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Typed TemplateHaskell?
by Ilya Portnov 25 May '12

25 May '12

Hello Haskell world. One well-known missfeature of TemplateHaskell is it's «untypedness». I mean, that if you have say an expression (Exp or Q Exp), you do not know it's type (does it represent String or Int or whatever). And GHC does not know too. So, one could easily construct bad-typed expressions using TH, and GHC will complain only on generated code, not on TH code. Such a missfeature was a reasoned decision, because adding some typing into TH will add much complexity to compiler and to language. As far as can I see, using features of last GHC one could write typed TH library relatively easily, and saving backwards compatibility. For example, now we have Q monad and Exp type in "template-haskell" package. Let's imagine some new package, say "typed-template-haskell", with new TQ monad and new polymorphic type Exp :: * -> *. Using last GHC's features, one will easily write something like "expr :: Exp String", which will mean that "expr" represents a string expression. And we will need a new function, say runTQ :: TQ a -> Q a (or some more complicated type), which will turn TypedTemplateHaskell's constructs into plain TH. One question will be on quotations and antiquotations: they are implemented to work untyped TH constructs, how to implement compatibility with new typed TH? We could add some support of them to GHC... Or, maybe define a phantom type (say, Any), and some conversion functions, say untyped :: Language.Haskell.TH.Syntax.Exp -> Language.Haskell.TH.Typed.Syntax.Exp Any. Main idea is: seems new GHC enables us to implement "typed TH" without modifying GHC itself, only by writing a library. Is it so? Any further thoughts? With best regards, Ilya Portnov.

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"Generic figures and their glueings" book ..... please help
by Vasili I. Galchin 24 May '12

24 May '12

Hello, I think the above book by Reyes, Reyes and Zolfaghari is quite intriguing. At the same time I can't get my mind around the notions of "generic figures" and "change of figures"! The first seems to be objects in the base category and the latter morphisms in the base category. Whoever has read this book and understands it please give me some pointers. Sorry the vague call for help. Kind regards, Vasili

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Re: [Haskell-cafe] Can Haskell outperform C++?
by Roman Werpachowski 24 May '12

24 May '12

> From: "Richard O'Keefe" <ok(a)cs.otago.ac.nz> > Subject: Re: [Haskell-cafe] Can Haskell outperform C++? > To: Haskell Cafe <haskell-cafe(a)haskell.org> > Message-ID: <5F6605A2-DFE0-4AEA-9987-3B07DEF34C42(a)cs.otago.ac.nz> > Content-Type: text/plain; charset=us-ascii > > On 17/05/2012, at 2:04 PM, Gregg Lebovitz wrote: > >> Richard, >> >> Thank you. This is an example of what I had in mind when I talked about changing the playing field. Do you have a slide deck for this lecture that you would be willing to share with me? I am very interested in learning more. > > > No slide deck required. The task is "generating alternating permutations". > > Method 1: generate permutations using a backtracking search; > when a permutation is generated, check if it is alternating. > > Method 2: use the same backtracking search, but only allow extensions > that preserve the property of being an alternating permutation. Gregg, what makes Method 2 so much harder than Method 1 to implement in C or C++? Regards, RW

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Re: [Haskell-cafe] Converting wiki pages into pdf
by do_not_reply_to_this_address＠t-online.de 24 May '12

24 May '12

I invested an enormous amount of time into this problem. Accordingly I got a very well working solution. http://de.wikibooks.org/wiki/Benutzer:Dirk_Huenniger/wb2pdf http://en.wikibooks.org/wiki/File:Haskell.pdf I am happy If you find it useful. Yours Dirk Hünniger > Thu, 08 Sep 2011 05:36:44 -0700 > Hello all > I am trying to write a Haskell program which download html pages from > wikipedia including images and convert them into pdf . I wrote a > small script > > import Network.HTTP > import Data.Maybe > import Data.List > > main = do > x<- getLine > htmlpage<- getResponseBody =<< simpleHTTP ( getRequest x ) -- > open url > --print.words $ htmlpage > let ind_1 = fromJust . ( \n -> findIndex ( n `isPrefixOf`) . > tails $ htmlpage ) $ "" > ind_2 = fromJust . ( \n -> findIndex ( n `isPrefixOf`) . > tails $ htmlpage ) $ "" > tmphtml = drop ind_1 $ take ind_2 htmlpage > writeFile "down.html" tmphtml > > and its working fine except some symbols are not rendering as it > should be. Could some one please suggest me how to accomplish this > task. > > Thank you > Mukesh Tiwari >

