- Haskell-Cafe - Haskell.org

Making extra GMP bindings
by Robin Morisset 26 Jul '12

26 Jul '12

Hello, We haven't found mpz_nextprime in the list of gmp functions that have bindings in integer-gmp, and we would like to call it from a Haskell project. We have first looked at a few simple examples of using the FFI but found several difficulties: - The mapping between Integer and the C data types isn't explicited anywhere - The gmp function gives its result by writing it in its second argument, instead of returning it. Building a wrapper for it that allocates the resulting Integer wouldn't be trivial because of how it could interact with the GC Then we looked at how integer-gmp itself does it, it seems to be writing lots of cmm wrappers that call inside of the GHC runtime. Is there an easier way to build a binding for a gmp function than to hack integer-gmp and recompile ghc ? Thanks, Robin Morisset & Florian Bourse

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Re: [Haskell-cafe] Is (==) commutative?
by Christian Sternagel 25 Jul '12

25 Jul '12

On 07/26/2012 11:53 AM, Alexander Solla wrote: > The classically valid inference: > > (x == y) = _|_ => (y == x) = _|_ Btw: whether this inference is valid or not depends on the semantics of (==) and that's exactly what I was asking about. In Haskell we just have type classes, thus (==) is more or less arbitrary (apart from its type). But usually there are certain intentions and in HOLCF we can make those intentions formal by using axiomatic type classes (i.e., type classes together with axioms that have to be satisfied by all instances; and when establishing an instance we have to formally prove that all the axioms are indeed satisfied). cheers chris

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Re: [Haskell-cafe] Is (==) commutative?
by Christian Sternagel 25 Jul '12

25 Jul '12

Sorry, I forgot to reply to the list. On 07/26/2012 11:53 AM, Alexander Solla wrote: >> (consider, e.g., 'head undefined' vs. 'head [undefined]') This was a typo, I meant 'tail undefined' vs. 'tail [undefined]'. > I'm not arguing one way or the other (I know what I do, unless there's > a need to do the other). What I'm saying is there are lots of ways to > interpret a language, and we can choose between them. If you want to > analyze Haskell, you will have to choose an interpretation for it. > Haskell, as a logic, is paraconsistent (i.e., it's unsound. So your > theory for Haskell either needs to keep contradiction in the object > language (the "partial" case where we do not identify bottoms) or take > it out of the object language and put it into the analytical > meta-language. The intention is not to use Haskell as a logic, but to use an existing logic (HOLCF, reference in an earlier mail; thus the interpretation is already fixed to a high degree) to formally verify Haskell programs. In order to do that the program to be verified has to be "rewritten" inside the logic and everything that is proved about this "formal variant" of the program should be transferable to the real program (assuming that "Haskell's semantics" was captured correctly). Another problem is if the formalization is too strict, i.e., requires properties from functions that are not satisfied in practice and that's where my original question about commutativity of (==) came from. If there are "reasonable" Haskell programs for which (==) is non-commutative, we should not require it, since we could not formalize those programs then. cheers chris > > The classically valid inference: > > (x == y) = _|_ => (y == x) = _|_ > > On 7/25/12, Christian Sternagel <c.sternagel(a)gmail.com> wrote: >> Dear Alexander, >> >> Let me clarify again: by "we identify all bottoms" I mean that any error >> and also nontermination is identified with _|_ . However, I did not mean >> that [_|_] is identified with _|_ (since the point of a formal treatment >> is exactly to correctly consider the lazy semantics of Haskell). In >> Haskell there definitely is a difference between 'undefined' and >> '[undefined]' (consider, e.g., 'head undefined' vs. 'head [undefined]') >> so this must be taken in to account in a rigorous treatment. >> >> Btw: I didn't get which inference you meant when you where saying it is >> "classically valid". >> >> cheers >> >> chris >> >> On 07/26/2012 11:13 AM, Alexander Solla wrote: >>> Depending on the context, it may or may not be wise to distinguish >>> between undefined and [undefined]. This is a matter of strictness, >>> laziness, and totality. If you identify all bottoms as one, you >>> essentially restrict yourself to (what might as well be, for the >>> purposes of this discussion) a strict subset of the Haskell language. >>> >>> If you distinguish between values that "contain" bottom and "are" >>> bottom, then you need to deal with the semantics of laziness. >>> >>> Like I said, it is classically valid, so choosing these semantics >>> means that it will be safe. But the undecidability of comparing >>> distinct bottoms means that you have to make a choice. >>> >>> On 7/24/12, Christian Sternagel <c.sternagel(a)gmail.com> wrote: >>>>> It's a classically valid inference, so you're "safe" in that respect, >>>>> and it is true that evaluating x == y requires traversing x and y, so >>>>> that if x or y "are" bottom, (x == y) and (y == x) will both be bottom. >>>> Well, (x == y) could result in bottom, even if neither x nor y are >>>> bottom, e.g., [undefined] == [undefined]. -cheers chris >>>> >> >>

