- Haskell-Cafe - Haskell.org

Community-wide RFP
by Black Mephistopheles 03 Aug '12

03 Aug '12

Is there a place - on the Haskell Wiki perhaps - with a list of desired Haskell-related projects? Both for programs written in Haskell, as well as things to help, and enhance the programming experience? Walt "BMeph" Rorie-Baety I am an eyewitness to what you committed in that location.

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ANNOUNCE: time-recurrence-0.9.0
by Chris Heller 03 Aug '12

03 Aug '12

I am happy to announce a new version of time-recurrence, a library of functions for generating recurring sequences of dates: http://hackage.haskell.org/package/time-recurrence New in version 0.9.0 is a redesigned top layer API. Previously recurrences were described by combining generating functions to get the desired effect. This was quite functional but had the drawback that a recurrence was difficult to persist outside of the Haskell run-time. As of version 0.9.0 this limitation is no more. Now the actual recurrence rule is defined by combining types into a data structure which can either be persisted (and later read back) or evaluated into a function to generate dates. This also means that the type system now enforces correct rule creation! Additionally some convenience functions are available to construct the recurrence rule in a format familiar to previous users of this library. Here is a familiar example: Generate the 15th and 30th of the month, but only during the work week: > jan2011 = ptime "Sat, 01 Jan 2012 00:00:00 -0400" > jan2012 = ptime "Sun, 01 Jan 2013 00:00:00 -0400" > takeWhile (<= jan2013) $ starting jan2012 $ > recur monthly > >==> enum (Days [15,30]) > >==> filter (WeekDays [Monday .. Friday]) [2011-02-15 04:00:00 UTC ,2011-03-15 04:00:00 UTC, 2011-03-30 04:00:00 UTC ,2011-04-15 04:00:00 UTC ,2011-05-30 04:00:00 UTC ,2011-06-15 04:00:00 UTC, 2011-06-30 04:00:00 UTC ,2011-07-15 04:00:00 UTC ,2011-08-15 04:00:00 UTC, 2011-08-30 04:00:00 UTC ,2011-09-15 04:00:00 UTC, 2011-09-30 04:00:00 UTC ,2011-11-15 04:00:00 UTC, 2011-11-30 04:00:00 UTC ,2011-12-15 04:00:00 UTC, 2011-12-30 04:00:00 UTC ] On the road to version 1.0, I plan to add another type layer which strictly enforces RFC 5545 section 3.3.10. As always I encourage feedback, criticism, suggestions and pull requests. I would especially welcome any type system experts feedback: quite a few GHC extensions needed to be turned on to get this to work. -Chris

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Re: [Haskell-cafe] CRIP: the Curiously Reoccuring Instance Pattern
by oleg＠okmij.org 03 Aug '12

03 Aug '12

> did you see this, and the discussion around that time? > http://www.haskell.org/pipermail/haskell-prime/2012-May/003688.html > > I implemented hDeleteMany without FunDeps -- and it works in Hugs (using > TypeCast -- but looks prettier in GHC with equality constraints). I'm afraid I didn't see that -- I was on travel during that time. It is nice that hDeleteMany works on Hugs. I forgot if we tried it back in 2004. To be sure some HList code did work on Hugs. We even had a special subset of HList specifically for Hugs (which I removed from the distribution some time ago). The problem was that Hugs inexplicably would fail in some other HList code. Perhaps 2006 version is better in that respect, and more or all of HList would work on it. Thank you.

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Is (==) commutative?
by Christian Sternagel 03 Aug '12

03 Aug '12

Dear all, with respect to formal verification of Haskell code I was wondering whether (==) of the Eq class is intended to be commutative (for many classes such requirements are informally stated in their description, since Eq does not have such a statement, I'm asking here). Or are there any known cases where commutativity of (==) is violated (due to strictness issues)? cheers chris

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Re: [Haskell-cafe] [Haskell] ALife category
by Brent Yorgey 02 Aug '12

02 Aug '12

On Thu, Aug 02, 2012 at 01:50:50PM +0000, Amy de Buitléir wrote: > I'm developing a framework for artificial life experiments that I would like to > make available on Hackage eventually. I've read all the guidance and proposals I > could find for where to place packages in the hierarchy: > > http://www.haskell.org/haskellwiki/Hierarchical_module_names > http://www.cs.york.ac.uk/fp/libraries/layout.html > http://www.cs.york.ac.uk/fp/libraries/layoutSM.html > > I've also examined the current hierarchy of modules: > > http://hackage.haskell.org/packages/archive/pkg-list.html > > The current hierarchy has a top-level AI category, so it would seem logical to > me to add a top-level ALife category and put my package under that. But perhaps > it would be best not to start a new top-level category? I could put my package > under AI, because my framework does support the use of AI, and wouldn't look too > out of place with the other packages there. (Moving the discussion to haskell-cafe; haskell@haskell is generally for announcements and such.) It seems to me that you might be confusing two separate things. Cabal packages may list one or more "categories", which are used to group packages on the main list of packages: http://hackage.haskell.org/package/ . In general, I would say that you should avoid creating a new top-level category. Hierarchical module names are something different. The 'Category' specified in the .cabal file does NOT have to be the same as the first component of your module names (though for many packages it is). For example, it would be perfectly fine to specify 'AI' as the category, and then have module names like ALife.Foo and so on. -Brent

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Key-Parametrized Lookup Table
by Alexander Foremny 02 Aug '12

02 Aug '12

Hello list, I am currently thinking that a problem of mine would best be solved if there was a Map-like data structure in which the value returned is parametrized over the lookup type. I wonder is this makes sense and if such a data structure exists or if it could be created while still being well typed. I essentially want to statically define a scope of Key values and dynamically define a list of keys. > -- Scope of possible keys. > type Label = String > data Key a where > KeyStr :: Label -> Key String > KeyInt :: Label -> Key Int > KeyChoice :: Label -> [a] -> Key a > -- Some key values, to be extended at runtime. > strKey "Some String" > strKey' "Another String" > intKey "Some integer" > choiceKey "Chose one" [ "a", "b", "c" ] :: KeyChoice String Now I need a data structure to possibly associate a value to the key. > data MapG = ... > type Value a = a > insert :: Key a -> Value a -> MapG Key Value -> MapG Key Value > lookup :: Key a -> MapG Key Value -> Maybe (Value a) I tried implementing this with multiple Map k a's. I tried adding a phantom type on some storage type of to implement KeyChoice as of type Key Int, but I ran into troubles with this approach. I wonder if Dynamic or Type Families could achieve this, but I am quite at a loss and would like to hear your opinion. I did try to search for this a bit, but I don't quite know how to phrase my problem. I'd like to apologize in advance if this question has been asked already. Regards, Alexander Foremny

