September 2010 - Haskell-Cafe

parsec manyTill documentation question
by Peter Schmitz 27 Sep '10

27 Sep '10

I am new to parsec and having difficulty understanding the explanation of manyTill in http://legacy.cs.uu.nl/daan/download/parsec/parsec.html. (I really appreciate having this doc by the way; great reference.) I.e.: > manyTill :: GenParser tok st a -> GenParser tok st end -> GenParser tok st [a] > > (manyTill p end) applies parser p zero or more times until parser > end succeeds. Returns the list of values returned by p . This parser > can be used to scan comments: > > simpleComment = do{ string "")) > } > > Note the overlapping parsers anyChar and string "", ... since anyChar begins reading input beginning with the char *after* string ""). Second, manyTill, by definition, keeps applying p (anyChar) until end (string "-->") is satisfied, so I would expect one could just write: manyTill anyChar (string "-->") Assuming the documentation is correct on both counts, I would really appreciate any explanation someone could offer. Thanks very much, (really like Haskell & parsec) -- Peter (If anyone knows of a collection of parsec demos or good examples, I would appreciate a link; thanks)

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Re: [Haskell-cafe] parsec manyTill documentation question
by Peter Schmitz 27 Sep '10

27 Sep '10

On Fri, Sep 24, 2010 at 9:28 PM, Evan Laforge <qdunkan(a)gmail.com> wrote: >>> simpleComment = do{ string "")) >>> } >>> >>> Note the overlapping parsers anyChar and string "", ... > > Yes, I think the doc just made a mistake there. In fact, it looks > like the same mistake is in the current doc at > http://hackage.haskell.org/packages/archive/parsec/3.1.0/doc/html/Text-Pars… Evan, Thanks very much for the typo confirmation, the explanation about backtracking below, and the tip about the source distribution for the examples. I need to remember that multiple char strings imply backtracking, and that backtracking is not the default, hence "try". Thanks. -- Peter > >> Second, manyTill, by definition, keeps applying p (anyChar) until >> end (string "-->") is satisfied, so I would expect one could just >> write: >> >> manyTill anyChar (string "-->") > > The problem is that "-->" has multiple characters. So if you have > "-not end comment", it will match the '-' against the (string "-->"). > Since it doesn't backtrack by default, it's committed now and will > fail when it hits the 'n'. The 'try' will make it backtrack to > 'anyChar' when the second '-' fails to match. > >> (If anyone knows of a collection of parsec demos or good examples, I >> would appreciate a link; thanks) > > I thought the parsec source included some example parsers for simple > languages? In any case, there is lots of material floating around, > though I found parsec so intuitive and the docs so good that I just > started hacking. I think the 'build scheme in haskell' tutorial uses > parsec for the parsing. >

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Help us test gtk2hs darcs!
by Andy Stewart 27 Sep '10

27 Sep '10

Hi all, We have add many APIs in gtk2hs darcs, and ready to release gtk2hs-0.12.0 Please help us test gtk2hs darcs, we can fix it before release gtk2hs-0.12.0 You can get gtk2hs darcs with below command: darcs get code.haskell.org/gtk2hs Please send any bug report to gtk2hs-users(a)lists.sourceforge.net Thanks! -- Gtk2hs Team

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BelHac: A Hackaton in Belgium, 5-7 November
by Jasper Van der Jeugt 27 Sep '10

27 Sep '10

Hello all, We are very pleased to officially announce an international Hackaton/Get-together in Ghent, Belgium, on 5, 6 and 7 November 2010. If you are interested in attending, we have put some information on the wiki [1]. We will soon put up more details about transportation, accommodation and food. (And Belgian beer, of course!) You can register here [2]. WHEN Friday November 5: 2pm - 7pm Saturday November 6: 10am - 6pm Sunday November 7: 10am - 6pm WHERE In “The Therminal”, in Ghent, Belgium. Please see the wiki [1] for more details. We hope to see you in Belgium! [1]: http://www.haskell.org/haskellwiki/Ghent_Functional_Programming_Group/BelHac [2]: http://www.haskell.org/haskellwiki/Ghent_Functional_Programming_Group/BelHa… On behalf of the GhentFPG organizing committee, Kind regards, Jasper Van der Jeugt