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Re: [Haskell-cafe] Is (==) commutative?
by Christian Sternagel 25 Jul '12

25 Jul '12

> It's a classically valid inference, so you're "safe" in that respect, > and it is true that evaluating x == y requires traversing x and y, so > that if x or y "are" bottom, (x == y) and (y == x) will both be bottom. Well, (x == y) could result in bottom, even if neither x nor y are bottom, e.g., [undefined] == [undefined]. -cheers chris

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Haskell Weekly News: Issue 237
by Daniel Santa Cruz 25 Jul '12

25 Jul '12

Welcome to issue 237 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of July 15 to 21, 2012. Quotes of the Week * Eduard_Munteanu: Sufficiently advanced category theory is indistinguishable from trolling * Ralith just wait till I publish foldl'' / it is enterprise-ready Top Reddit Stories * Fay programming language — A strict subset of Haskell that compiles to JavaScript Domain: chrisdone.com, Score: 146, Comments: 83 On Reddit: [1] http://goo.gl/hQdvT Original: [2] http://goo.gl/0DDKv * Lambda-case and multi-way if added to GHC head (for 7.6.1) Domain: hackage.haskell.org, Score: 68, Comments: 71 On Reddit: [3] http://goo.gl/YGZeV Original: [4] http://goo.gl/hulnj * Purify code using free monads Domain: haskellforall.com, Score: 60, Comments: 47 On Reddit: [5] http://goo.gl/v8UPp Original: [6] http://goo.gl/fVtQY * Cloud Haskell Appetiser: an introductory glimpse at the new distributed-process package Domain: well-typed.com, Score: 38, Comments: 14 On Reddit: [7] http://goo.gl/zGI6e Original: [8] http://goo.gl/BlDQJ * Code Us Some Roguelike in Haskell (Part 2)! Domain: jamiltron.com, Score: 37, Comments: 6 On Reddit: [9] http://goo.gl/xPekY Original: [10] http://goo.gl/ZraR4 * The Power of Types Compels You Domain: paczesiowa.blogspot.com.au, Score: 33, Comments: 14 On Reddit: [11] http://goo.gl/NuZkM Original: [12] http://goo.gl/8djHL * RecordWildCards for localised module imports — what do you think? Domain: hpaste.org, Score: 30, Comments: 48 On Reddit: [13] http://goo.gl/L0WLp Original: [14] http://goo.gl/O8y1M * FRP - Translated slides from tutorial session Domain: apfelmus.nfshost.com, Score: 28, Comments: On Reddit: [15] http://goo.gl/9sNHU Original: [16] http://goo.gl/kqCWu * [PDF] First-Class Modules -- What happened? Domain: research.microsoft.com, Score: 27, Comments: 9 On Reddit: [17] http://goo.gl/V53sj Original: [18] http://goo.gl/JSFjJ * First class modules without defaults Domain: haskellforall.com, Score: 23, Comments: 27 On Reddit: [19] http://goo.gl/1Q7B8 Original: [20] http://goo.gl/rbS9u * gotchas with -threaded GHC runtime: accessing a MVar after forkprocess Domain: joeyh.name, Score: 22, Comments: 4 On Reddit: [21] http://goo.gl/gpu1j Original: [22] http://goo.gl/ievD1 * A Tutorial on the Curry-Howard Correspondence Domain: wellnowwhat.net, Score: 20, Comments: 11 On Reddit: [23] http://goo.gl/5jHMT Original: [24] http://goo.gl/2lDg8 * The Haskell Implementors Workshop 2012 Programme is out! Domain: haskell.org, Score: 20, Comments: 6 On Reddit: [25] http://goo.gl/3q0Y3 Original: [26] http://goo.gl/pEvww * Minimize your cloud costs with GLPK: linear programming Domain: chrisstucchio.com, Score: 18, Comments: 2 On Reddit: [27] http://goo.gl/a1Awm Original: [28] http://goo.gl/pJUrt Top StackOverflow Questions * Profiling the cost of polymorphism? votes: 16, answers: 1 Read on SO: [29] http://goo.gl/en8Ab * hackage.haskell.org documentation convention meaning votes: 13, answers: 1 Read on SO: [30] http://goo.gl/knlpX * What is the 'Const' applicative functor useful for? votes: 13, answers: 2 Read on SO: [31] http://goo.gl/iGFgu * Why does