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[iteratee] empty chunk as special case of input
by Sergey Mironov 14 Jul '11

14 Jul '11

Hi community, hi John. I find myself reading bindIteratee[1] function for a several days.. there is something that keeps me away from completely understanding of the concept. The most noticeble thing is \nullC\ guard in the definition. To demonstate the consequences of this solution, let me define an iterator like myI = Iteratee $ \onDone _ -> onDone 'a' (Chunk "xyz") It is a bit unusial, since myI substitutes real stream with a fake one (xyz). Now lets define two actions producing different results in unusual manner: printI i = enumPure1Chunk ['a'..'g'] i >>= run >>= print i1 = (return 'b' >> myI >> I.head) -- myI substitutes the stream, last /I.head/ produces 'x', OK i2 = (I.head >> myI >> I.head) -- produces 'b'! I expected another 'x' here but myI's stream was ignored by >>= Well, I understand that this is probably an expected behaviour, but what is it for? Why we can't handle null input like non-null? Iterator may just stay in it's current state in that case. Thanks in advance Sergey -- [1] - bindIteratee (basically, >>=) code from Data.Iteratee.Base.hs bindIteratee :: (Monad m, Nullable s) => Iteratee s m a -> (a -> Iteratee s m b) -> Iteratee s m b bindIteratee = self where self m f = Iteratee $ \onDone onCont -> let m_done a (Chunk s) | nullC s = runIter (f a) onDone onCont m_done a stream = runIter (f a) (const . flip onDone stream) f_cont where f_cont k Nothing = runIter (k stream) onDone onCont f_cont k e = onCont k e in runIter m m_done (onCont . (flip self f .))

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Compiler support for orphan instances
by Henning Thielemann 14 Jul '11

14 Jul '11

Yesterday I tried to install fix-imports and ran into another example of problems with changing transitive closures of orphan instance propagation: $ cabal install fix-imports Resolving dependencies... Configuring fix-imports-0.1.2... Preprocessing executables for fix-imports-0.1.2... Building fix-imports-0.1.2... [1 of 6] Compiling Types ( src/Types.hs, dist/build/FixImports/FixImports-tmp/Types.o ) [2 of 6] Compiling Util ( src/Util.hs, dist/build/FixImports/FixImports-tmp/Util.o ) [3 of 6] Compiling Index ( src/Index.hs, dist/build/FixImports/FixImports-tmp/Index.o ) src/Index.hs:48:15: No instance for (Monad (Either [Char])) arising from a use of `sequence' at src/Index.hs:48:15-40 Possible fix: add an instance declaration for (Monad (Either [Char])) ... It is a repeating problem that you use an instance without noting it (here Monad (Either [Char])) and import it without noting it (you import a module, that possibly by accident imported and thus re-exported that instance). If the modules you import, decide to no longer use the Monad Either instance, then your imports will ignore the instance. I thought that some compiler support could reduce this problem. Proposal 1 GHC might warn, if you use an orphan instance but import it from a module, that does not define it. Example: module A where instance Msg s => Monad (Either s) ... module B where import A module C where {- B does not define instance Monad Either, but propagates that instance from A. -} import B test :: Either String Int test = do fail "bla" return 5 {- Here GHC should emit a warning, since the Monad Either instance is imported from B but not directly from A. If B drops the 'import A' this will no longer work. -} Proposal 2 Sometimes a module is actually intended for re-exporting an instance. Sometimes an instance is moved from module A to module Z, but A still re-exports the instance in order to maintain compatibility. In these cases proposal 1 would yield false alarms. So maybe there should be a pragma for tagging instances that are intentionally re-exported. Problem: How to identify instances? This is certainly related to earlier ideas of explicit named instance imports. module A where instance Msg s => Monad (Either s) ... module B where import A {- Explicitly propagate the instance from A, which is as good as defining it here. If the instance is not in scope, then compilation should fail at this pragma. This way it is asserted, that the instance is really exported. -} {-# INSTANCE-EXPORT Monad (Either s) #-} module C where import B test :: Either String Int test = do fail "bla" return 5 {- No warning here, because B intentionally propagated the Monad Either instance. -} Proposal 3 Forget about all those proposals and implement named instances. However the proposals 1 and 2 let you write code that works on all Haskell 98 compilers. Compilers may just not warn or ignore the INSTANCE-EXPORT pragma. Proposal 3 requires a language extension and using it will work only on the compilers that support that extension. Proposal 4 Forget about flexible, undecidable, overlapping, orphan and other instances and create a system, where classes are replaced by explicit method dictionaries and some rules for inserting these dictionaries, where the rules can be declared by the package author in the modules.

