- Haskell-Cafe - Haskell.org

what are the points in pointsfree?
by Steve Downey 16 Dec '06

16 Dec '06

i'm not naive enough to think they are the composition function, and i've gathered it has something to do with free terms, but beyond that i'm not sure. unless it also has something to do with fix points?

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Composing monads (sort of)
by Mark Wassell 16 Dec '06

16 Dec '06

Hi, I have a set of functions: f1 :: DBRecord -> Maybe Int f2 :: Int -> IO Maybe DBRecord f3 :: DBRecord -> Maybe Int The odd numbered functions are field accessors, accessing a field that might hold an identifier for another record. The even numbered functions are record fetch functions that get records from the database. I want to compose these so that I can navigate structures of joined records in the database. How can I concisely compose these functions without having to write a cascade of case statements such as: case f1 rec1 of Nothing -> return Nothing Just id1 -> do rec2 <- f2 id2 return $ case rec2 of Nothing -> return Nothing Just rec2' -> case f3 rec2' of .... I understand that if I was just dealing with Maybe I could use the fact that Maybe is a monad. I am also not sure if composing the IO and the Maybe will get me what I want (some of the functions only return Maybe Int). Cheers Mark PS Heard this on the 'West Wing' and thought it was appropriate in a way: A coach goes up to a player and asks "Are you ignorant or apathetic?". The player replies "I don't know and I don't care".

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Fwd: Build failure on FC6, ghc66
by Conrad Parker 16 Dec '06

16 Dec '06

Hi Haskellers, I received the following bug report from someone trying to build HOgg on Fedora Core 6 (FC6) with its ghc66 package. The build error is: Could not find module `Data.Map': it is a member of package base, which is hidden I've built HOgg on Debian unstable with its ghc6.6 package without problems. I know that non-base libraries have been separated out in the GHC 6.6 release. Is this problem FC6 specific or a bug in HOgg's cabal setup? any clues? cheers, Conrad. ---------- Forwarded message ---------- From: Ian Malone <ibmalone(a)gmail.com> Date: 11-Dec-2006 06:36 Subject: Re: [annodex-announce] HOgg 0.2.0 Released To: Conrad Parker <conrad(a)metadecks.org> Hi, On 06/12/06, Conrad Parker <conrad(a)metadecks.org> wrote: > HOgg 0.2.0 Released > ------------------- > > The HOgg package provides a commandline tool for manipulating Ogg files, > and a corresponding Haskell library. > > http://snapper.kfish.org/~conrad/software/hogg/ > Can't get it to build on FC6, using their ghc66 package, tried installing hugs98 and haddock as well (although they don't seem to be related to the problem). "Could not find module `Data.Map'" [ian@atlas hogg-0.2.0]$ ./Setup.hs configure Configuring hogg-0.2.0... configure: /usr/bin/ghc-pkg configure: Using install prefix: /usr/local configure: Binaries installed in: /usr/local/bin configure: Libraries installed in: /usr/local/lib/hogg-0.2.0/ghc-6.6 configure: Private binaries installed in: /usr/local/libexec configure: Data files installed in: /usr/local/share/hogg-0.2.0 configure: Using compiler: /usr/bin/ghc configure: Compiler flavor: GHC configure: Compiler version: 6.6 configure: Using package tool: /usr/bin/ghc-pkg configure: Using ar found on system at: /usr/bin/ar configure: Using haddock found on system at: /usr/bin/haddock configure: No pfesetup found configure: Using ranlib found on system at: /usr/bin/ranlib configure: Using runghc found on system at: /usr/bin/runghc configure: Using runhugs found on system at: /usr/bin/runhugs configure: No happy found configure: No alex found configure: Using hsc2hs: /usr/bin/hsc2hs configure: No c2hs found configure: No cpphs found configure: No greencard found [ian@atlas hogg-0.2.0]$ ./Setup.hs build Preprocessing library hogg-0.2.0... Preprocessing executables for hogg-0.2.0... Building hogg-0.2.0... Codec/Container/Ogg/MessageHeaders.hs:17:7: Could not find module `Data.Map': it is a member of package base, which is hidden -- imalone