State need a value? votes: 11, answers: 6 Read on SO: [32] http://goo.gl/BBvLN * Haskell module naming conventions votes: 9, answers: 2 Read on SO: [33] http://goo.gl/bTPSr * Haskell RSS, mail and XML library choices votes: 8, answers: 0 Read on SO: [34] http://goo.gl/4Jj0u * Haskell: Equality constraint in instance votes: 8, answers: 1 Read on SO: [35] http://goo.gl/GnS3X * Profiling a Haskell program votes: 8, answers: 4 Read on SO: [36] http://goo.gl/Z3DXo * How can I turn either a Unix POSIX file descriptor or standard input Handle into a Socket? votes: 7, answers: 2 Read on SO: [37] http://goo.gl/sIyf2 * Reify a module into a record votes: 7, answers: 1 Read on SO: [38] http://goo.gl/Q9QDQ * ByteStrings in Haskell: should I use Put or Builder? votes: 6, answers: 2 Read on SO: [39] http://goo.gl/lhtGW Until next time, Daniel Santa Cruz References 1. http://chrisdone.com/fay/ 2. http://www.reddit.com/r/haskell/comments/wxi3l/fay_programming_language_a_s… 3. http://hackage.haskell.org/trac/ghc/ticket/4359#comment:87 4. http://www.reddit.com/r/haskell/comments/wp70x/lambdacase_and_multiway_if_a… 5. http://www.haskellforall.com/2012/07/purify-code-using-free-monads.html 6. http://www.reddit.com/r/haskell/comments/wtix3/purify_code_using_free_monad… 7. http://www.well-typed.com/blog/68 8. http://www.reddit.com/r/haskell/comments/wx0im/cloud_haskell_appetiser_an_i… 9. http://jamiltron.com/2012/07/Code_Us_Some_Roguelike2.html 10. http://www.reddit.com/r/haskell/comments/wwiar/code_us_some_roguelike_in_ha… 11. http://paczesiowa.blogspot.com.au/2010/03/generalized-zipwithn.html 12. http://www.reddit.com/r/haskell/comments/wr52t/the_power_of_types_compels_y… 13. http://hpaste.org/71626 14. http://www.reddit.com/r/haskell/comments/wp229/recordwildcards_for_localise… 15. http://apfelmus.nfshost.com/blog/2012/07/15-frp-tutorial-slides.html 16. http://www.reddit.com/r/haskell/comments/wmt58/frp_translated_slides_from_t… 17. http://research.microsoft.com/en-us/um/people/simonpj/Papers/first-class-mo… 18. http://www.reddit.com/r/haskell/comments/wpeam/pdf_firstclass_modules_what_… 19. http://www.haskellforall.com/2012/07/first-class-modules-without-defaults.h… 20. http://www.reddit.com/r/haskell/comments/wtx32/first_class_modules_without_… 21. http://joeyh.name/blog/entry/ghc_threaded_runtime_gotchas/ 22. http://www.reddit.com/r/haskell/comments/wxfp9/gotchas_with_threaded_ghc_ru… 23. http://wellnowwhat.net/Programming/Curry-Howard.pdf 24. http://www.reddit.com/r/haskell/comments/wpu3e/a_tutorial_on_the_curryhowar… 25. http://www.haskell.org/haskellwiki/HaskellImplementorsWorkshop/2012#Program… 26. http://www.reddit.com/r/haskell/comments/wr2h6/the_haskell_implementors_wor… 27. http://www.chrisstucchio.com/blog/2012/linear_programming.html 28. http://www.reddit.com/r/haskell/comments/wnegn/minimize_your_cloud_costs_wi… 29. http://stackoverflow.com/questions/11538977/profiling-the-cost-of-polymorph… 30. http://stackoverflow.com/questions/11526701/hackage-haskell-org-documentati… 31. http://stackoverflow.com/questions/11532050/what-is-the-const-applicative-f… 32. http://stackoverflow.com/questions/11583489/why-does-state-need-a-value 33. http://stackoverflow.com/questions/11594983/haskell-module-naming-conventio… 34. http://stackoverflow.com/questions/11495877/haskell-rss-mail-and-xml-librar… 35. http://stackoverflow.com/questions/11553705/haskell-equality-constraint-in-… 36. http://stackoverflow.com/questions/11594481/profiling-a-haskell-program 37. http://stackoverflow.com/questions/11558584/how-can-i-turn-either-a-unix-po… 38. http://stackoverflow.com/questions/11571520/reify-a-module-into-a-record 39. http://stackoverflow.com/questions/11512313/bytestrings-in-haskell-should-i…

