January 2009 - Haskell-Cafe

Re: [Haskell-cafe] teaching functional programming at work
by Warren Harris 10 Jan '09

10 Jan '09

Thank you all for your responses on this. These ideas/materials are very helpful. In particular, the John Harrison book looks excellent (http://www.cl.cam.ac.uk/teaching/Lectures/funprog-jrh-1996/index.html ) -- a comprehensive and understandable introduction to all the concepts I had in mind. (I'm not sure what my audience's attention span will be though.) The WhyFP paper is also very good, although doesn't go into monads or types, it does a good job of illustrating modularity/composability. I'll send an update if this study group gets off the ground, to let you all know how it worked out. Warren

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Shared library creating on Mac OS X
by Phil 10 Jan '09

10 Jan '09

Hi, I¹m hitting a problem trying create shared haskell libs to be linked into a C program on Mac OS X. I¹m using the latest download for Leopard from the GHC page: http://www.haskell.org/ghc/dist/6.10.1/witten/ghc-6.10.1-powerpc-apple-darwi n.tar.bz2 I can get basic executables working fine (with a C main() #including ghc¹s stub header), using something like: ghc -optc-O invnorm.c InverseNormal.o InverseNormal_stub.o -o cTest I started off using the following line to try to create a shared lib: ghc --make -no-hs-main -optl '-shared' -o Inv.so InverseNormal.hs This doesn¹t work on mac os x because Apple¹s gcc annoyingly takes different switches, so I changed it to: ghc --make -no-hs-main -optl '-dynamiclib' -o Inv.dylib InverseNormal.hs Which still fails at the final link giving: Linking Inv.dylib ... Undefined symbols: "_environ", referenced from: _environ$non_lazy_ptr in libHSbase-4.0.0.0.a(PrelIOUtils.o) ld: symbol(s) not found I¹ve seen similar things before, and I believe if you have full control over the source you just slip in a: #define environ (*_NSGetEnviron()) Sure enough, I can find references to environ in, for example HsBase.h Problem (as I see it) is that references to environ are already wrapped up in the static lib libHSbase-4.0.0.0.a, so without recompiling Haskell we can¹t alter the C definition now. However, given that packager must have made this behave when he compiled the distribution there must be a way to make Mac gcc accept _envrion symbols?? Has anyone seen this before / can confirm my analysis / and by any chance have a solution? Many thanks, Phil.

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Re: [Haskell-cafe] Haskell not ready for Foo [was: Re: Hypothetical Haskell job in New York]
by John A. De Goes 10 Jan '09

10 Jan '09

Hi John, Take two examples I gave up getting to work: a Haskell wrapper for a popular GUI library; and a Haskell wrapper for a database. I understand the former has a new team of developers, so perhaps it's time to revisit the library. Then again, writing 5000 line GUIs in an imperative style in a Haskell program just doesn't seem very appealing to me. And that's one of the problem with FFI-based libraries: it exposes the functionality (when it works), but in a strictly imperative way. Selection bias applies to us both. To quote myself, "Which means your statements come from your experience, and my statements come from my experience, and the truth is probably somewhere in between." Regards, John On Jan 9, 2009, at 1:42 PM, John Goerzen wrote: > John A. De Goes wrote: >> >> Hi Austin, >> >> How do you know it's not your experience with FFI code that isn't >> biased? As far as I know, there has been no systematic attempt to >> document whether pure Haskell or FFI-based libraries are better >> designed >> and better maintained. Which means your statements come from your >> experience, and my statements come from my experience, and the >> truth is >> probably somewhere in between. >> >> In my experience, FFI code is often based on bulk translation of C >> header files into the IO monad. It requires an exact version of a >> library to work (usually much older than the current), it does not >> compile out of the box, there's scant documentation, and very little >> high-level design has been imposed on the low-level C interface >> (may as >> well use C!). Exceptions to this rule, there are, but as I said >> before, >> my experience leads me to believe they *are* exceptions to a >> *general* rule. > > Which libraries are you talking about? I haven't ever used *any* like > that, as far as I know. > > Current libraries almost always do build right out of the box with > standard cabal commands. > > Maybe part of what you state was accurate a few years ago, but right > now? > > As far as selection bias is concerned, that would apply equally well > to > both of us, would it not? > > -- John