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Re: [Haskell-cafe] FFI for a beginner
by Jason Dagit 14 Jul '11

14 Jul '11

I don't know how to make ghc load them without using either hsc2hs or c2hs. I've had better experiences with hsc, but your mileage may vary. On Apr 8, 2011 3:34 AM, "Andrew Pennebaker" <andrew.pennebaker(a)gmail.com> wrote: ncurses is proving too difficult to setup, so I'm working on a new library called charm. The C code works by itself, but I can't compile a Haskell wrapper for it. While the tutorials at HaskellWiki<http://www.haskell.org/haskellwiki/FFI_complete_examples>are helpful, they're outdated. Argh! The docs<http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi-ghc.html#glasgo…>say that -#include pragmas no longer work, but fail to explain how to load code without them. Suffice to say I have no recourse but trial and error. GitHub: charm <https://github.com/mcandre/charm> and hscharm<https://github.com/mcandre/hscharm> <https://github.com/mcandre/charm>$ make cp /usr/include/charm.c . ghc --make -fforce-recomp -o hellocharm hellocharm.hs charm.hs charm.c -I/usr/include -dylib-install-name /usr/lib/libcharm.dynlib [1 of 2] Compiling Charm ( charm.hs, charm.o ) [2 of 2] Compiling Main ( hellocharm.hs, hellocharm.o ) Linking hellocharm ... ld: duplicate symbol _Charm_getWidth_info in charm.o and charm.o collect2: ld returned 1 exit status make: *** [hellocharm] Error 1 Cheers, Andrew Pennebaker www.yellosoft.us _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Haskell Weekly News: Issue 190
by Daniel Santa Cruz 14 Jul '11