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Haskell Platform - BSD License?
by Sai Hemanth K 02 Aug '12

02 Aug '12

Hi, I am trying to use haskell for building a tool (in a commercial setting). I am trying to figure out what all licenses are involved here. Is there a single license for the entire haskell platform (and the runtime) or is it that I need to look at the individual licenses of all the libraries and tools that make up the platform and point to them separately? The wikipedia page on haskell platform [0] says Haskell Platform is BSD licensed. But I do not find any such info elsewhere. Any pointers on this would be greatly appreciated, Thanks, Hemanth K [0] Haskell platform on wikipedia : http://en.wikipedia.org/wiki/Haskell_Platform

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What Haskell Records Need
by Jonathan Geddes 02 Aug '12

02 Aug '12

Greetings, tl;dr - What Haskell Records need are semantic editor combinators for free. I know this is yet another Record proposal among many, but none of them out there strike me as being exactly what I want in Haskell. Take the following types from a contrived example. >type Salary = Integer > >data Job = Job > { title :: String > , salary :: Salary > } > >data Person = Person > { name :: String > , job :: Job > } Since I've used record syntax, I get getter/accessor functions (title, salary, name, job) for free. Now suppose I want to create an aggregate getter function: return the salary of a given person. Piece of cake, it's just function composition >getSalary :: Person -> Salary >getSalary = salary . job Done! Now suppose I want to write a setter/mutator function for the same nested field >setSalaryMessy :: Salary -> Person -> Person >setSalaryMessy newSalary person = > person { > job = (job person) { > salary = newSalary > } > } Ouch! And that's not even very deeply nested. Imagine 4 or 5 levels deep. It really makes Haskell feel clunky next to `a.b.c.d = val` that you see in other languages. Of course immutability means that the semantics of Haskell are quite different (we're creating new values here, not updating old ones) but it's still common to model change using these kinds of updates. What if along with the free getters that the compiler generates when we use record syntax, we also got semantic editor combinator (SEC) functions[0] that could be used as follows? >setSalary newSalary = job' $ salary' (const newSalary) > >giveRaise amount = job' $ salary' (+amount) > >givePercentRaise percent = job' $ salary' (*(1+percent)) For each field x, the compiler generates a function x' (the tic is mnemonic for change). These little functions aren't hard to write, but they're classic boilerplate. >job' :: (Job -> Job) -> Person -> Person >job' f person = person {job = f $ job person} >salary' :: (Salary -> Salary) -> Job -> Job >salary' f job = job { salary = f $ salary job} These type of utility functions are a dream when working with any reference type or State Monad. > modify $ givePercentRaise 0.25 The compiler could also generate polymorphic SEC functions for polymorphic fields. Further, the compiler could disallow using old-style update syntax for fields whose SEC update function is not in scope, giving us fine-grained control over access and update. On the other hand we currently have to create new functions to achieve this (exporting the getter means exporting the ability to update as well, currently). Of course this doesn't address the namespacing issues with records, but it is likely nicely orthogonal to other proposals which do. Also note that there's a package on hackage [1] that will generate SEC functions using TH. It's nice, but I prefer the style of field names used above for updaters (field' vs editField). Let me know what you think. I'll write up an official proposal if there's a bit of general interest around this. Thanks for reading, --Jonathan [0] - http://conal.net/blog/posts/semantic-editor-combinators [1] - http://hackage.haskell.org/packages/archive/sec/0.0.1/doc/html/Data-Semanti…

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Proposal: Technique to handle package dependency gridlock
by Tom Murphy 02 Aug '12

02 Aug '12

We've got a problem with dependencies: http://cdsmith.wordpress.com/2011/01/21/a-recap-about-cabal-and-haskell-lib… http://cdsmith.wordpress.com/2011/01/17/the-butterfly-effect-in-cabal/ http://www.reddit.com/r/haskell/comments/x4knd/what_is_the_reason_for_haske… I'd like to present a proto-proposal for another arrow in our quiver. First, a few... Principles: This problem isn't uniquely Haskell's ...Although it may be uniquely Haskell's to solve. Lots of languages have a problem managing their package dependencies. To quote Chris Smith, "it’s fair to say that perhaps Haskell is one of the few environments where we’ve got a prayer at solving the problem." It's a magnitude problem The trick is to not find one perfect solution; it's to use enough effective solutions that the remaining tricky cases can be swept away individually. Solving the problem will most likely involve several techniques (good versioning policy, a little work on the part of package maintainers, smarter Cabal, new tools, etc.). Once we get the problem down to a manageable level, then it's a problem that can be dealt with package-by-package. Haskell packages are not black boxes Inheriting - from the imperative world - the idea of packages as indivisible units may be a mistake. A language like Ruby may have to import a full library, because it's nearly-impossible to reason about the behavior of part of it in isolation from the rest of it. Haskell's not like that, though! Referential transparency gives us awesome powers to reason about a pure function's behavior in nearly all cases. We could define package dependency in terms of providing functions-that-behave-like-the-ones-we-used. I mean, for god's sake, we can write QuickCheck properties like `quickSort == bubbleSort` and test it with a very high degree of assurance. The ability to pass the same suite of QuickCheck tests, or even the ability to pass a test like `fmap-3.1.2 == fmap 3.1.3`, could be strong enough proof - in the vast majority of cases - of functions' equivalence and therefore interchangeability. This could ease a significant number of package version constraints. Call it Property-Based Versioning - where your packages dependencies are based on which other versions of the package can provide the same functionality *for the functions and types that your package uses*. Programmer time is precious Thus far, the package dependency issue has been more or less a tug-of-war between package maintainers not wanting to do more work, and package users not wanting to do more work. Users just want to use the packages. Maintainers have a potentially-endless list of versions to check compatibility for ("I'm using foo-2.5.3. I assume anything 2.3 or greater is fine. Should I check through and see if 2.4 works? 2.3? 2.2? [...]"). Not to mention they can't know what the future will hold for later versions of the dependent packages, and they have to make a usually-pretty-uneducated guess about the future. Instead of maintainers and users, maybe computers should lift some of the load. Huge swaths of the most-used packages on Hackage are pure functions and data types. These are relatively easy to reason about, and if we could come close to reducing our dependency problem by, say, 30-50%, that would in my view put us within striking distance of having a very smoothly-running package ecosystem. These are decisions the community has to make There are lots of decisions to make and tradeoffs to balance. The main reason that I'm presenting such an embryonic description of this technique is to see what the Haskell community values most. In my view the ideal would be to be able to write a dependency description like "This package needs functions that behave like the ones I used in foo-3.1.2". A potentially less hair-shirty way to get some of the same functionality is for package authors to have a cabal-install or pre-cabal-install tool that determines which functions and data types are used from a package, finds equivalent ones in other packages, and write a dependency spec based on its findings (this wouldn't have anything to say about compatibility of not-yet-written packages, of course). Tom

