December 2011 - Haskell-Cafe

Convert Double to Data.Fixed
by Eugene Kirpichov 29 Dec '11

29 Dec '11

Hi cafe, How do I most efficiently convert a Double to a Data.Fixed? I'm asking because I want to convert fractional seconds to the seconds field of Data.Time.TimeOfDay, which is Pico = Data.Fixed.Fixed E12. For now the fastest thing I came up with was fromIntegral (round (sec*1000000)) / fromIntegral 1000000, but this frankly sucks and is rather slow, there must be a better way. -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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Windows: openFile gives permission denied when file in use
by Michael Snoyman 29 Dec '11

29 Dec '11

Hi all, I just received a bug report from a client that, when an input file is open in FrameMaker, my program gives a "permission denied error". This bug is reproducible with a simple Haskell program: import System.IO main = do putStrLn "here1" h <- openFile "filename.txt" ReadMode putStrLn "here2" I tried writing a simple C program using fopen, and it ran just fine. Does anyone have experience with this issue, and know of a workaround? Thanks, Michael

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data-structure-inferrer, standard library on steroids
by Aleksander Balicki 29 Dec '11

29 Dec '11

Hi, I'm writing a project and was wondering what other people think about it. This project is meant to be a compiler feature/wrapper that analyzes your code and chooses the best data structure depending on your source code. It analyzes the functions used on a wildcard data structure and chooses the type of structure that minimizes the time complexity. It nearly supports C language and hopefully some other languages too. The purpose is that you don't have to care which data structure to use ever again :D (hopefully, if it works well, lol). Maybe some illustrative examples: int main() { ds d; //data structure while(something) { insert(d, 5); } while(!empty(d)) { printf("%d\n, max(d)); delete_max(d); } } Here dsinf would infer some kind of a heap (or possibly a rbt with max element cache). int main() { ds d; //data structure while(something) insert(d, 5); while(something) update(d, 2, 7); while(something) delete(d, 1); printf("25 is in the set", search(d, 25)); } Here it would be hashtable (it differs from the earlier, because we don't need element ordering). if anybody wants to hack it, the C api is in dsimp/ds.h and some tests in C/tests/. I have some ideas how to extend it from trivial cases, the ideas are in github issues. repo: git clone https://github.com/alistra/data-structure-inferrer.git hackage: http://hackage.haskell.org/package/data-structure-inferrer-1.0 So just let me know what you think ;] Also does anybody know a good ~MIT licenced library of data structures written in C (they're needed for the auto-compile option)?

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[ANNOUNCEMENT] SBV 0.9.24 is out
by Levent Erkok 28 Dec '11

28 Dec '11

Hello all, SBV 0.9.24 is out: http://hackage.haskell.org/package/sbv In short, SBV allows for scripting SMT solvers directly within Haskell, with built-in support for bit-vectors and unbounded integers. (Microsoft's Z3 SMT solver, and SRI's Yices can be used as backends.) New in this release are SMT based optimization (both quantified and iterative), and support for explicit domain constraints with vacuity check. Full release notes: http://github.com/LeventErkok/sbv/blob/master/RELEASENOTES Happy new year! -Levent.

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[ANNOUNCE] HaskellNet-0.3 released
by Jonathan Daugherty 28 Dec '11

28 Dec '11

Hi all, I'm pleased to announce the release of HaskellNet, version 0.3. Get it on Hackage: http://hackage.haskell.org/package/HaskellNet-0.3 This release has quite a few changes; to get a comprehensive summary, see the CHANGELOG included in the package. My aim with this release was to get the code to a state where I could do further work on it and split it up into protocol-specific packages. As a result, 0.3 is likely to be the last release of this code under this name. -- Jonathan Daugherty

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Haskell meta-programming
by John Lato 28 Dec '11

28 Dec '11

> From: Heinrich Apfelmus <apfelmus(a)quantentunnel.de> > > * Meta-programming / partial evaluation. When designing a DSL, it is > often the case that you know how to write an optimizing compiler for > your DSL because it's usually a first-order language. However, trying to > squeeze that into GHC rules is hopeless. Having some way of compiling > code at run-time would solve that. Examples: > ** Conal Elliott's image description language Pan > ** Henning Thielemann's synthesizer-llvm I've been thinking about this, and I suspect that meta-programming in Haskell may not be that far off. Suppose you have a Meta monad data Meta a = Meta { runMeta :: a} with the magical property that the compiler will optimize/recompile expressions of type `Meta a` when they are run via `runMeta`. That would provide usable metaprogramming, and I think it would have all the desired type properties etc. Of course no compiler currently has that facility, but we could use a different monad, perhaps something like this: data ThMeta a = ThMeta { runThMeta :: Language.Haskell.TH.ExpQ } now we just need to get the compiler to run an arbitrary TH splice and check that the types match after `runThMeta` is called. I think this is already possible via the GHC api. It would have the undesirable property that some expressions could be ill-typed, and this wouldn't be known until run-time, but it's a start. That's about as far as I got before I discovered a much more worked-out version on Oleg's site (with Chung-chieh Shan and Yukiyoshi Kameyama). Of course they've tackled a lot of the awkward typing issues that my simple construct rudely pushes onto the user. I'm probably showing my naivety here, and I haven't fully digested their papers yet, but I wouldn't be surprised to see applications using Haskell metaprogramming within a relatively short time. John L.

