February 2013 - Haskell-Cafe

Problem installing cabal-dev
by David Turner 13 Feb '13

13 Feb '13

Hi, From a clean install of Haskell Platform 2012.4.0.0 (on Windows) I have issued just: > cabal update > cabal install cabal-install > cabal install cabal-dev The last command fails with: Resolving dependencies... In order, the following would be installed: tar-0.3.2.0 (new package) transformers-0.2.2.0 (new version) mtl-2.0.1.0 (new version) parsec-3.1.3 (reinstall) changes: mtl-2.1.2 -> 2.0.1.0 network-2.3.2.0 (new version) HTTP-4000.2.8 (new version) cabal-dev-0.9.1 (new package) cabal: The following packages are likely to be broken by the reinstalls: network-2.3.1.0 haskell-platform-2012.4.0.0 cgi-3001.1.7.4 HTTP-4000.2.5 Use --force-reinstalls if you want to install anyway. I *think* the problem is that cabal-dev has these dependencies: mtl >= 1.1 && < 2.1, transformers >= 0.2 && < 0.3, where the latest version of mtl and transformers are 2.1.2 and 0.3 respectively. At least, if I download the latest cabal-dev package and relax those upper bounds I can get it to install. The irony is, of course, that I'm trying to install cabal-dev to get me out of a totally unrelated dependencies hole! What should I do now? Cheers, -- David Turner Senior Consultant Tracsis PLC Tracsis PLC is a company registered in England and Wales with number 5019106.

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How far compilers are allowed to go with optimizations?
by Jan Stolarek 13 Feb '13

13 Feb '13

Hi all, some time ago me and my friend had a discussion about optimizations performed by GHC. We wrote a bunch of nested list comprehensions that selected objects based on their properties (e.g. finding products with a price higher than 50). Then we rewrote our queries in a more efficient form by using tree-based maps as indices over the data. My friend knows how to turn nested list comprehensions into queries that use maps and claims this could be automated. He argued that it would be fine if compiler did such an optimization automatically, since we can guarantee that both versions have exactly the same semantics and if they have the same semantics we are free to use faster version. From a functional point of view this is a good point, but nevertheless I objected to his opinion, claiming that if compiler performed such a high-level optimization - replace underlying data structure with a different one and turn one algorithm into a completely different one - programmer wouldn't be able to reason about space behaviour of a program. I concluded that such a solution should not be built into a compiler but instead turned into an EDSL. I would like to hear your opinion on this. How far a compiler can go with transforming code? I don't want to concentrate on discussing whether such an optimization is possible to implement from a technical point of view. What I'm interested in is whether such kind of high-level optimization could be accepted. Janek

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[Announcement] hArduino: Control your Arduino board from Haskell
by Levent Erkok 12 Feb '13

12 Feb '13

I'm happy to announce hArduino: a library that allows Arduino boards to be controlled directly from Haskell. hArduino uses the Firmata protocol ( http://firmata.org) to communicate with and control Arduino boards, making it possible to implement many controller projects directly in Haskell. Home page: http://leventerkok.github.com/hArduino/ Hackage: http://hackage.haskell.org/package/hArduino Some example programs: Blink the led: http://hackage.haskell.org/packages/archive/hArduino/latest/doc/html/System… Digital counter: http://hackage.haskell.org/packages/archive/hArduino/latest/doc/html/System… Control an LCD: http://hackage.haskell.org/packages/archive/hArduino/latest/doc/html/System… Short (4.5 mins) youtube video of the blink example: http://www.youtube.com/watch?v=PPa3im44t2g hArduino is work-in-progress: Patches, bug-reports, and feedback is most welcome. -Levent.

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arrow notation
by Ross Paterson 12 Feb '13

12 Feb '13

I'd like to hear from anyone who's using arrow notation as supported by GHC, because I'm planning a couple of changes to parts of it.

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[Announcement] HsLlvm v0.1: Using Hoopl to optimize LLVM Code
by Ning Wang 12 Feb '13

12 Feb '13

Hi all, It is my dream for years to be able to manipulate LLVM in type safe language. I has explored the possibility of reimplementing LLVM in Haskell with Hoopl and made a small step to realize this dream. I am writing to announce the release of HsLlvm v0.1, which is a pure Haskell implementation (Not LLVM C++ binding) of LLVM. It is available through the link below. https://github.com/mlite/HsLlvm Hightlight of this release: - Support LLVM 3.1 syntax - LLVM normalization - Dominator, mem2reg, DCE passes Project goals: - port all LLVM optimizations - provide functionalities to generate and manipulate LLVM code. Since LLVM is a complex IR and there are many sophisticated optimizations, this job won't be easy. Your contribution would be of value to eventually achieve this goal. Cheers, Ning

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Fwd: Google Summer of Code 2013
by Johan Tibell 11 Feb '13