14 Jul '11

Welcome to issue 190 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of July 3 to 9, 2011. You can find the HTML version of this issue at: http://contemplatecode.blogspot.com/2011/07/haskell-weekly-news-issue-190.h… Announcements Karel Gardas announced that he, Stephen Blackheath, and David Terei are "working together on ARM registerised port of GHC. The port is using LLVM as a code generator and in kind of working already." Testing is much appreciated! [1] http://goo.gl/TVvGD Matthias Fischmann inveted people to the HacPhi satellite in Berlin on July 29-31. See the post for details. Admission is free, but there is limited space, so don't wait too long! [2] http://goo.gl/XqdvI Quotes of the Week * chrisdone: [on hpaste as motivational tool] When I added hlint I was like 'hey Neil, check this', and he was all 'Oh noes! I must fix all the bugs!' * *amacleod started excising GOTOs from his BASIC code when he was 10. *roconnor started excising GOTOs from his Pascal code when he was 11. *luite used goto's in his C code last year. * AlanPerlis: One can't proceed from the informal to the formal by formal means. * Axman6: well, if C++ is anything to go by, the next Haskell will have lambdas and easy concurrency! * Cale: Constructing a definition from a type is formally the same thing as constructing a proof from a theorem. (Just with different logical systems in place :) * xplat: im in ur Sierpinski space, classifyin' ur subobjects * cmccann: in ContT, flow controls you * [Kron] What is the meaning of life? D: [joe6] learning agda? * camccann: Applying readFile to a file path doesn't return a String value any more than putting a steak next to a meat grinder magically turns them into a hamburger. * blackdog: the type system makes you strong, makes you hard. Top Reddit Stories * GHC on ARM Domain: ghcarm.wordpress.com, Score: 35, Comments: 4 On Reddit: [3] http://goo.gl/idJ7s Original: [4] http://goo.gl/D5zjE * The Monad.Reader Issue 18 Domain: themonadreader.wordpress.com, Score: 32, Comments: 3 On Reddit: [5] http://goo.gl/XA67w Original: [6] http://goo.gl/tzVX2 * Haskell vs. ML in the context of "Worse is Better" Domain: news.ycombinator.com, Score: 31, Comments: 25 On Reddit: [7] http://goo.gl/LpTbP Original: [8] http://goo.gl/bNU4w * Do (side) effects really need a total order? Domain: pchiusano.blogspot.com, Score: 31, Comments: 8 On Reddit: [9] http://goo.gl/jfxWl Original: [10] http://goo.gl/5tmUZ * Call for GUI examples - Any small GUI applications that you would like to see implemented with Functional Reactive Programming? Domain: apfelmus.nfshost.com, Score: 30, Comments: 13 On Reddit: [11] http://goo.gl/MMZJb Original: [12] http://goo.gl/X7IXP * Impredicative Polymorphism - A Use Case Domain: augustss.blogspot.com, Score: 23, Comments: 6 On Reddit: [13] http://goo.gl/QCwpt Original: [14] http://goo.gl/owsJT * Yesod :: Testimonials from a developer and designer Domain: yesodweb.com, Score: 19, Comments: 9 On Reddit: [15] http://goo.gl/5HOKo Original: [16] http://goo.gl/Zad2d * Parallelizing a nonogram solver Domain: jaspervdj.be, Score: 14, Comments: 2 On Reddit: [17] http://goo.gl/5LFlF Original: [18] http://goo.gl/BUbUA * A Haskell newbie's guide to Snap, Part 2 Domain: learnmeahaskell.blogspot.com, Score: 13, Comments: 0 On Reddit: [19] http://goo.gl/KTW50 Original: [20] http://goo.gl/rF1B1 * Losing my mind: Is Rhask (R language+Haskell) a real entity? Domain: self.haskell, Score: 11, Comments: 8 On Reddit: [21] http://goo.gl/O95KN Original: [22] http://goo.gl/O95KN Top StackOverflow Questions * How to gain control of a 5GB heap in Haskell? votes: 27, answers: 1 Read on SO: [23] http://goo.gl/OdZIG * Using higher-order Haskell types in C# votes: 16, answers: 2 Read on SO: [24] http://goo.gl/qzvok * Why applicative functors in Haskell and functional programming votes: 16, answers: 6 Read on SO: [25] http://goo.gl/72kE7 * Is there OS written in Haskell? votes: 16, answers: 3 Read on SO: [26] http://goo.gl/PgOeO * A good way to avoid “sharing”? votes: 15, answers: 1 Read on SO: [27] http://goo.gl/9GLzu * Hard Time Learning Haskell [migrated] votes: 13, answers: 0 Read on SO: [28] http://goo.gl/hhonY * Creating a Haskell REPL within a Haskell application votes: 11, answers: 1 Read on SO: [29] http://goo.gl/jZU7I About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/90890 2. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/90922 3. http://www.reddit.com/r/haskell/comments/ija21/ghc_on_arm/ 4. http://ghcarm.wordpress.com/2011/07/06/armv7-thumb-vfpv3-support-and-github… 5. http://www.reddit.com/r/haskell/comments/ihz6h/the_monadreader_issue_18/ 6. http://themonadreader.wordpress.com/2011/07/05/issue-18/ 7. http://www.reddit.com/r/haskell/comments/ig4yk/haskell_vs_ml_in_the_context… 8. http://news.ycombinator.com/item?id=2725386 9. http://www.reddit.com/r/haskell/comments/iihrl/do_side_effects_really_need_… 10. http://pchiusano.blogspot.com/2011/07/do-side-effects-really-need-total-ord… 11. http://www.reddit.com/r/haskell/comments/ijsnv/call_for_gui_examples_any_sm… 12. http://apfelmus.nfshost.com/blog/2011/07/08-call-for-gui-examples.html 13. http://www.reddit.com/r/haskell/comments/il5g5/impredicative_polymorphism_a… 14. http://augustss.blogspot.com/2011/07/impredicative-polymorphism-use-case-in… 15. http://www.reddit.com/r/haskell/comments/iitr1/yesod_testimonials_from_a_de… 16. http://www.yesodweb.com/page/testimonials 17. http://www.reddit.com/r/haskell/comments/ihdbh/parallelizing_a_nonogram_sol… 18. http://jaspervdj.be/posts/2011-07-05-parallelizing-a-nonogram-solver.html 19. http://www.reddit.com/r/haskell/comments/iiksr/a_haskell_newbies_guide_to_s… 20. http://learnmeahaskell.blogspot.com/2011/07/haskell-newbies-guide-to-snap-p… 21. http://www.reddit.com/r/haskell/comments/ig5mk/losing_my_mind_is_rhask_r_la… 22. http://www.reddit.com/r/haskell/comments/ig5mk/losing_my_mind_is_rhask_r_la… 23. http://stackoverflow.com/questions/6623391/how-to-gain-control-of-a-5gb-hea… 24. http://stackoverflow.com/questions/6562378/using-higher-order-haskell-types… 25. http://stackoverflow.com/questions/6570779/why-applicative-functors-in-hask… 26. http://stackoverflow.com/questions/6638080/is-there-os-written-in-haskell 27. http://stackoverflow.com/questions/6614023/a-good-way-to-avoid-sharing 28. http://stackoverflow.com/questions/6618081/hard-time-learning-haskell-migra… 29. http://stackoverflow.com/questions/6588999/creating-a-haskell-repl-within-a…