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Formalisation for types of monads
by Yves Parès 24 May '12

24 May '12

When explaining monads to beginners (having an imperative background), I found myself to say that there is *roughly* three "groups" of monads (because they're always worried about their cost, i.e. their incidental complexity): - Function-oriented monads (e.g. State, Reader, Cont) - Reductible data-oriented monads (e.g. Maybe, Identity, Writer) - Irreductible data-oriented monads (e.g. List, free monads) (which, as I understood, have a (>>=) operation that has a quadratic complexity if not used properly) Are there others "groups" I'm missing and is there terms to formalize them?

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question about linking with sqlite3
by Canhua 24 May '12

24 May '12

Hi, I made a cabal package that use HDBC-sqlite3 on Windows. When cabal install it, I got an error from ld.exe: cannot find -lsqlite3. How can I tell ld.exe where sqlite3.dll is plcaed? Thanks.

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Parallel cooperative multithreading?
by Benjamin Ylvisaker 24 May '12

24 May '12

Has anyone ever worked on implementing something like this in Haskell? http://www.cs.hmc.edu/~stone/papers/ocm-unpublished.pdf The outline of the idea: - Concurrent programming is really hard with the popular frameworks today. - For most purposes parallel programming is hard, in some part because it requires concurrent programming. Of course there are attempts to do non-concurrent parallel programming, but I hope it's not too controversial to say that such frameworks are still on the fringe. - Cooperative concurrency is way easier than preemptive concurrency because between invocations of pause/yield/wait, sequential reasoning works. - Historically, cooperative concurrency only worked on a single processors, because running threads in parallel would break the atomicity of sequential blocks (between invocations of p/y/w). - Researchers have been poring tons of effort into efficiently running blocks of code atomically. - Hey, we can do parallel cooperative multithreading! The paper discusses implementations in Lua, C++ and C, but I think Haskell could be an awesome substrate for such a framework. Has anyone thought about this? Ben

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Re: [Haskell-cafe] [Haskell-beginners] Most C++ compilers will not optimize x^2.0 as x*x but instead will do an expensive ...
by Tom Murphy 24 May '12

24 May '12

Just to play devil's advocate, if you look back at the list, KC has written a lot of helpful and informative messages in the past. Tom On 5/23/12, Bardur Arantsson <spam(a)scientician.net> wrote: > On 05/24/2012 04:31 AM, Brandon Allbery wrote: >> On Wed, May 23, 2012 at 10:19 PM, Bardur Arantsson >> <spam(a)scientician.net>wrote: >> >>> This has come up before -- this KC person probably has a broken mail >>> client which doesn't set appropriate References headers. >>> >> >> That, however, ignores the rest of it; the lack of references in this >> case >> forms a pattern with the other things I noted, in that a conversation is >> apparently being held in the form of single observations emitted at the >> point of observation instead of being collected and presented *as* a >> conversation. >> > > Right. I was actually just about to respond to (only) KC in person, but > perhaps unwisely, decided to "hijack" your response to add a little > explanation for everyone. > > You are of course right that not quoting context and just randomly > spewing out small bits of text is not really suitable for a mailing list. > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

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Most C++ compilers will not optimize x^2.0 as x*x but instead will do an expensive ...
by KC 24 May '12