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WANTED: Haskell web developer for two Yesod projects
by Michael Snoyman 27 Sep '10

27 Sep '10

Hi all, I have been working on two projects for a client, but will not be able to continue on them. I am currently looking for someone interested in some freelance work maintaining and adding features to two Yesod web applications. Please contact me if interested. Michael

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ANN: Atom 1.0.7
by Lee Pike 26 Sep '10

26 Sep '10

Announcing Atom 1.0.7 http://hackage.haskell.org/package/atom/ Recent changes include: atom 1.0.6: Support for math.h expressions (contributed by Sebastian Niller) atom 1.0.7: Support for using a HW clock to control the duration of phases. The patch to atom 1.0.7 adds the ability to use your HW to control precisely the duration of phases. Previously, this would have to be done using "ad-hoc" C code. For example, on an AVR microprocessor[1], I might call compile[2] with compile name defaults { cCode = prePostCode , hardwareClock = Just defaultClock { clockName = "millis" , clockType = Word32 , delta = 50 , delay = "delay" , err = Just "error" } } atomFn to have phases of 50 milliseconds each, calling the user-defined error() C function if that bound is violated. See Code.hs[2] for documentation. It'll generate code that looks something like the following: void main_atom_loop() { __global_clock = millis(); ... Scheduled Atom functions ... // In the following we sample the hardware clock, waiting for the next phase. uint32_t const __phase_len = 30; uint32_t const __max = 255; uint32_t __curr_time = millis(); // An error check for when the phase has already expired. // The first disjunct is for when the current time has not overflowed, // and the second for when it has. if ( ( (__curr_time >= __global_clock) && (__curr_time - __global_clock > __phase_len)) || ( (__curr_time < __global_clock) && ((__max - __global_clock) + __curr_time > __phase_len))) { error(); } __curr_time = millis(); // Update the current time. // Wait until the phase has expired. If the current time hasn't // overflowed, execute the first branch; otherwise, the second. if (__curr_time >= __global_clock) { delay(__phase_len - (__curr_time - __global_clock)); } else { delay(__phase_len - (__curr_time + (__max - __global_clock))); } } Suggestions, bug reports, improvements welcomed. Regards, Lee [1] http://www.nongnu.org/avr-libc/ [2] http://hackage.haskell.org/packages/archive/atom/1.0.7/doc/html/Language-At… [3] http://hackage.haskell.org/packages/archive/atom/1.0.7/doc/html/Language-At… -- <http://www.galois.com> <http://www.cs.indiana.edu/~lepike/> Phone: +1 503.808.7185 Fax: +1 503.350.0833

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ANNOUNCE: Utrecht Haskell Compiler (UHC), version 1.1.0
by Atze Dijkstra 26 Sep '10

26 Sep '10

Utrecht Haskell Compiler -- second release 1.1.0 ================================================ The UHC team is happy to announce the second public release of the Utrecht Haskell Compiler (UHC). UHC supports almost allmost all Haskell98 and Haskell2010 features plus some experimental extensions. The compiler runs on MacOSX, Windows (cygwin), and various Unix flavors. This second release has the following highlights relative to the previous first release: * Supports most of the Haskell2010 features. * Generic deriving [1]. * UHC knows about packages, and Cabal version 1.9.3 and later have basic support for UHC, so that Cabal can be used to build UHC packages. * For the default backend libraries are provided until including package haskell98. * The runtime system is no longer dependent on Boehm garbage collector, but uses UHCs own, a swap space collector allowing parameterization with backend specifics. * More programs from the nofib test suite compile and run. * Many smaller and larger problems fixed. For more info see http://www.cs.uu.nl/wiki/UHC Getting started & Download -------------------------- UHC is available for download as source distribution via the UHC home page: http://www.cs.uu.nl/wiki/UHC Here you will also find instructions to get started and the specifics of what UHC can and cannot do. Status of the implementation ---------------------------- Over the past year much effort by many people has been put into improving UHC, in particular to make it compile and run more library modules and test programs. Although the UHC project is very much work in progress overall reliability has improved, bugs have been fixed, and some Hackage packages compile. Warning ------- Although we think that the compiler is stable enough to compile substantial Haskell programs, we do not recommend yet to use it for any serious development work in Haskell. We ourselves use the GHC as a development platform! We think however that it provides a great platform for experimenting with language implementations, language extensions, etc. Mailing lists ------------- For UHC users and developers respectively: http://mail.cs.uu.nl/mailman/listinfo/uhc-users http://mail.cs.uu.nl/mailman/listinfo/uhc-developers Bug reporting ------------- Please report bugs at: http://code.google.com/p/uhc/issues/list The UHC Team [1] Jose Pedro Magalhaes, Atze Dijkstra, Johan Jeuring, and Andres Loeh. A generic deriving mechanism for Haskell. http://dreixel.net/research/pdf/gdmh_draft.pdf (see Section 7.1 for the discussion on constrained datatypes) - Atze - Atze Dijkstra, Department of Information and Computing Sciences. /|\ Utrecht University, PO Box 80089, 3508 TB Utrecht, Netherlands. / | \ Tel.: +31-30-2534118/1454 | WWW : http://www.cs.uu.nl/~atze . /--| \ Fax : +31-30-2513971 .... | Email: atze(a)cs.uu.nl ............ / |___\