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ANN: fix-imports-0.1.2
by Evan Laforge 13 Jul '11

13 Jul '11

Not sure if anyone else is using this, but I've fixed some bugs and bumped fix-imports by a few more versions. Here's copy paste from the package description: A small standalone program to manage the import block of a haskell program. It will try to add import lines for qualified names with no corresponding import, remove unused import lines, and sort the import block according to some heuristic you can define. This only works for qualified imports! Unqualified imports are left untouched. It's most convenient if bound to an editor key. Changes since last announcement: Mon Jul 11 23:00:19 PDT 2011 qdunkan(a)gmail.com * version 0.1.2 Mon Jul 11 22:58:56 PDT 2011 qdunkan(a)gmail.com * fix but where multiple comments above an import are lost Fri Jul 1 15:16:40 PDT 2011 qdunkan(a)gmail.com * don't reload file if no changes were made Fri Jul 1 14:43:02 PDT 2011 qdunkan(a)gmail.com * sort and format the import block even if imports weren't added or removed Wed Jun 15 12:39:12 PDT 2011 qdunkan(a)gmail.com * version 0.1.1 Wed Jun 15 12:14:31 PDT 2011 qdunkan(a)gmail.com * upgrade to haskell-src-exts 1.11, which removes the mandatory fixities Wed Jun 15 12:11:27 PDT 2011 qdunkan(a)gmail.com * fix bug causing qualified vs. non-qualified imports to be sorted inconsistently Thu Jun 2 12:10:08 PDT 2011 qdunkan(a)gmail.com * more specific versions for base

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Paid work available in functional web programming
by Adam Chlipala 13 Jul '11

13 Jul '11

First, I apologize in advance if this post is deemed off-topic. It's about Ur/Web, a statically typed, pure functional programming languages that is much influenced by Haskell. For instance, Ur/Web includes monadic IO and type classes. I write to mention briefly that I'm looking for people interested in writing Ur/Web programs for pay. Ur/Web is a DSL for building modern web applications, and I believe it is truly a secret weapon for that domain, and one that should appeal to many Haskell fans. I have one customer now for whom I'm leading a project to develop a particular web application, and I'd like to have more. The current project would benefit from more programming help, and I would also like to develop a network of people interested in future projects. More information on Ur/Web can be found here: http://www.impredicative.com/ur/ So, are you interested in being paid at least market rates to develop real web applications in a cool Haskell-like language, thus helping to push functional programming further into the real world? Might you have any leads for possible customers for this kind of work, possibly including start-ups that want to do in-house development using a cool language with close support from the language creator (which happens to be me!)? I would love to receive reply e-mails with positive answers to either question!