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XMonad-screenshot
by Dmitry Malikov 25 Jul '12

25 Jul '12

Hi. My friend Matvey Aksenov create some useful screen capturing utility for XMonad. https://github.com/supki/xmonad-screenshot It allows capturing all workspaces in single image. Also chosen workspaces could be filtered by some predicate. Result image could be filled with horizontal or vertical layout. Are there some other possible use cases for that utility? What are you expecting from the ideal screenshot module for XMonad? By the way, is there some another available screenshot utility specially for XMonad? -- Best regards, dmitry malikov !

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A Question about withFile
by Bin Shi 25 Jul '12

25 Jul '12

Hello, I wrote a simple test program as main = do withFile "a.txt" ReadMode (\h -> do c <- hGetContents h putStrLn c) then I got my expected results: I'm a.txt but if I changed to main = do c <- withFile "a.txt" ReadMode hGetContents putStrLn c I got just a empty line. Where am I wrong? Thanks.

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Re: [Haskell-cafe] COBOL-85 parser, anyone?
by Richard O'Keefe 24 Jul '12

24 Jul '12

On 25/07/2012, at 1:47 AM, S D Swierstra wrote: > You can find a CFG at: http://www.cs.vu.nl/~x/grammars/vs-cobol-ii/index.html Thank you. I see it's closer to COBOL-85 than I realised.

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Which ghc binary does ghc-mod use?
by Peter Simons 24 Jul '12

24 Jul '12

Hi, I am a happy user of Emacs with ghc-mod for Haskell programming. There is just one issue I've run into: I have multiple versions of GHC installed on my machine. Now, ghc-mod seems to use the GHC binary that was used to compile ghc-mod itself, but that is not the version I want it to use for syntax checking, etc. In fact, I want to be able to switch ghc-mod between different GHC binaries depending on which project I'm working on, but I have no idea how to do that. Is there maybe some Elisp guru reading this list who can help me out? Can I somehow configure which GHC binary ghc-mod uses? Take care, Peter

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Two-day Haskell tutorial, Copenhagen, September 13-14
by Simon Thompson 24 Jul '12

24 Jul '12

This year CUFP is putting on a series of longer tutorials to introduce functional programming languages in depth, and in particular the organisers are delighted to announce a TWO DAY Haskell tutorial, taught by leading experts and educators Jeremy Gibbons (Oxford) and Simon Marlow (Microsoft Research). This two-day tutorial is designed to introduce you to Haskell without assuming that you know anything about the language in advance, and will introduce you to the underlying ideas and concepts of Haskell as well as giving you hands-on experience of programming in Haskell. So, if you're interested we do hope that you are able to attend, or if not, that you'll pass this information on to colleagues and friends who might be interested. There's more information and registration instructions at http://cufp.org/conference/2012 Hoping to see you there! Duncan Coutts, Simon Thompson, Michael Sperber CUFP tutorials organisers More information … Abstract Pure functional programming is about programming with expressions rather than actions, data rather than control. Many programs are more naturally expressed in this way, and functional programs are often an order of magnitude shorter than equivalent imperative programs - just as programs in traditional high-level languages like C and Java are typically much shorter and clearer than their equivalents in assembly language. Many exciting ideas in mainstream programming arose first in FP, including garbage collection, generics, closures, and list comprehensions. FP is a fertile laboratory for language innovation, and familiarity with FP is good preparation for future developments in programming. In particular, multicore chips and cloud computing architectures are forcing us all into dealing with concurrency, and there is good evidence that the breakthroughs here will come from FP, with its emphasis on immutability and independence of evaluation order. Content This two-day tutorial introduces the essentials of Haskell, a pure lazy functional language with a mature compiler, a wealth of libraries, and an active community. We will focus particularly on concurrent programming - handling multiple interactions at the same time - which is increasingly important in a wide range of applications today, from GUI programming to server-side web development. There will be a balance between lectures and exercises, so that you will be exposed to both the theory and the practice of FP. Day 1 will focus on the basics of FP with Haskell, introducing the concepts necessary to get you writing simple programs, including an introduction to monads. On Day 2 you will get a little taste of the joys of concurrent programming in Haskell. By the end, you will be writing some simple (but real!) Concurrent Haskell programs. We will also cover some of the practical aspects of day-to-day programming in Haskell, such as working with third-party libraries. Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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