Date: Sun, 28 Jan 2007 19:01:57 -0800 > >How do people stumble on Haskell? -- snip >What's folks most interesting ways to arrive at FP? > >Cheers, >Alexy I have been programming other stuff for years (APL, C, Assembly, Visual Basic, Java, C#) and a year or two ago I started thinking of going back to study computing a bit more formally. Reading web pages of lecturers on courses that looked interesting I found a few whose favorite language was a functional programming language called Haskell. I didn't know anything about functional programming so I just moved on. More recently I started to work my way through 'Modern compiler implementation in Java' by Andrew Appel and he seemed keen on functional programming and the book covers extending the basic language to a functional programming language. (I have have read the relevant chapter but not got as far as implementing it yet!) Just before Christmas I decided to investigate Haskell. I got hold of a copy of Graham Hutton's new book 'Programming in Haskell', which I am thoroughly enjoying. Haskell is great fun but as I dig deeper I am finding my lack of mathematical sophistication to be a problem. This leads me off thread to ask if anyone could recommend reading for someone who has done mathematics to college level, but nearly 30 years ago when many English schools didn't cover 20th century mathematics. I thought calculus was about differentiation and integration and was very surprised to discover that there were such things as 'predicate calculus', 'propositional calculus', and various flavours of 'lambda calculus'. I also have little or no idea of set theory, group theory, domain theory, combinatory logic, ... (I can just imagine the surprised looks on the faces of the mathematicians reading this. You never know computer programmers could be so ignorant, did you?) I have no idea how much of this stuff I need to know but I would certainly like to be able to learn more of this facinating new world and not just be content with learning how to write a Haskell program. I just don't know where to start. Thanks, Bob Davison _________________________________________________________________ Turn searches into helpful donations. Make your search count. http://click4thecause.live.com/search/charity/default.aspx?source=hmemtagli…

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type and class problems compiling code on GHC 6.6
by José Miguel Vilaça 29 Jan '07

29 Jan '07

Dear Haskellers, I had used some code which worked fine on GHC 6.4 and now it dont compile on GHC 6.6. Can anyone please give me some workarounds and/or explanations about these 2 errors? instance InfoKind a b => InfoKind (Maybe a) b where blank = Nothing check n _ Nothing = ["No info value stored with "++n] check n g (Just a) = check n g a GHC complains that Illegal instance declaration for `InfoKind (Maybe a) b' (the Coverage Condition fails for one of the functional dependencies) In the instance declaration for `InfoKind (Maybe a) b' multiListViewGetTSelections :: MultiListView x () -> IO [x] multiListViewGetTSelections multiListView = do { Just ((model, _) :: (Var [x], x -> String)) <- unsafeObjectGetClientData multiListView ; -- more and more code } GHC complains that A pattern type signature cannot bind scoped type variables `x' unless the pattern has a rigid type context In the pattern: (model, _) :: (Var [x], x -> String) I would really appreciate some help. Best regards Miguel Vilaça

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c2hs failed to process __extension__
by 向井淳 29 Jan '07

29 Jan '07

Hello all, I have a trouble when compiling a program with c2hs. The program can be seen at http://www.city5.org/prog/hstraier/, which is a binding with HyperEstraier (a fulltext search system). When I compile it at FreeBSD 6.2, I encountered the following error: | % runghc Setup.hs build | Preprocessing library hstraier-0.2... | c2hs: Error in C header file. | | /usr/include/machine/endian.h:146: (column 25) [FATAL] | >>> Syntax error! | The symbol `{' does not fit here. From the dumped file Node.i (you can see from http://www.city5.org/ Node.i), the error line is as follows: | return (__extension__ ({ register __uint32_t __X = (_x); __asm ("bswap %0" : "+r" (__X)); __X; })); c2hs fails to process __extension__? From the changelog, c2hs now can understand __extension. Is this an exceptional case? Best Regards, Jun Mukai mukai(a)jmuk.org

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Re: [Haskell-cafe] Simple HTTP lib for Windows?
by brad clawsie 29 Jan '07