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Haskell Implementors' Workshop 2012: Call for Participation
by Johan Tibell 02 Aug '12

02 Aug '12

===================================================================== Call for Participation ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2012 Copenhagen, Denmark, September 14th, 2012 The workshop will be held in conjunction with ICFP 2012 http://www.icfpconference.org/icfp2012/ ===================================================================== There are less than nine days left until the early registration expires! The Haskell Implementors' Workshop is to be held alongside ICFP 2012 this year in Copenhagen, Denmark. There will be no proceedings; it is an informal gathering of people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure. This relatively new workshop reflects the growth of the user community: there is a clear need for a well-supported tool chain for the development, distribution, deployment, and configuration of Haskell software. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. We intend the workshop to have an informal and interactive feel, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. * Program: http://www.haskell.org/haskellwiki/HaskellImplementorsWorkshop/2012 * Local arrangements (including travel and accommodation): http://icfpconference.org/icfp2012/local.html * Registration is available via (workshop abbreviation: HIM): http://icfpconference.org/icfp2012/registration.html Workshop organizers: * Workshop Co-Chairs: Gregory Collins, Google Johan Tibell, Google * Program Committee: Lennart Augustsson, Standard Chartered Bank Manuel M T Chakravarty, University of New South Wales Simon Marlow, Microsoft Research David Terei, Stanford University =====================================================================

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Haskell Weekly News: Issue 238
by Daniel Santa Cruz 02 Aug '12