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capture of idioms and patterns
by Luc TAESCH 26 Sep '10

26 Sep '10

in real life I am doing architecture (appication and system) and I tend to see things differently with my haskell background. when reading what system XYZ does, I see folds, maps, lazy sort, memoisation , monads, etc...ie my mind apply idioms learned at code design level to architecture level So the following questions raised in me : - are there any prior art in documenting idioms and patterns in FP, and especially haskell. - have any work been done refering to architecture level , using these idioms, or "FP specific architecture"... (the idea germinated to extend the FP contiunum from code, design to architecture (and requirement) thank you, folks luc

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ANN: ieee version 0.7
by Patrick Perry 26 Sep '10

26 Sep '10

ieee is a Haskell library for dealing with IEEE floating numbers. It was originally written to make testing with floating point values less painful. The library provides an "approximate equality" type class, AEq, with approximate equality operator "~==". One property of "~==" is that nearby floating point numbers are deemed equivalent, so that, for example, "1 ~== 1.00000001" evaluates to True. Documentation is on hackage: http://hackage.haskell.org/package/ieee Changes since the last release: * Add IEEE type class with instances for Double, Float, CDouble, and CFloat * Add functions for getting/setting NaN payloads * Add succIEEE/predIEEE for advancing up and down the IEEE number line (ported from Tango's nextUp and nextDown) * Add bisectIEEE for midpoints of two numbers on the IEEE number line (ported from Tango's ieeeMean) * Add identicalIEEE for exact (bitwise) equality of IEEE numbers * Add copySign for setting the sign bit of an IEEE number * Add sameSignificandBits for seeing how many significand bits of two IEEE numbers agree, ported from Tango's feqrel * Add nan, infinity, maxFinite, minNormal constants for IEEE numbers * Add maxNum and minNum * Rename maxF and minF to maxNaN and minNaN * Switch to a simpler "~==" comparison for complex numbers * Make "~==" comparison use sameSignificandBits for IEEE types * Make "===" comparison use bitwise equality for IEEE types * Remove old "eqRel" comparisons. * Remove old epsilon' and delta constants * Remove (RealFloat a) => AEq (Complex a) instance in favor of explicit instances for Complex {Double,Float,CDouble,CFloat}

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Polymorphic record field?
by Kevin Jardine 26 Sep '10

26 Sep '10

I have an almost finished Haskell program and I discovered that I needed to change the type of one record field. The problem is that the new type is polymorphic. Being new to Haskell, I simply changed: data MyStruct = MyStruct {myField :: myType} to data MyStruct = MyStruct {myField :: MyTypeClass a => a} and then ended up in a maze of compiler messages about Rank N types and "Inferred type is less polymorphic than expected". Upon Googling for this issue, I have discovered that Haskell records do not support polymorphic types and that there is a large literature about options to fix this. Although the literature is interesting, what I am looking for right now is a simple fix. I really don't want to have to rewrite all the many type and value references to MyStruct in my code. Is there a way to redefine the definition of MyStruct without changing any of my other code? If I have to change the way myField is accessed, I can handle that. What I don't really want to do is change the type signature of functions that take MyStruct parameters (of which there are many). Is this possible? What is the simplest way to do this? Kevin

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