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Stack, Heap and GHC
by Felix Breuer 16 Dec '06

16 Dec '06

Hello everyone, I have been trying to run a Haskell program of mine that does an extensive computation with very large amounts of data. I compiled the program with ghc --make. When I run it it terminates after some time with the message: Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. The program isn't that well written so the overflow did not surprise me, I expected that it might run out of memory. What did surprise me was the *stack* overflow. I do not use recursion in my program except for a couple of fold operations over very large lists. So I have a number of questions: 1) Which Haskell operations cost space on the stack, which cost space on the heap? I guess this is implementation dependent, so I looked into the GHC manual but did not find an answer. Where can I look these things up? 2) What could be possible sources of a stack overflow? (Apart from a recursive but not tail-recursive function.) 3) I tried using +RTS -K<size> as suggested, but these options do not seem to be passed through if I use --make. How can I use both, these compilation flags and --make? Thanks, Felix

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Re: [Haskel-cafe] Re: what are the points in pointsfree?
by Steve Downey 16 Dec '06

16 Dec '06

I spent time working with cubic mappings. With two critical points, there are Julia sets that consist of infinitely many disconnected components that are still locally connected, corresponding two one critical point escaping and one falling into one of the basins of attraction. Of course that was about 15 years ago, and I haven't really kept up. Too much time keeping up with software. On 12/16/06, Jacques Carette <carette(a)mcmaster.ca> wrote: > > Steve Downey wrote: > > No fair. Although I've a B.S. in Mathematics, I spent most of my time > > in complex analytic dynamical systems, rather than hanging with the > > algebraists. Except for a bit where I did some graph theory. > > Never thought I'd run into a fellow dynamicist on haskell-cafe! I did > my PhD on the links between the geometry and dynamics of Julia sets (co > supervised by the (late) Adrien Douady and John Hubbard). What > 'flavour' of this stuff are you interested in? > > Jacques > > PS: now I have converted to being a computer scientist, which is why I > hang out on haskell-cafe! >

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Well-typed functions with nonexisting signatures [Was: type variable question]
by oleg＠pobox.com 16 Dec '06

16 Dec '06

Nicolas Frisby wrote > The commented out signature is the signature that GHC infers for the > function. When I uncomment that signature, it will no longer type > check. Why does this happen? Even with the forall and the explicit > signatures in the where clause, it chokes. Alas, this is the property of Haskell, even of Haskell98. Half a year ago I even wanted to write a message on the Haskell' list, pointing out the embarrassment. One one hand, writing signatures is highly recommended. On the other hand, there exist, even in Haskell98, well-typed functions for which signatures simply do not exist. The number of such exceptions multiply when we move to higher-rank types (where such anomalies are to be expected). The reason for the anomalies is the difference between the 'internal' language that expresses inferred types and the external type language. > Is there a canonical example example that exhibits this behavior? Yes, it was discussed back in 2003. Here's the canonical form: g::(Show a,Show b) => a->b g = undefined --h :: Show a => b -> a h x = g(h x) Both Hugs and GHC (in the pure Haskell98 mode!) are happy with h. Both compilers infer the type for h, shown in the comment. If you give h the type signature -- the one that is inferred by the compilers themselves -- both compilers complain. This example has been distilled from practical problems (the earliest one was by Levent Erkok reported back on 2000) http://www.mail-archive.com/haskell@haskell.org/msg06754.html The following thread gives more detail and discussion: http://www.haskell.org/pipermail/haskell/2003-May/011730.html The final solution to the problem is in http://www.haskell.org/pipermail/haskell/2003-May/011762.html I have actually managed to give h the type signature. The following message indicates that the problem is actually quite common: http://www.haskell.org/pipermail/haskell-cafe/2003-May/004302.html Here's an example of an anomaly with higher-rank types: http://www.haskell.org/pipermail/haskell-cafe/2004-October/007207.html http://www.haskell.org/pipermail/haskell-cafe/2004-October/007212.html we can define functions Foo and Bar (data constructors are functions) such that we can write > myFoo :: Int -> Foo > myFoo i = Foo (Bar run op) where > run :: Monad m => StateT Int m a -> m a > run prog = do (a, s) <- runStateT prog i > return a > op :: Monad m => StateT Int m Int > op = get but we cannot write the composition of Foo and Bar more directly: > foo run op = Foo (Bar run op) The latter is not accepted: it must have a signature but no signature can ever be given to this function.