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Splitting Hackage Packages and re-exporting entire modules (with same module name)
by Thomas DuBuisson 13 Jul '11

13 Jul '11

All, I have decided it would be beneficial to split System.Crypto.Random and the rest of crypto-api into different packages. Is there I way I can create a package, "entropy", with System.Crypto.Random but continue to expose that module from crypto-api (allowing people who use that module some time to move)? If so, how? If not, does anyone else see value in this and how it can be added to the infrastructure? Cheers, Thomas

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Seeking Don Stewart
by wren ng thornton 13 Jul '11

13 Jul '11

Hello all, Sorry for the spam. I'm trying to get ahold of Don Stewart, but it looks like there's some hiccup at Galois. When I mail him I get: <dons(a)galois.com>: host mail.galois.com[69.30.63.196] said: 550 5.1.1 <dons(a)galois.com>... User unknown (in reply to RCPT TO command) Does anyone know what's up, or if there's another (non-emergency) way to get in touch with him? -- Live well, ~wren

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Baffling Hackage build log
by wren ng thornton 13 Jul '11

13 Jul '11

As a dutiful package maintainer I just checked to make sure unification-fd built correctly on Hackage. Unfortunately it didn't, and it gives this inscrutable error log: cabal: There is no package named unification-fd. Perhaps you need to run 'cabal update' first? Something awry on Hackage? -- Live well, ~wren

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Re: [Haskell-cafe] Type checking oddity -- maybe my own confusion
by oleg＠okmij.org 13 Jul '11

13 Jul '11

Ryan Newton wrote: > The desired goal was that everywhere I construct a value using the Assign > constructor, that the resulting value's type to be tainted by the AssignCap > constraint. Your code essentially accomplishes the goal: > data E m where > Assign :: AssignCap m => V -> E m -> E m -> E m > Varref :: V -> E m > -- ... Every time you construct the value of the type (E m) with the Assign constructor, you have to constructively prove -- provide evidence for -- that the type m is a member of AssignCap. That evidence is stored within the E m value and so can be reused whenever it is needed later. Let us modify the example slightly: > class AssignCap m > class MutateCap m > > data E m where > Assign :: AssignCap m => V -> E m -> E m -> E m > Mutate :: MutateCap m => V -> E m -> E m -> E m > Varref :: V -> E m > -- ... > > type V = String > > -- test1 :: E t -> E t > test1 (Assign v x1 x2) = Assign v x1 x2 > > -- test2 :: MutateCap t => E t -> E t > test2 (Assign v x1 x2) = Mutate v x1 x2 To use the constructor Assign, we need the evidence that AssignCap m holds. The deconstructed value (Assign v x1 x2) provides such an evidence (as a `hidden' field of the data type). Therefore, the inferred type for test1 has no constraints. In contrast, in test2 nothing provides Mutate with the evidence that MutateCap m holds. Therefore, the inferred type has the MutateCap m constraint. If storing the evidence is not desired, then one should use ordinary ADT with smart constructors: > -- Data constructors should not be exported > data E1 m = Assign1 V (E1 m) (E1 m) | Varref1 V > isAssign (Assign1 v x1 x2) = Just (v,x1,x2) > isAssign _ = Nothing > > assign :: AssignCap m => V -> E1 m -> E1 m -> E1 m > assign = Assign1 > > -- test3 :: AssignCap m => E1 m -> E1 m > test3 e | Just (v,x1,x2) <- isAssign e = assign v x1 x2 OCaml has a so-called private constructors, which permit deconstruction but prohibit construction. They are quite handy, saving us trouble writing the views like isAssign. > Actually... to go a bit further, I thought there was some way to arrange > this so that, upon GHC type-checking the case statement, it would realize > that certain cases are forbidden based on the type, and would consider the > pattern match complete without them (or issue an error if they are present). The key word in the phrase ``certain cases are forbidden based on the type'' is _type_ -- rather than a type class. If you wish to use this special GADT feature, you have to `invert' your design. Rather than specifying what is _required_ to construct an (E m) value, you should specify what is _provided_. > data E2 m where > Assign2 :: V -> E2 m -> E2 m -> E2 IOT > Varref2 :: V -> E2 PureT > > test4 :: E2 IOT -> E2 IOT > test4 (Assign2 v x1 x2) = Assign2 v x1 x2 > test4 (Varref2 _) = undefined The compiler complaints Couldn't match type `IOT' with `PureT' Inaccessible code in a pattern with constructor Varref2 :: V -> E2 PureT, in an equation for `test4' In the pattern: Varref2 _ In an equation for `test4': test4 (Varref2 _) = undefined as desired.

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