24 May '12

exponentiation and logarithm. So, I believe this C++ versus Haskell versus (your language of choice) is a Penn & Teller misdirection. Whereas, another level of indirection solves everything. -- -- Regards, KC

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Haskell Weekly News: Issue 228
by Daniel Santa Cruz 24 May '12

24 May '12

Welcome to issue 228 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of May 13 to 19, 2012. Announcements Stefan Wehr issued a call for presentations for the Commercial Users of Functional Programmers (CUFP 2012) conference to be held in Copenhagen, Denmark during September 13 to 15, 2012. [1] http://goo.gl/Rl1T9 Quotes of the Week * hpc measures understanding in angstroms per (second * liter) ^ (1/15) Top Reddit Stories * ANNOUNCE: forthcoming O'Reilly book on Parallel and Concurrent Haskell Domain: haskell.org, Score: 101, Comments: 13 On Reddit: [2] http://goo.gl/UY9L6 Original: [3] http://goo.gl/oqLo6 * GHC 7.6 will have support for deferred type errors Domain: hackage.haskell.org, Score: 55, Comments: 4 On Reddit: [4] http://goo.gl/sPknX Original: [5] http://goo.gl/ieM9s * What happens when you mix three research programming languages together Domain: blog.ezyang.com, Score: 41, Comments: 7 On Reddit: [6] http://goo.gl/ujmF1 Original: [7] http://goo.gl/jME65 * Reactive-Banana 0.6: FRP in Haskell, supports both WxHaskell and generation to Javascript! Domain: apfelmus.nfshost.com, Score: 39, Comments: 3 On Reddit: [8] http://goo.gl/XrFxl Original: [9] http://goo.gl/Uanl6 * Keter: It's Alive! (Web app deployment system for Haskell) Domain: yesodweb.com, Score: 35, Comments: 6 On Reddit: [10] http://goo.gl/RyuLI Original: [11] http://goo.gl/EVcDT * Reasoning about Stream Processing with Effects Domain: personal.cis.strath.ac.uk, Score: 26, Comments: 26 On Reddit: [12] http://goo.gl/TI8bj Original: [13] http://goo.gl/n2rS0 * Patches for the new GHC-iPhone (v4) builds but needs a little more debugging Domain: hip-to-be-square.com, Score: 26, Comments: 27 On Reddit: [14] http://goo.gl/7QvqK Original: [15] http://goo.gl/zLCzr * Commutative Edits and Exceptions -- Categorically Dual(ish)? Domain: bosker.wordpress.com, Score: 24, Comments: 2 On Reddit: [16] http://goo.gl/1quI5 Original: [17] http://goo.gl/ET2ym * Parallel Haskell Digest 10: SPJ on Cloud Haskell, concurrent channels, meta-par, Accelerate, and StackOverflow'ing like crazy Domain: well-typed.com, Score: 21, Comments: 3 On Reddit: [18] http://goo.gl/z1UiV Original: [19] http://goo.gl/iPiAx * Interactive Programming with LambdaCalc Domain: dynamicaspects.org, Score: 20, Comments: 6 On Reddit: [20] http://goo.gl/IeEis Original: [21] http://goo.gl/n3Bqo * BitParser - RFC on a new parsing library for parsing data at the bit level. Similar in functionality to Erlang's bit syntax Domain: github.com, Score: 18, Comments: 8 On Reddit: [22] http://goo.gl/a7Qmq Original: [23] http://goo.gl/SL1u4 Top StackOverflow Questions * Haskell: Between a list and a tuple votes: 10, answers: 3 Read on SO: [24] http://goo.gl/6eiPg * Why is it so uncommon to use type signatures in where clauses? votes: 9, answers: 4 Read on SO: [25] http://goo.gl/ih3cK * Is there any good Haskell indentation script for Vim? votes: 9, answers: 2 Read on SO: [26] http://goo.gl/t1D3Y * Haskell “exceptions” votes: 8, answers: 2 Read on