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Re: [Haskell-cafe] Looking for Haskellers on Windows
by Günther Schmidt 10 Jan '09

10 Jan '09

Hi Kyra, thanks for your reply. The problem I was facing is using an MS-Access backend with HDBC-ODBC. I try to export data to MS-Access and I can't get it to work. Otherwise HDBC works fine in other parts of my app but with MS-Access it blows up. I did of course post this on the haskell-cafe list first, but no response and the HDBC package maintainer himself was unable to help too since he's not using Windows. Günther Am 10.01.2009, 15:48 Uhr, schrieb kyra <kyrab(a)mail.ru>: > GЭnther Schmidt wrote: >> Hi all, >> I'm looking for Haskell programmer that use Windows as their >> OS-Plattform. I developing an application in Haskell on Windows and run >> into problems that seem to have a lower priority with the greater >> Haskell community as most of them are using Linux where the problems do >> not occur or are already solved. >> So it'd be nice to get in touch with other Haskellers that face the >> same problems. >> GЭnther > > I don't know what problems You are talking about, but, since Windows is > my development platform and I'm a haskeller, I hope to be able to help > You. > > What problems are your facing? > > Kyra -- Erstellt mit Operas revolutionärem E-Mail-Modul: http://www.opera.com/mail/

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data declarations should provide fold functions
by Tim Newsham 10 Jan '09

10 Jan '09

I've had to use some fold functions recently and I've come to the conclusion that Haskell should automatically generate fold functions for data definitions. To clarify what I mean, for the data definition: data MyData = This Int Char | That String Int Int there should be a matching function definition: foldMyData f g (This a b) = f a b foldMyData f g (That a b c) = g a b c This definition is as natural as the constructors "This" and "That". Consider the tuple definition and its fold: data (,) a b = (a, b) foldTuple f (x, y) = f x y and the definition of Either and its fold: data Either a b = Left a | Right b foldEither f g (Left x) = f a foldEither f g (Right x) = g x In logic these constructors define the introduction rules ((,), Left and Right) and the folds define the elimination rules (exactly for Either, indirectly for tuples) for conjunction and disjunction. Further, while pattern-matching is very convenient for accessing the constituents of the data type, patterns are not first-class objects in Haskell. Fold functions, on the othe hand, are. They can be passed around and used in higher-order functions to extract constituents in a points-free manner. I know the short-term answer is "use TH" to derive folds if I want them, but I think such an important concept should probably be part of the language. Tim Newsham http://www.thenewsh.com/~newsham/

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ANN: Haskell BLAS bindings version 0.7
by Patrick Perry 10 Jan '09

10 Jan '09

New version! The blas package is a set of bindings to the BLAS (Basic Linear Algebra Subprograms) library. On Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/blas-0.7 What's new: * Get rid of most functional dependencies in favor of type families. There is one remaining functional dependency that cannot be gotten rid of until GHC implements equality constraints in superclass contexts. * Put the right superclass constraints in ReadMatrix/ReadBanded. * Fix freeze/thaw functions. They used to be awkward to use. * Fix a bug in getting a row view of a banded matrix. * Make sure no NaNs/Infinities/Denormals are used when testing. * Documentation. * Export functions for writing QuickCheck tests. * Remove Perm and Diag. These will be in the LAPACK bindings. * Get rid of "UnsafeIOToM" type class. * Lots of INLINE everywhere. This will improve performance (?) * Switch to autoconf for build system. Requisite blog post: http://quantile95.com/2009/01/09/new-haskell-blas-bindings/ Not much example code. Here's some old stuff: http://github.com/patperry/blas/tree/master/examples/LU.hs Thanks for listening. Any feedback is always welcome. Patrick

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Hypothetical Haskell job in New York
by Tony Hannan 09 Jan '09

09 Jan '09

Hello Haskellers, I'm trying to convince my boss to use Haskell. His main concern is finding people who know Haskell when hiring. He is comfortable with Java because he knows he can always find a Java developer. So if I advertised we were looking for Haskell programmers in New York City, how many would respond? Please reply to this email if you would respond. Email me privately if you like and I will post the results to haskell-cafe later. Thanks, Tony