02 Aug '12

Welcome to issue 238 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of July 22 to 28, 2012. Quotes of the Week * clsmith: i find programming in haskell is mostly about reinventing things and then discovering them in Prelude * EvanR: ruby, cobol of the 2000s * monochrom: "customers who used == also liked: foldr, map, Ord, continuation passing style" * jmcarthur: we calmly talk trolls to death Top Reddit Stories * Haskell logo QR code Domain: self.haskell, Score: 44, Comments: 5 On Reddit: [1] http://goo.gl/DuSSU Original: [2] http://goo.gl/DuSSU * In Praise of EitherT Domain: ocharles.org.uk, Score: 43, Comments: 9 On Reddit: [3] http://goo.gl/V2R9B Original: [4] http://goo.gl/M7rTb * The must-know list of packages on hackage? Domain: self.haskell, Score: 39, Comments: 29 On Reddit: [5] http://goo.gl/N7i6J Original: [6] http://goo.gl/N7i6J * SmartCheck: a better QuickCheck for testing algebraic data and generalizing counterexamples found Domain: leepike.wordpress.com, Score: 39, Comments: 6 On Reddit: [7] http://goo.gl/ENFxH Original: [8] http://goo.gl/SZMCG * Compile Haskell (Fay) to Javascript using Javascript compiled with GHCJS Domain: ghcjs.github.com, Score: 36, Comments: 8 On Reddit: [9] http://goo.gl/YOSuG Original: [10] http://goo.gl/d7Af4 * Programming in Fay - Simple example Domain: youtube.com, Score: 30, Comments: 17 On Reddit: [11] http://goo.gl/Ieswf Original: [12] http://goo.gl/4DcLd * Flexibly-Phased Constraints in Haskell Domain: jonmsterling.com, Score: 28, Comments: 19 On Reddit: [13] http://goo.gl/uvYY4 Original: [14] http://goo.gl/3cRD4 * Do you use Haskell in business? What do you use it for? Domain: self.haskell, Score: 24, Comments: 57 On Reddit: [15] http://goo.gl/AcGlL Original: [16] http://goo.gl/AcGlL * ANN: CmdTheLine 0.2 released -- with new tutorial. Domain: elifrey.com, Score: 23, Comments: 11 On Reddit: [17] http://goo.gl/yezpc Original: [18] http://goo.gl/cM9kZ * State of the Shelly (Haskell scripting lib) Domain: yesodweb.com, Score: 23, Comments: 1 On Reddit: [19] http://goo.gl/siupj Original: [20] http://goo.gl/FyrDC * Hackage b0rked for cabal 0.10.2 Domain: self.haskell, Score: 22, Comments: 4 On Reddit: [21] http://goo.gl/HzgX8 Original: [22] http://goo.gl/HzgX8 * Pipes to Conduits part 0: Combining Functors Domain: unknownparallel.wordpress.com, Score: 22, Comments: 18 On Reddit: [23] http://goo.gl/sF95u Original: [24] http://goo.gl/Jepr5 * Deconstraining DSLs [PDF] Domain: doc.ic.ac.uk, Score: 19, Comments: 2 On Reddit: [25] http://goo.gl/Ur7Lz Original: [26] http://goo.gl/E2DhL * What is the reason for Haskell's cabal package problems? Domain: self.haskell, Score: 17, Comments: 41 On Reddit: [27] http://goo.gl/O0x9z Original: [28] http://goo.gl/O0x9z * Pipes to Conduits part 4: Recovering from Abort Domain: unknownparallel.wordpress.com, Score: 16, Comments: On Reddit: [29] http://goo.gl/tr9Jy Original: [30] http://goo.gl/XbAUR * Pipes to Conduits part 1: Yield and Await Domain: unknownparallel.wordpress.com, Score: 15, Comments: 5 On Reddit: [31] http://goo.gl/KV0io Original: [32] http://goo.gl/GuFqX * Pipes to Conduits part 2: Upstream Results Domain: unknownparallel.wordpress.com, Score: 15, Comments: 9 On Reddit: [33] http://goo.gl/ONYzN Original: [34] http://goo.gl/Tf3Si * Instance Templates: a proposed feature to change type class hierarchies without breaking compatibility Domain: github.com, Score: 15, Comments: 6 On Reddit: [35] http://goo.gl/7Jx8c Original: [36] http://goo.gl/LG1YK * Categories over pairs of types Domain: twanvl.nl, Score: 15, Comments: 1 On Reddit: [37] http://goo.gl/8ee77 Original: [38] http://goo.gl/jxLPR * Pipes to Conduits part 3: Abort Domain: unknownparallel.wordpress.com, Score: 14, Comments: 12 On Reddit: [39] http://goo.gl/IZbij Original: [40] http://goo.gl/2kgLg Top StackOverflow Questions * What does “Pure” mean, in the context of programming languages and paradigms? votes: 21, answers: 2 Read on SO: [41] http://goo.gl/a0IUe * Can a thunk be duplicated to improve memory performance? votes: 17, answers: 3 Read on SO: [42] http://goo.gl/rznKJ * What's the current status of restricted monads? votes: 14, answers: 1 Read on SO: [43] http://goo.gl/UnMbE * Is it good style to use a case expression on an Unit value just to use guards? votes: 13, answers: 3 Read on SO: [44] http://goo.gl/E8xn3 * Temporary namespace/context in Haskell votes: 12, answers: 3 Read on SO: [45] http://goo.gl/AWg4O * Using the LogicT Monad Transformer in Haskell votes: 12, answers: 1 Read on SO: [46] http://goo.gl/DOw0H * Irrefutable pattern does not leak memory in recursion, but why? votes: 11, answers: 1 Read on SO: [47] http://goo.gl/qK7rx * How to determine type of Haskell functions? votes: 11, answers: 3 Read on SO: [48] http://goo.gl/Rn5F7 * Haskell: map runST votes: 9, answers: 3 Read on SO: [49] http://goo.gl/mWnvL * How to play with Control.Monad.Writer in haskell? votes: 9, answers: 1 Read on SO: [50] http://goo.gl/GLTJc Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/xb3ig/haskell_logo_qr_code/ 2. http://www.reddit.com/r/haskell/comments/xb3ig/haskell_logo_qr_code/ 3. http://ocharles.org.uk/blog/posts/2012-07-24-in-praise-of-EitherT.html 4. http://www.reddit.com/r/haskell/comments/x3ikr/in_praise_of_eithert/ 5. http://www.reddit.com/r/haskell/comments/x4g18/the_mustknow_list_of_package… 6. http://www.reddit.com/r/haskell/comments/x4g18/the_mustknow_list_of_package… 7. http://leepike.wordpress.com/2012/07/26/smartcheck/ 8. http://www.reddit.com/r/haskell/comments/x7fyt/smartcheck_a_better_quickche… 9. http://ghcjs.github.com/fay-hello-js.jsexe/ 10. http://www.reddit.com/r/haskell/comments/x43nd/compile_haskell_fay_to_javas… 11. https://www.youtube.com/watch?v=dhFZ8vx9byc 12. http://www.reddit.com/r/haskell/comments/xbihk/programming_in_fay_simple_ex… 13. http://www.jonmsterling.com/posts/2012-07-22-flexibly-phased-constraints-in… 14. http://www.reddit.com/r/haskell/comments/wzwda/flexiblyphased_constraints_i… 15. http://www.reddit.com/r/haskell/comments/x8qnh/do_you_use_haskell_in_busine… 16. http://www.reddit.com/r/haskell/comments/x8qnh/do_you_use_haskell_in_busine… 17. http://elifrey.com/2012/07/23/CmdTheLine-Tutorial/ 18. http://www.reddit.com/r/haskell/comments/x0xda/ann_cmdtheline_02_released_w… 19. http://www.yesodweb.com/blog/2012/07/shelly-update 20. http://www.reddit.com/r/haskell/comments/x9etz/state_of_the_shelly_haskell_… 21. http://www.reddit.com/r/haskell/comments/x16h7/hackage_b0rked_for_cabal_010… 22. http://www.reddit.com/r/haskell/comments/x16h7/hackage_b0rked_for_cabal_010… 23. http://unknownparallel.wordpress.com/2012/07/24/pipes-to-conduits-part-0-co… 24. http://www.reddit.com/r/haskell/comments/x2lao/pipes_to_conduits_part_0_com… 25. http://www.doc.ic.ac.uk/~wlj05/files/Deconstraining.pdf 26. http://www.reddit.com/r/haskell/comments/x3bta/deconstraining_dsls_pdf/ 27. http://www.reddit.com/r/haskell/comments/x4knd/what_is_the_reason_for_haske… 28. http://www.reddit.com/r/haskell/comments/x4knd/what_is_the_reason_for_haske… 29. http://unknownparallel.wordpress.com/2012/07/28/pipes-to-conduits-part-4-re… 30. http://www.reddit.com/r/haskell/comments/xar9r/pipes_to_conduits_part_4_rec… 31. http://unknownparallel.wordpress.com/2012/07/25/pipes-to-conduits-part-1-yi… 32. http://www.reddit.com/r/haskell/comments/x4si1/pipes_to_conduits_part_1_yie… 33. http://unknownparallel.wordpress.com/2012/07/26/pipes-and-conduits-part-2-u… 34. http://www.reddit.com/r/haskell/comments/x6w9k/pipes_to_conduits_part_2_ups… 35. https://github.com/mgsloan/instance-templates 36. http://www.reddit.com/r/haskell/comments/x71ll/instance_templates_a_propose… 37. http://twanvl.nl/blog/haskell/categories-over-pairs-of-types 38. http://www.reddit.com/r/haskell/comments/xab88/categories_over_pairs_of_typ… 39. http://unknownparallel.wordpress.com/2012/07/27/pipes-to-conduits-part-3-ab… 40. http://www.reddit.com/r/haskell/comments/x8yx2/pipes_to_conduits_part_3_abo… 41. http://stackoverflow.com/questions/11607927/what-does-pure-mean-in-the-cont… 42. http://stackoverflow.com/questions/11675807/can-a-thunk-be-duplicated-to-im… 43. http://stackoverflow.com/questions/11599685/whats-the-current-status-of-res… 44. http://stackoverflow.com/questions/11620649/is-it-good-style-to-use-a-case-… 45. http://stackoverflow.com/questions/11661963/temporary-namespace-context-in-… 46. http://stackoverflow.com/questions/11695961/using-the-logict-monad-transfor… 47. http://stackoverflow.com/questions/11625840/irrefutable-pattern-does-not-le… 48. http://stackoverflow.com/questions/11638232/how-to-determine-type-of-haskel… 49. http://stackoverflow.com/questions/11662696/haskell-map-runst 50. http://stackoverflow.com/questions/11684321/how-to-play-with-control-monad-…