29 Jan '07

hi, i have popped in on this thread before to mention my own extension to Network.HTTP (http://www.b7j0c.org/content/haskell-http.html, providing get() and head()). i would like to thank bjorn for his work on Network.HTTP and echo his observation that this package needs some work and active maintainence. i would also suggest that people needing http functionality extend his package instead of writing their own or providing quick and dirty hacks that cover 80% of the problem space, as i think others have discussed. http is a simple protocol, but it is not trivial, and reference implementations need to address the standard as completely as possible. no one is going to take haskell seriously as a practical tool if the libraries don't address the corner cases, and http has some. reading through the Network.HTTP code, it does appear that the original authors and present maintainers are concerned with standards conformance, which means reading the standard. http is becoming as integral to the development environment as file access. haskell needs an authoritative native implementation or a ffi wrapper to libcurl. developers also need to know that there is *one* reference library for http support (like perl's LWP)...likewise i think the lack of a single reference library for sql/db access is also hurting haskell adoption. perl wins here again with DBI. p.s. i would gladly volunteer to maintain the http package if i thought my haskell was "export quality". it isn't. thanks brad

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Error and warning messages with -O
by Marco Túlio Gontijo e Silva 29 Jan '07

29 Jan '07

Hello, I was trying to compile a library I wrote with cabal, but I couldn't because of -O. I tried removing it and it worked ok. Then I noticed that -O was giving some warnings and an error that are not present without it: marcot@quindinho:~/codigo/haskell/haskellgpc/haskellgpc$ /usr/bin/ghc -package-name HaskellGPC-0.0.1 --make -hide-all-packages -i -idist/build/autogen -i. -odir dist/build -hidir dist/build -package base-2.0 -O -Wall -Werror -fffi Algebra.Geometric.Gpc [1 of 1] Compiling Algebra.Geometric.Gpc ( Algebra/Geometric/Gpc.hs, dist/build/Algebra/Geometric/Gpc.o ) /tmp/ghc5871_0/ghc5871_0.hc:5:35: error: Algebra/Geometric/Gpc.h: Arquivo ou diretório não encontrado /tmp/ghc5871_0/ghc5871_0.hc: In function ‘r2Hk_entry’: /tmp/ghc5871_0/ghc5871_0.hc:290:0: warning: implicit declaration of function ‘gpc_polygon_clip’ /tmp/ghc5871_0/ghc5871_0.hc: In function ‘r2Hm_entry’: /tmp/ghc5871_0/ghc5871_0.hc:327:0: warning: implicit declaration of function ‘gpc_free_polygon’ marcot@quindinho:~/codigo/haskell/haskellgpc/haskellgpc$ /usr/bin/ghc -package-name HaskellGPC-0.0.1 --make -hide-all-packages -i -idist/build/autogen -i. -odir dist/build -hidir dist/build -package base-2.0 -Wall -Werror -fffi Algebra.Geometric.Gpc [1 of 1] Compiling Algebra.Geometric.Gpc ( Algebra/Geometric/Gpc.hs, dist/build/Algebra/Geometric/Gpc.o ) marcot@quindinho:~/codigo/haskell/haskellgpc/haskellgpc$ Why can't I use -O? Thanks. -- malebria Marco Túlio Gontijo e Silva Correio (MSN): malebria(a)riseup.net Jabber (GTalk): malebria(a)jabber.org Ekiga: malebria(a)ekiga.net IRC: malebria(a)irc.freenode.net malebria(a)irc.oftc.org Skype: marcotmarcot Telefone: 33346720 Celular: 98116720 Endereço: Rua Paula Cândido, 257/201 Gutierrez 30430-260 Belo Horizonte/MG Brasil

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Happy outputs "parE" - ideas on what to change in my .y file?
by Mark Wassell 29 Jan '07

29 Jan '07

Sometimes happy outputs 'parE' when I run it on my .y file? I believe it is coming from a part of Grammer.lhs which has the comment line "This bit is a real mess, mainly because of the error message support." Are there any suggestions on what to change in my .y file to get over this? Thanks Mark

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