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RE: [Haskell-cafe] HookedBuildInfo and Cabal
by Duncan Coutts 09 Jan '09

09 Jan '09

On Sat, 2009-01-10 at 01:40 +0100, Brian B wrote: > Hi Duncan, > > This works for me too, many thanks. Great. > The only change I needed was that the real pg_config outputs a newline > where your fake one didn't: Ah yes, good point. Duncan

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Possible GC bug
by Anish Muttreja 09 Jan '09

09 Jan '09

Hi, I have a program that seems to run into occasional garbage collection-related core dumps. The problem typically only occurs after the program has been running for a while and is consuming a large amount of memory (5 - 16GB). The large memory consumption is expected because the program analyzes very large traces from verilog simulation and needs to maintain IntMaps with hundreds of thousands of entries. Is this a bug that I should report?I am afraid that my employer will not allow me to share my source code. I do have a stack trace, below. This was obtained using ghc 6.10.1, RTS -N2 on an RHEL 4 machine. Is there something I can do trace the problem or avoid it? Thanks. PS: This is my first Haskell program and one of the most complicated I ever wrote, in any language. Using Haskell has been (mostly :-)) a joy. (gdb) where #0 0x0000000000612f40 in slowIsHeapAlloced () #1 0x000000000060f868 in evacuate () #2 0x0000000000618d12 in scavenge_block () #3 0x0000000000617c8d in scavenge_loop () #4 0x0000000000610b25 in scavenge_until_all_done () #5 0x0000000000610d02 in gc_thread_entry () #6 0x000000000064859d in start_thread (arg=<value optimized out>) at pthread_create.c:297 #7 0x000000000069e739 in clone () #8 0x0000000000000000 in ?? () -- As an adolescent I aspired to lasting fame, I craved factual certainty, and I thirsted for a meaningful vision of human life - so I became a scientist. This is like becoming an archbishop so you can meet girls. -Matt Cartmill, anthropology professor and author (1943- )

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Re: [Haskell-cafe] Haskell not ready for Foo [was: Re: Hypothetical Haskell job in New York]
by John A. De Goes 09 Jan '09

09 Jan '09

Hi Austin, How do you know it's not your experience with FFI code that isn't biased? As far as I know, there has been no systematic attempt to document whether pure Haskell or FFI-based libraries are better designed and better maintained. Which means your statements come from your experience, and my statements come from my experience, and the truth is probably somewhere in between. In my experience, FFI code is often based on bulk translation of C header files into the IO monad. It requires an exact version of a library to work (usually much older than the current), it does not compile out of the box, there's scant documentation, and very little high-level design has been imposed on the low-level C interface (may as well use C!). Exceptions to this rule, there are, but as I said before, my experience leads me to believe they *are* exceptions to a *general* rule. Regards, John On Jan 8, 2009, at 3:47 PM, Austin Seipp wrote: > Excerpts from John A. De Goes's message of Thu Jan 08 12:14:18 -0600 > 2009: >> But really, what's the point? FFI code is fragile, often uncompilable >> and unsupported, and doesn't observe the idioms of Haskell nor take >> advantage of its powerful language features. > > This is a completely unfair generalization. The FFI is an excellent > way to interoperate with an extraordinary amount of external > libraries, and if you ask me, it's *worth* taking those pieces of C > code and wrapping them up in a nice, safe haskell interface. I will > also > mention that Haskell has *the* simplest FFI I have ever used, which to > me only means it's easier to get it right (the fact that there are > customized *languages* like e.g. cython to make writing python > extensions easier makes me wonder.) > > I suggest you take a look at the haskell llvm bindings - they are > extraordinarily well documented, and the high level interface uses > *many* haskell idioms that make the library safe and easy to use: > > http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ > llvm-0.4.2.0 > > This code most certainly takes advantage of powerful features and > idioms that only Haskell can provide. Please do not take your bad > experiences with a few crappy binding (or not even crappy bindings, > perhaps bindings that just aren't very abstract) and extend them to > the bindings that are excellent with a sweeping statement like that. > > Austin > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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