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GHC rendering of non-ASCII characters configurable?
by Richard Cobbe 01 Aug '12

01 Aug '12

I'm working on an application that involves processing a lot of Unicode data, and I'm finding the built-in Show implementation for Char to be really inconvenient. Specifically, it renders all characters at U+0080 and above with decimal escapes: Prelude> '\x80' '\128' This is annoying because all of the Unicode charts give the code points in hex, and indeed the charts are split into different PDFs at numbers that are nice and round in hex but not in decimal. So in order to figure out which character I'm looking at, I have to convert back to hex and then look it up in the charts. Is there any way to ask GHC to render super-ASCII characters with their hexadecimal escapes, instead? I'm perfectly happy to write my own custom Show instance, but I don't know how to hook that into ghci's REPL (or, for that matter, the routines that HUnit uses to generate the messages on failed tests, etc.). I'm using GHC 7.4.1 on MacOS 10.7.4. Thanks, Richard

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Re: [Haskell-cafe] specifying using type class
by Patrick Browne 01 Aug '12

01 Aug '12

2 2

Re: [Haskell-cafe] The following is supposed to be initializing a 2D array but it doesn't seem to work.
by KC 01 Aug '12

01 Aug '12

That was it! On Tue, Jul 31, 2012 at 3:34 PM, Jan-Willem Maessen <jmaessen(a)alum.mit.edu>wrote: > > > On Tue, Jul 31, 2012 at 5:52 PM, KC <kc1956(a)gmail.com> wrote: > >> All I am getting is this: >> >> array ((1,5),(1,5)) [((1,5),1.0)] >> >> Maybe the behaviour of ghc was changed since the article was written. >> > > I think you've made a common mistake here. Array bounds are (lower in all > dimensions, upper in all dimensions). So perhaps you intended to use the > bounds ((1,1), (5,5)) in your code? > > I make this mistake frequently. > > -Jan-Willem Maessen > -- -- Regards, KC

1 0

CRIP: the Curiously Reoccuring Instance Pattern
by oleg＠okmij.org 01 Aug '12

01 Aug '12

Ryan Ingram wrote: > I've been seeing this pattern in a surprising number of instance > definitions lately: > > instance (a ~ ar, b ~ br) => Mcomp a ar b br [1] > instance (b ~ c, CanFilterFunc b a) => CanFilter (b -> c) a [2] And here are a few more earlier instances of the same occurrence: http://okmij.org/ftp/Haskell/typecast.html > What I'm wondering is--are there many cases where you really want the > non-constraint-generating behavior? It seems like (aside from contrived, > ahem, instances) whenever you have instance CLASS A B where A and B share > some type variables, that there aren't any valid instances of the same > class where they don't share the types in that way. Instances of the form class C a b class C a a class C a b are very characteristic of (emulation) of disequality constraints. Such instances occur, in a hidden form, all the time in HList -- when checking for membership in a type-level list, for example. There are naturally two cases: when the sought type is at the head of the list, or it is (probably) at the tail of the list. class Member (h :: *) (t :: List *) instance Member h (Cons h t) instance Member h t => Member h (Cons h' t) Of course instances above are overlapping. And when we add functional dependencies (since we really want type-functions rather type relations), they stop working at all. We had to employ work-arounds, which are described in detail in the HList paper (which is 8 years old already).

2 1

The following is supposed to be initializing a 2D array but it doesn't seem to work.
by KC 01 Aug '12

01 Aug '12

newArr :: (Ix i) => i -> i -> e -> Array i e newArr n m x = listArray (n,m) (repeat <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:repeat> x) ----- Prelude <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html> Data.Array> newArr 0 10 0 array (0,10) [(0,0),(1,0),(2,0),(3,0),(4,0),(5,0),(6,0),(7,0),(8,0),(9,0),(10,0)] Prelude <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html> Data.Array> newArr (0,0) (5,5) 0 array ((0,0),(5,5)) [((0,0),0),((0,1),0),((0,2),0),((0,3),0),((0,4),0),((0,5),0),((1,0),0),((1,1),0),((1,2),0),((1,3),0),((1,4),0),((1,5),0),((2,0),0),((2,1),0),((2,2),0),((2,3),0),((2,4),0),((2,5),0),((3,0),0),((3,1),0),((3,2),0),((3,3),0),((3,4),0),((3,5),0),((4,0),0),((4,1),0),((4,2),0),((4,3),0),((4,4),0),((4,5),0),((5,0),0),((5,1),0),((5,2),0),((5,3),0),((5,4),0),((5,5),0)] All I am getting is this: array ((1,5),(1,5)) [((1,5),1.0)] Maybe the behaviour of ghc was changed since the article was written. -- -- Regards, KC