SO: [27] http://goo.gl/5WfTT * Is it possible to program and check invariants in haskell? votes: 8, answers: 3 Read on SO: [28] http://goo.gl/5wNE6 * The “handle” function and Real World Haskell votes: 7, answers: 2 Read on SO: [29] http://goo.gl/K8WK0 * Haskell: Flatten binary tree votes: 7, answers: 2 Read on SO: [30] http://goo.gl/eSokk * How can I uninstall a version of a Cabal package? votes: 6, answers: 1 Read on SO: [31] http://goo.gl/Gl1Mi * Fault tolerant JSON parsing votes: 6, answers: 1 Read on SO: [32] http://goo.gl/NBsVH * Can a Haskell or Haskell OS thread waiting on Network.Socket.accept not be killed on Windows? votes: 6, answers: 1 Read on SO: [33] http://goo.gl/8NHKB * efficient functional data structure for finite bijections votes: 6, answers: 2 Read on SO: [34] http://goo.gl/bXA1Ke Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.general/19157 2. http://www.haskell.org/pipermail/haskell/2012-May/023328.html 3. http://www.reddit.com/r/haskell/comments/trf72/announce_forthcoming_oreilly… 4. http://hackage.haskell.org/trac/ghc/wiki/Status/May12 5. http://www.reddit.com/r/haskell/comments/tm1mc/ghc_76_will_have_support_for… 6. http://blog.ezyang.com/2012/05/what-happens-when-you-mix-three-research-pro… 7. http://www.reddit.com/r/haskell/comments/tptuo/what_happens_when_you_mix_th… 8. http://apfelmus.nfshost.com/blog/2012/05/15-frp-banana-0-6.html 9. http://www.reddit.com/r/haskell/comments/toghk/reactivebanana_06_frp_in_has… 10. http://www.yesodweb.com/blog/2012/05/keter-its-alive 11. http://www.reddit.com/r/haskell/comments/tsyv5/keter_its_alive_web_app_depl… 12. https://personal.cis.strath.ac.uk/~raa/posts/2012-01-06-streams.html 13. http://www.reddit.com/r/haskell/comments/tm7ur/reasoning_about_stream_proce… 14. http://hip-to-be-square.com/~blackh/ghc-iphone-new/v4/ 15. http://www.reddit.com/r/haskell/comments/tnnqc/patches_for_the_new_ghciphon… 16. http://bosker.wordpress.com/2012/05/10/on-editing-text/ 17. http://www.reddit.com/r/haskell/comments/ts3sz/commutative_edits_and_except… 18. http://www.well-typed.com/blog/66 19. http://www.reddit.com/r/haskell/comments/tta8y/parallel_haskell_digest_10_s… 20. http://dynamicaspects.org/blog/2012/05/15/back-to-the-future/ 21. http://www.reddit.com/r/haskell/comments/tokmh/interactive_programming_with… 22. https://github.com/axman6/BitParser/tree/bs-state 23. http://www.reddit.com/r/haskell/comments/tmhca/bitparser_rfc_on_a_new_parsi… 24. http://stackoverflow.com/questions/10626943/haskell-between-a-list-and-a-tu… 25. http://stackoverflow.com/questions/10609936/why-is-it-so-uncommon-to-use-ty… 26. http://stackoverflow.com/questions/10663888/is-there-any-good-haskell-inden… 27. http://stackoverflow.com/questions/10611382/haskell-exceptions 28. http://stackoverflow.com/questions/10656402/is-it-possible-to-program-and-c… 29. http://stackoverflow.com/questions/10578572/the-handle-function-and-real-wo… 30. http://stackoverflow.com/questions/10592920/haskell-flatten-binary-tree 31. http://stackoverflow.com/questions/10576868/how-can-i-uninstall-a-version-o… 32. http://stackoverflow.com/questions/10604808/fault-tolerant-json-parsing 33. http://stackoverflow.com/questions/10608602/can-a-haskell-or-haskell-os-thr… 34. http://stackoverflow.com/questions/10669217/efficient-functional-data-struc…

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