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Re: [Haskell-cafe] If you'd design a Haskell-like language, what would you do different?
by AUGER Cédric 28 Dec '11

28 Dec '11

Le Mon, 26 Dec 2011 19:30:20 -0800, Alexander Solla <alex.solla(a)gmail.com> a écrit : > So we give meaning to syntax through our semantics. That is what this > whole conversation is all about. I am proposing we give Haskell > bottoms semantics that bring it in line with the bottoms from various > theories including lattice theory, the theory of sets, the theory of > logic, as opposed to using denotational semantics' bottom semantic, > which is unrealistic for a variety of reasons. Haskell bottoms can't > be compared, due to Rice's theorem. Haskell bottoms cannot be given > an interpretation as a Haskell value. There is no problem with Rice Theorem, you can perfectly "compare" bottoms between them, all you cannot do is to make this "compare" function both "correct" and "complete" in Haskell. By correct, I mean "(⊥==⊥)≠False", and by complete, I mean "(⊥==⊥)≠⊥". But incompleteness is very common in Haskell, and correctness is the only "real worry" when computing (comparing two non terminating values, and expecting the comparison to terminate is a mistake from the programmer). On purely theoretical side (so not in Haskell), there is no problem either. For instance, the "halting-problem" can be turned into a mathematical function φ (ie., φ ∈ ⅋(TM×{true,false}) satisfying the criteria of functionnal total relation); the only thing is that this set is not "computable". > as opposed to using denotational semantics' bottom semantic, > which is unrealistic for a variety of reasons. Indeed, it is realistic, since in real world, things are not always expected to terminate, so we have to take this into account into the semantics. Don't forget that semantics is something "upper" Haskell evaluation, it is in the purely mathematical world and can rely on non computable things as far as they are consistent with the theory. That is you cannot define a complete and correct function in pure Haskell which takes a String which can be interpreted as a Haskell function, and return a String which can be interpreted as the semantics of the Haskell function. (Of course the following function would be correct: semantics :: String -> String semantics s = semantics s and this one would be complete semantics :: String -> String semantics s = [] and there is many possible functions which are complete and correct for many inputs and many possible functions which are correct and terminate for many inputs) > Haskell bottoms cannot be given an interpretation as a Haskell > value. As a lot of others said, Haskell bottoms ARE values, so they have an interpretation as a Haskell value (itself). If a whole community doesn't agree with you, either you are right and know it and should probably run away from this community or publish a well argumented book (you won't solve in 2 lines, what the community believes in for years) either the other case (you are right but not sure, or simply you are wrong) and in this case don't try to impose your point of view, rather ask questions on the points you don't understand. For me your actual behaviour can be considered as trolling. Note that I don't say the semantics you have in mind is "wrong", I only say that the semantics of ⊥ as described in the community's paper is not "wrong"; but two different "formal" semantics can exist which give the same "observationnal" semantics.

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MIDI-controlled application
by Tim Baumgartner 28 Dec '11

28 Dec '11

Hi Haskellers! I'm writing my first non-trivial Haskell application. I have an electronic drum set that generates MIDI events that I process with Haskell. A simple application of this kind might have fixed drums associated with fixed commands (I've done that). The next step would be to display menus (with very large font...) that show commands and the associated drums. The menu structure should be derived from the commands active in each context. Up to this point, I implemented this already in Java. But now after some successful attempts in Haskell, I plan for more features: the user should ultimately be able to record his own "triggers", i.e. short drum rhythms, and associate them with actions. Since I'm still a beginner with only some basic experience in Monads, Arrows and their transformers, there is infinite knowledge to be gained by working on this problem (both library and concrete apps). Currently I'm using a monad that combines Parsec (with MIDI event stream) and a Writer (that writes commands that should result in IO). It's done in a way that during running the monad, many parses can be done and failing parses roll back the parser state so that a new parse can be tried. Now my questions: I have read about Yampa, but I have not mastered it yet. E.g. I don't understand switches. Could my "triggers" be realized with Yampa's events and switches? Would you recommend any other approach? Is there something similar somewhere? Regards Tim

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How hard is it to start a web startup using Haskell?
by Gracjan Polak 28 Dec '11

28 Dec '11

Hi all, The question 'How hard is it to start a technical startup with Haskell?' happened a couple of times on this list. Sometimes it was in the form 'How hard is to find Haskell programmers?' or 'Are there any Haskell jobs?'. I'd like to provide one data point as an answer: http://www.reddit.com/r/haskell/comments/ngbbp/haskell_only_esigning_startu… Full disclosure: I'm one of two that founded this startup. How are others doing businesses using Haskell doing these days? -- Gracjan

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GHC 7.4 and binutils-gold
by Michael Snoyman 27 Dec '11

27 Dec '11

Hi all, One other little GHC 7.4 note. When I first tried building code with it, I got the following error message: /usr/bin/ld: --hash-size=31: unknown option Once I uninstalled binutils-gold, everything went just fine. Has anyone else experienced this? I'm running Ubuntu Oneiric. Michael

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