2 1

Reddy on Referential Transparency
by Chris Dornan 31 Jul '12

31 Jul '12

Café, For those who havent seen it Uday Reddy has a comprehensive answer to a request to explain referential transparency on Stack Overflow. http://stackoverflow.com/questions/210835/what-is-referential-transparency/9 859966#9859966 For good measure he finishes with a rather scathing assessment of functional programmers claim to ownership of RT: Functional programmers don't know much of this research. Their ideas on referential transparency are to be taken with a large grain of salt. Marvellous! 'Tis rare to find such a robust assessment combined with understanding (and it certainly makes me think a bit). Have we become a bit complacent about RT? Chris

8 12

Explicit forall - Strange Error
by Shayan Najd Javadipour 31 Jul '12

31 Jul '12

Hi, I wonder why the following code doesn't typecheck in GHC 7.4.1: {-# LANGUAGE GADTs,RankNTypes #-}data T a where T1 :: (forall b. b -> b) -> (forall a. Int -> T a) {- Error: Data constructor `T1' returns type `forall a. Int -> T a' instead of an instance of its parent type `T a' In the definition of data constructor `T1' In the data type declaration for `T' Failed, modules loaded: none. -} While: {-# LANGUAGE GADTs,RankNTypes #-} f :: (forall b. b -> b) -> (forall a. Int -> Maybe a)f = undefined {- ghci> :t f f :: (forall b. b -> b) -> Int -> Maybe a -} Thanks, Shayan

3 3

Detecting numeric overflows
by Евгений Пермяков 31 Jul '12

31 Jul '12

Can someone tell me if there are any primitives, that used to detect machine type overflows, in ghc haskell ? I perfectly understand, that I can build something based on preconditioning of variables, but this will kill any performance, if needed.

4 5

ANN: stm-sbchan-0.1 - STM channel with maximum total size of items
by Joey Adams 31 Jul '12

31 Jul '12

This package provides a bounded channel type for STM. TBChan (in stm-chans) and TBQueue (introduced in stm 2.4) are bounded channels that limit the number of items in the channel. SBChan, on the other hand, limits the total size of items in the channel, where "size" is defined by providing an instance of the ItemSize class: data Entry = Entry Int64 ByteString Time -- | Estimated amount of memory an 'Entry' requires, -- including channel overhead instance ItemSize Entry where itemSize (Entry _ s _) = B.length s + 200 Then, "SBChan Entry" is a channel that limits (approximately) the amount of memory the entries take up. SBChan can also be used as a regular count-bounded channel by using the SBCItem newtype wrapper, where itemSize is always 1. http://hackage.haskell.org/package/stm-sbchan Enjoy! -Joey --- Implementation details --- itemSize returns an Int. I originally considered using an associated type, so users could pick their own number type to use. However, this would have made the implementation harder to reason about, if we had to worry about the user picking an ill-behaved number type like Float. Besides, Int is usually adequate for representing in-memory sizes. This library takes a lot of ideas from TChan and TBChan. I decided to use the linked list of TVars approach that TChan uses, rather than the pair of lists approach T(B)Queue uses, to avoid a potential problem with code like this: msg <- readTBQueue case msg of Foo -> ... Bar -> ... If the transaction is repeated a lot due to retries or invalidation, and readTBQueue needs to turn around the list at this point, then we'll end up repeating O(n) work a bunch of times. SBChan uses stm-tlist, a library I wrote that is based on TChan's internal representation. Also, SBChan uses the usual trick for reducing reader-writer contention by having two counters for capacity, one for readers and one for writers.

1 0

What does unpacking an MVar really mean?
by Leon Smith 31 Jul '12

31 Jul '12

I admit I don't know exactly how MVars are implemented, but given that they can be aliased and have indefinite extent, I would think that they look something vaguely like a cdatatype ** var, basically a pointer to an MVar (which is itself a pointer, modulo some other things such as a thread queue.) And, I would think that "unpacking" such an structure would basically be eliminating one layer of indirection, so it would then look vague like a cdatatype * var. But again, given aliasing and indefinite extent, this would seem to be a difficult thing to do. Actually this isn't too difficult if an MVar only exists in a single unpacked structure: other references to the MVar can simply be pointers into the structure. But the case where an MVar is unpacked into two different structures suggests that, at least in some cases, an "unpacked" MVar is still a cdatatype ** var; So, is my understanding more or less correct? Does anybody have a good, succinct explanation of how MVars are implemented, and how they are unpacked? One final question, assuming that unpacking an MVar really does eliminate a layer of indirection, and that other references to that MVar are simply pointers into a larger structure, what happens to that larger structure when there are no more references to it (but still some references to the MVar?) Given the complications that must arise out of a doubly "unpacked" MVar, I'm going to guess that the larger structure does get garbage collected in this case, and that the MVar becomes dislodged from this structure. Would that MVar then be placed directly inside another unpacked reference, if one is available? Best, Leon

3 4

IMAGE_FILE_LARGE_ADDRESS_AWARE (4GB on Win64) ... any best practices??
by Nick Rudnick 31 Jul '12

31 Jul '12

Dear Haskellers, did anybody of you stumble about surprisingly having a 2GB memory limit on Win64? I admit I didn't get it at once (just about to finish a complete memcheck... ;-) -- but of course there already is a discussion of this: http://stackoverflow.com/questions/10743041/making-use-of-all-available-ram… Unfortunately, this left me a little stupid about how to actually get a program running with IMAGE_FILE_LARGE_ADDRESS_AWARE -- did anybody try this successfully (or even not...) and has experience to share? Usually a Linuxer, I am just about to begin exploring the possibilities of Haskell on Windows, please forgive. Thanks a lot advance, Nick