11 Feb '13

Hi all, Summer of code has always been a good way for us to get some important work, that no one has time to do, done. I encourage everyone to come up with good summer of code projects so we have a good number when the time for students to apply comes around. Empirically projects that focus on existing infrastructure (e.g. Cabal) work well. They limit the scope enough for students to 1) make something that's impacts lots of people and 2) doesn't give them too much rope*. * Building something good from scratch requires lots of experience, something most students don't have, almost per definition. -- Johan ---------- Forwarded message ---------- From: Carol Smith <carols(a)google.com> Date: Mon, Feb 11, 2013 at 11:02 AM Subject: Google Summer of Code 2013 To: Google Summer of Code Announce < google-summer-of-code-announce(a)googlegroups.com> Hi all, We're pleased to announce that Google Summer of Code will be happening for its ninth year this year. Please check out the blog post [1] about the program and read the FAQs [2] and Timeline [3] on Melange for more information. [1] - http://google-opensource.blogspot.com/2013/02/flip-bits-not-burgers-google-… [2] - http://www.google-melange.com/gsoc/document/show/gsoc_program/google/gsoc20… [3] - http://www.google-melange.com/gsoc/events/google/gsoc2013 Cheers, Carol -- You received this message because you are subscribed to the Google Groups "Google Summer of Code Announce" group. To unsubscribe from this group and stop receiving emails from it, send an email to google-summer-of-code-announce+unsubscribe(a)googlegroups.com. To post to this group, send email to google-summer-of-code-announce(a)googlegroups.com. Visit this group at http://groups.google.com/group/google-summer-of-code-announce?hl=en. For more options, visit https://groups.google.com/groups/opt_out.

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"Error: junk" while building GHC
by Corentin Dupont 11 Feb '13

11 Feb '13

Hi all, I'm trying to build GHC HEAD snapshot 7.7.20130210, I have the following error: "inplace/bin/ghc-stage1" -optc-Ilibraries/ghc-prim/. -optc-I'/home/cdupont/Haskell/ghc-7.7.20130210/rts/dist/build' -optc-I'/home/cdupont/Haskell/ghc-7.7.20130210/includes' -optc-I'/home/cdupont/Haskell/ghc-7.7.20130210/includes/dist-derivedconstants/header' -fPIC -dynamic -H32m -O -package-name ghc-prim-0.3.1.0 -hide-all-packages -i -ilibraries/ghc-prim/. -ilibraries/ghc-prim/dist-install/build -ilibraries/ghc-prim/dist-install/build/autogen -Ilibraries/ghc-prim/dist-install/build -Ilibraries/ghc-prim/dist-install/build/autogen -Ilibraries/ghc-prim/. -optP-include -optPlibraries/ghc-prim/dist-install/build/autogen/cabal_macros.h -package rts-1.0 -package-name ghc-prim -XHaskell98 -XCPP -XMagicHash -XForeignFunctionInterface -XUnliftedFFITypes -XUnboxedTuples -XEmptyDataDecls -XNoImplicitPrelude -O2 -no-user-package-db -rtsopts -c libraries/ghc-prim/cbits/debug.c -o libraries/ghc-prim/dist-install/build/cbits/debug.dyn_o debug.c: Assembler messages: debug.c:12:0: Error: junk `.get_pc_thunk.bx' after expression debug.c:41:0: Error: junk `.get_pc_thunk.bx' after expression debug.c:63:0: Error: junk at end of line, first unrecognized character is `1' debug.c:64:0: Error: junk at end of line, first unrecognized character is `1' debug.c:65:0: Error: Missing symbol name in directive debug.c:65:0: Error: junk at end of line, first unrecognized character is `1' debug.c:66:0: Error: Missing symbol name in directive debug.c:66:0: Error: junk at end of line, first unrecognized character is `.' debug.c:67:0: Error: junk at end of line, first unrecognized character is `1' The file debug.c is not even 12 lines long (and it looks correct)... Any ideas? Thanks, Corentin

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xml conduit
by grant 11 Feb '13

11 Feb '13

Hi, Is there a nice way to update xml. I want to be able to use xml-conduit to find a location in the xml and then add/update that node. eg xpath from //d/e/f and then change the content at 'f' or add a new node <a> ... <d> <e> <f>some data to change </f> </e> </d> ... </a> Thanks for any help, Grant

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Control.Monad provisional?
by Petr Pudlák 11 Feb '13

11 Feb '13

Dear Haskellers, Looking at Control.Monad: http://hackage.haskell.org/packages/archive/base/4.6.0.0/doc/html/Control-M… I see: "Stability provisional". I checked some older versions and it's the same. This feel somewhat unsettling - if Control.Monad is provisional, do we have any "stable" packages at all? Best regards, Petr Pudlak

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CoArbitrary
by Tony Morris 10 Feb '13

10 Feb '13

Hello, In the QuickCheck library, there is a type-class called CoArbitrary. It is defined like so: class CoArbitrary a where coarbitrary :: a -> Gen b -> Gen b -- Gen is a monad Its purpose is to allow generation of functions. In other words, for taking Gen x -> Gen (a -> x), which could be done rather degenerately (fmap const) but QuickCheck constrains with (CoArbitrary a) to perturb the resulting value instead of ignoring (const) it. It has always puzzled me in the general sense of thinking about the scenario: f x -> f (a -> x) and whether the CoArbitrary is a good/general solution, perhaps for the specific case of f=Gen or maybe even more generally. -- approximate (a -> f x -> f x) -- e.g. CoArbitrary to perturb the result -> f x -- e.g. Gen x -> f (a -> x) -- e.g. Gen (a -> x) So I often wonder about what might be a better (and perhaps more general) constraint to produce functions f x -> f (a -> x) for a given Monad f. I was wondering if there is an existing abstraction (or paper) that might point me in that direction. It is a problem that I encounter on occasion in general programming and I am using Arbitrary/CoArbitrary as an example to help make my point, but I am dissatisfied (for reasons that I am unsure about) with the solution provided by CoArbitrary. What about other monads? For example, what is a general constraint to be placed to permit a function Maybe x -> Maybe (a -> x) that does not simply "const" away the argument. I hope I have phrased this in a way to make the point. I found it a bit difficult to articulate and I do wonder (hope!) that others encounter similar scenarios. Thanks for any tips! -- Tony Morris http://tmorris.net/

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