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Why is this array use wrong?
by Maurício 16 Dec '06

16 Dec '06

Hi, I'm trying this in ghci: let b = array (1,33) [(i,False) | i <- [1..33]] after :m Data.Array.IArray. It gives me that error message: <interactive>:1:8: No instance for (IArray a Bool) arising from use of `array' at <interactive>:1:8-12 Probable fix: add an instance declaration for (IArray a Bool) In the definition of `b': b = array (1, 33) ([(i, False) | i <- [1 .. 33]]) Why? Thanks, Maurício

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GHC package lang not found
by Aditya Siram 16 Dec '06

16 Dec '06

Hi all, I am trying to install Hare but I cannot build it. GHC complains that package lang is not found. Here is the output: .... editors/GenEditorInterfaces pfeRefactoringCmds > refactorer/PfeRefactoringCmds.hs cd refactorer; rm -f hidir/`uname`/Main.hi; ./myghc--make -o pfe pfe.hs 2>&1 | tee log.txt ghc-6.6: unknown package: lang cd refactorer; rm -f hidir/`uname`/Main.hi; ./myghc--make -o pfe_client pfe_client.hs 2>&1 | tee -a log.txt ghc-6.6: unknown package: lang I've searched the net for lang but I cannot find it. Here is my ghc-pkg list: /usr/local/lib/ghc-6.6/package.conf: Cabal-1.1.6, HUnit-1.1, QuickCheck-1.0, base-2.0, cgi-2006.9.6, fgl-5.2, (ghc-6.6), haskell-src-1.0, haskell98-1.0, html-1.0, mtl-1.0, network-2.0, parsec-2.0, readline-1.0, regex-base-0.71, regex-compat-0.71, regex-posix-0.71, rts-1.0, stm-2.0, template-haskell-2.0, time-1.0, unix-1.0, xhtml-2006.9.13 /home/deech/.ghc/i386-linux-6.6/package.conf: (hat-2.4), (hat-2.5) What am I doing wrong? Thanks... Deech _________________________________________________________________ WIN up to $10,000 in cash or prizes enter the Microsoft Office Live Sweepstakes http://clk.atdmt.com/MRT/go/aub0050001581mrt/direct/01/

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[ANN] JoinCabal makes its brave entry into the world
by Dougal Stanton 15 Dec '06

15 Dec '06

Okay, after a bit of a wait (I believe it was several weeks, in fact) there's an alternative to Don Stewart's mkcabal. JoinCabal is available from here: <http://brokenhut.no-ip.org/~dougal/darcs/joincabal/> or <http://brokenhut.no-ip.org/~dougal/cgi-bin/darcsweb.cgi?r=JoinCabal;a=summa…> It follows on from my post yesterday about the default licences in cabal, in that it'll create a stub source file with a licence header and description relevant to the licence you chose. (Apart from the BSD licences, which are not well documented. I'll get round to it.) If you use the [L]GPL a lot then this'll save you a wee bit of time. In future I also hope to download licences where I can, straight into the LICENSE file. Full usage can be found in the README. I hope someone finds it useful. Cheers, D. -- Dougal Stanton

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Re[2]: [Haskell] Haskell alternatives: functional programming in rich IDEs
by Bulat Ziganshin 15 Dec '06

15 Dec '06

Hello Martin, Friday, December 15, 2006, 3:51:24 PM, you wrote: >>1) F#, is Ocaml dialect integrated in .NET environment >>2) Clean, very Haskell-like language with a commercial IDE, GUI libs and so on >>3) Business Objects, integration of Haskell-like language into Java > 4) Scala, which has a nice eclipse plugin, and I believe there's a > plugin for netbeans as well. is Scala FP language or imperative/OOP language with FP extensions like C#, for example? in other words, how it looks compared to F#, at least? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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