1 0

How to define a Monad instance
by Thiago Negri 31 Jul '12

31 Jul '12

Hello. I'm trying to understand Monads. In order to do so, I decided to create my own Monad for a simple domain-specific language. The idea is to define a way to describe a multi-value replacement inside do-notation. Example of a function doing what I want (without Monads): replaceAll :: (a -> Maybe a) -> [a] -> [a] replaceAll f xs = go f xs [] where go :: (a -> Maybe a) -> [a] -> [a] -> [a] go _ [] acc = acc go f (x:xs) acc = let acc' = acc ++ [fromMaybe x (f x)] in acc' `seq` go f xs acc' Example of a replacement table: table :: Char -> Maybe Char table x = case x of 'a' -> Just 'b' 'A' -> Just 'B' _ -> Nothing Example of use: \> replaceAll table "All I want" "Bll I wbnt" Now, want I tried to do... As Monads are used for sequencing, first thing I did was to define the following data type: data TableDefinition a = Match a a (TableDefinition a) | Restart So, to create a replacement table: table' :: TableDefinition Char table' = Match 'a' 'b' (Match 'A' 'B' Restart) It look like a Monad (for me), as I can sequence any number of replacement values: table'' :: TableDefinition Char table'' = Match 'a' 'c' (Match 'c' 'a' (Match 'b' 'e' (Match 'e' 'b' Restart))) In order to run the replacement over a list, I've defined the following function: runTable :: Eq a => TableDefinition a -> [a] -> [a] runTable t = go t t [] where go _ _ acc [] = acc go s Restart acc (x:xs) = let acc' = (acc ++ [x]) in acc' `seq` go s s acc' xs go s (Match a b m) acc ci@(x:xs) | x == a = let acc' = (acc ++ [b]) in acc' `seq` go s m acc' xs | otherwise = go s m acc ci The result is still the same: \> runTable table' "All I want" "Bll I wbnt" I'd like to define the same data structure as: newTable :: TableDefinition Char newTable = do 'a' :> 'b' 'A' :> 'B' But I can't figure a way to define a Monad instance for that. :( Can you help me? Thanks, Thiago.

4 5

Relational Algebra library: first version on GitHub
by Paul Visschers 31 Jul '12

31 Jul '12

Hello, A couple of weeks ago I asked if there was interest in a library that implements a type-safe relational algebra. The response was positive, so I have spruced up the code I had a bit and created a repository on GitHub at: https://github.com/PaulVisschers/relational-algebra It is a very rudimentary version. The code is not documented and there is only a very basic example database in Test.hs. It might be helpful to look at HaskellDB's PrimQuery and PrimExpr, as this library is mostly a direct copy from that (but typed). I will add some decent examples of expressions and queries shortly. If you check it out, please comment on it and let me know if you want to contribute. Since this is going to be my first release, any feedback is welcome. Paul Visschers

5 6

CRIP: the Curiously Reoccuring Instance Pattern
by Ryan Ingram 31 Jul '12

31 Jul '12

With apologies to Jim Coplien :) I've been seeing this pattern in a surprising number of instance definitions lately: instance (a ~ ar, b ~ br) => Mcomp a ar b br [1] instance (b ~ c, CanFilterFunc b a) => CanFilter (b -> c) a [2] The trick is that since instance selection is done entirely on the instance head, these instances are strictly more general than the ones they replace: instance Mcomp a a b b instance CanFilterFunc b => CanFilter (b -> b) a The compiler has to do a lot more work to select these instances; it has to prove that the matching types actually match before it can select the instance; if it can't, it won't select an instance, and instead will complain about no instance "CLASS Int a". But with the CRIP, you help the compiler--it chooses the general instance, and then gets a constraint to solve. The constraint forces the two types to unify, or else there is a type error. What I'm wondering is--are there many cases where you really want the non-constraint-generating behavior? It seems like (aside from contrived, ahem, instances) whenever you have instance CLASS A B where A and B share some type variables, that there aren't any valid instances of the same class where they don't share the types in that way. For example, I've never really seen a class in practice with instances like class Foo a b instance Foo a a instance Foo ConcreteTypeA ConcreteTypeB Note that it's very difficult to use polymorphic types in the second instance without risking overlap. TL;DR: I, for one, welcome our new type equality constraint overlords. -- ryan [1] http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/99611 [2] http://www.yesodweb.com/blog/2012/07/classy-prelude

1 0

Polyvariadic composition
by Artyom Kazak 31 Jul '12

31 Jul '12

Hello, I have accidentally written my version of polyvariadic composition combinator, `mcomp`. It differs from Oleg’s version ( http://okmij.org/ftp/Haskell/polyvariadic.html#polyvar-comp ) in three aspects: a) it is simpler, b) it works without enumerating basic cases (all existing types, in other words), and c) it needs more type extensions. > {-# LANGUAGE > MultiParamTypeClasses > , FunctionalDependencies > , FlexibleInstances > , UndecidableInstances > , TypeFamilies , OverlappingInstances > #-} > > class Mcomp a ar b br | a br -> b where > mcomp :: a -> (ar -> br) -> b > > instance (a ~ ar, b ~ br) => Mcomp a ar b br where > mcomp a f = f a > > instance (Mcomp a ar b br) => Mcomp (x -> a) ar (x -> b) br where > mcomp a f = \x -> mcomp (a x) f My question is: why doesn’t it work when I replace instance (a ~ ar, b ~ br) => Mcomp a ar b br with instance Mcomp a a b b ? I thought that equal letters mean equal types…

3 2

why does a foldr not have a space leak effect?
by Qi Qi 30 Jul '12

30 Jul '12

Hi, I wonder why a foldr does not have a space leak effect? Any hints? Thanks. Qi

5 8

AI - machine learning
by miro 30 Jul '12

30 Jul '12

Hi All, recently I started to take a look at haskell, especially at AI. I can see some email addresses of interested people there but not so much of other activity behind. Does it exist some mailing group especially for AI? Basically I'm interested in trying some machine learning algorithms. Start with reinforcement learning and value-based), and go towards AGI (Artificial General Intelligence). Does anybody know about some already existing haskell approaches, or is there anybody working on this? Cheers, m.

