July 2012 - Haskell-Cafe

FP Developers Conference
by Bartosz Milewski 27 Jul '12

27 Jul '12

Alex Miller is the organizer of a very successful Strange Loop developers conference in St Luis. He also happens to be a Closure programmer. He wants to organize a Functional Programming Developers Conference in the US (stress on _developers_). He will probably succeed, so it's very important that Haskell be well represented there. I would like to encourage everybody to take a minute and fill out a short preliminary poll at http://tech.puredanger.com/2012/07/24/functional-programming-conference/ . This is to give Alex the idea how many people are interested, what language they prefer, and what area of development they're interested in. -- [:Bartosz:]

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GHC type error pretty printing
by Christopher Done 27 Jul '12

27 Jul '12

Is it me or is this output not right? The \ syntax doesn't indicate a newline. chris@midnight:~$ ghci GHCi, version 7.4.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> "abc\n123" :: () <interactive>:2:1: Couldn't match expected type `()' with actual type `[Char]' In the expression: "abc\ \123" :: () In an equation for `it': it = "abc\ \123" :: ()

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Applicative functors with branch/choice ?
by Евгений Пермяков 26 Jul '12

26 Jul '12

Let assume, that some computation takes argument and produces value Either a b. This computation may be represented in for different forms ==== computePure :: a -> Either b c computeMonad :: a -> m (Either b c) computeApplicative :: app a -> app (Either b c) computeArrow :: arr a (Either b c) ===== And now, having result, we need to execute several actions, making a choice, what actions to perform for Left and Right tags of Either. Pure function and monads are easy, as there is way to pattern-match on value and take actions depending on result. There is an extension to Arrow class that do the job -- ArrowChoice. However, I cannot find any way to make choice for Applicative. It seems that both Applicative and Alternative are not suited for it. So, it seems for me, that Applicative API should be extended with typeclass for making choice what actions to execute depending on result of some test (pattern matching). Is there any reasonable definition of such typeclass or clear explanation, why such typeclass is unneeded? The possible extension may look somehow like this: class Applicative a => Branching a where branch :: a (Either b c) -> (a b -> a d) -> (a c -> a d) -> a d

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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 26 Jul '12

26 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 26 Jul '12

26 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 26 Jul '12

26 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 26 Jul '12

26 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 26 Jul '12

26 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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