4 3

Logging pure code
by Marco Túlio Gontijo e Silva 30 Jul '12

30 Jul '12

Hi. I'm using the hslogger library for logging pure code, using the following function: uLog :: Priority -> String -> a -> a uLog pri str x = unsafePerformIO $ logM rootLoggerName pri str >> return x Everything was working fine, till I used uLog on a very accessed function. Then the program started to fail with: thread blocked indefinitely in an MVar operation I've changed uLog to: uLog :: Priority -> String -> a -> a uLog pri str x = unsafePerformIO $ hPutStrLn stderr (show str) >> return x And I noticed on the log that two messages were sent "at the same time" on the point where the version with hslogger fails: "canonicate 4 "canonicate 1 \r\n\t\t\t\t\t\t\t\tRio Branco = rio branco" "canonicate 2 AC\r\n\t\t\t\t\t\t\t\t = ac" ac = ac" They were sent as (in any order): "canonicate 4 ac = ac" "canonicate 1 \r\n\t\t\t\t\t\t\t\tRio Branco = rio branco" "canonicate 2 AC\r\n\t\t\t\t\t\t\t\t = ac" I tried changing hslogger to avoid calling modifyMVar when getting logger for rootLoggerName, and using withMVar instead, but the problem persisted. I tried to create a simple test case to reproduce the problem, but I couldn't, all test cases I created worked as expected. As I was with no other ideas, I tried some things that make little sense. This happens with optimization or without it. Following http://hackage.haskell.org/trac/ghc/ticket/5859 I tried using -fno-state-hack, but the problem persisted. I tried adding NOINLINE to uLog, without success. I also tried changing unsafePerformIO to unsafeDupablePerformIO. I thought that the only thing I needed to take care while using unsafePerformIO was knowing that the time of execution is undetermined and that it could even run more than once. This is not a problem for my logging. Is there something else I should be aware while using unsafePerformIO? Greetings. -- marcot http://marcot.eti.br/

4 4

Current state of garbage collection in Haskell
by C K Kashyap 30 Jul '12

30 Jul '12

Hi, I was looking at a video that talks about GC pauses. That got me curious about the current state of GC in Haskell - say ghc 7.4.1. Would it suffer from lengthy pauses when we talk about memory in the range of 500M +? What would be a good way to keep abreast with the progress on haskell GC? Regards, Kashyap

3 3

Capturing the parent element as I parse XML using parsec
by C K Kashyap 30 Jul '12

30 Jul '12

Hi, With the help of the cafe I've been able to write up the xml parser using parsec - https://github.com/ckkashyap/really-simple-xml-parser/blob/master/RSXP.hs I am struggling with an idea though - How can I capture the parent element of each element as I parse? Is it possible or would I have to do a second pass to do the fixup? Regards, Kashyap

3 3

Munich Haskell Meeting
by Heinrich Hördegen 29 Jul '12

29 Jul '12

Dear all, next Tuesday, 31st of July, at 19h30, Munich's functional programming enthusiasts will meet at Max-Emanuell-Brauerei. If the weather is fine, we will sit outside. If you plan to join, please go to: http://www.haskell-munich.de/dates and click the button. Heinrich

1 0

Explaining instance declarations
by Patrick Browne 29 Jul '12

29 Jul '12

1 0

MonadPlus m => Maybe a -> m a
by Thiago Negri 28 Jul '12

28 Jul '12

I'm solving this exercise: http://www.haskell.org/haskellwiki/All_About_Monads#Exercise_4:_Using_the_M… I'm missing a function to transform a Maybe a into a MonadPlus m => m a. I did search on Hoogle with no luck. There is no standard definition for the "g" function I'm defining? My take on the exercise: data Sheep = Sheep { mother :: Maybe Sheep, father :: Maybe Sheep, name :: String } instance Show Sheep where -- for testing show = name g :: (MonadPlus m) => Maybe a -> m a g Nothing = mzero g (Just a) = return a mother' :: (MonadPlus m) => Sheep -> m Sheep mother' = g . mother father' :: (MonadPlus m) => Sheep -> m Sheep father' = g . father parent'' :: (MonadPlus m) => Sheep -> m Sheep parent'' s = mother' s `mplus` father' s grandparent'' :: (MonadPlus m) => Sheep -> m Sheep grandparent'' s = parent'' s >>= parent''

2 1

every Friday the 13th at midnight...
by Corentin Dupont 27 Jul '12

27 Jul '12

Hello! I'm looking for a library to be able to express, store and retrieve recurring appointments, like "every Monday at midnight". I saw Data.Time.Recurrence, which has a nice language, but how to store the appointments? A command like: *now <- getCurrentTime recur daily `begin` now* produces an infinite list of UTCTime. Best, Corentin

2 2

Haskell on Mac OS X Mountain Lion
by mrbuchmann＠googlemail.com 26 Jul '12

26 Jul '12

Hi all, does the Haskell Platform (2012.2.0.0 I suppose) work on 10.8. http://hackage.haskell.org/platform/mac.html only says something about 10.7. I don't want to break my development system due to upgrading to 10.8, so if anybody has any experience regarding this topic, please let me know. Best wishes

2 1

FP Developers Conference
by Bartosz Milewski 26 Jul '12

26 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:]

1 0

GHC type error pretty printing
by Christopher Done 26 Jul '12

26 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" :: ()

1 0

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

5 8

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

1 0

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

2 3

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 >>>> >> >>

1 0

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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Re: [Haskell-cafe] COBOL-85 parser, anyone?
by Richard O'Keefe 25 Jul '12

25 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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Re: [Haskell-cafe] Is (==) commutative?
by Tristan Seligmann 24 Jul '12

24 Jul '12

Forgot to copy the list. Sorry for the duplicates :( On Jul 24, 2012 1:35 PM, "Tristan Seligmann" <mithrandi(a)mithrandi.net> wrote: > On Jul 24, 2012 12:32 PM, "Twan van Laarhoven" <twanvl(a)gmail.com> wrote: > > > > On 2012-07-24 10:10, Christian Sternagel wrote: > >> > >> Dear all, > >> > >> with respect to formal verification of Haskell code I was wondering > whether (==) > >> of the Eq class is intended to be commutative (for many classes such > >> requirements are informally stated in their description, since Eq does > not have > >> such a statement, I'm asking here). Or are there any known cases where > >> commutativity of (==) is violated (due to strictness issues)? > > > > > > Strictness plays no role for Eq, since to test for equality both sides > will have to be fully evaluated. > > I don't think this is necessarily true. For example: > > (==) :: (Eq a) => Maybe a -> Maybe a -> Bool > Nothing == Nothing = True > Nothing == Just _ = False > Just _ == Nothing = False > Just x == Just y = x == y > > This particular example is still commutative, however (at least I think it > is). >

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