- Haskell-Cafe - Haskell.org

How to make callCC more dynamic
by bob zhang 25 Aug '11

25 Aug '11

Hi, all I thought the right type for ContT should be newtype ContT m a = ContT {runContT :: forall r. (a-> m r) -> m r} and other control operators shift :: Monad m => (forall r . (a-> ContT m r) -> ContT m r) -> ContT m a reset :: Monad m => ContT m a -> ContT m a callCC :: ((a-> (forall r . ContT m r)) -> ContT m a) -> ContT m a unfortunately, I can not make callCC type check, and don't know how to do it. I managed to make shift, reset type check reset :: Monad m => ContT m a -> ContT m a reset e = ContT $ \ k -> runContT e return >>= k shift :: Monad m => (forall r . (a-> ContT m r) -> ContT m r) -> ContT m a shift e = ContT $ \ (k :: a -> m r) -> runContT ((e $ \ v -> ContT $ \c -> k v >>= c) :: ContT m r) return but still, I cann't use shift, reset in recursive jumpings like this? newtype H r m = H (H r m -> ContT m r) unH (H x) = x test = flip runContT return $ reset $ do jump <- shift (\f -> f (H f)) lift . print $ "hello" unH jump jump Have anyone tried this before? Best, bob

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Re: [Haskell-cafe] Lifting an enumerator
by John Lato 25 Aug '11

25 Aug '11

> Message: 17 > Date: Wed, 24 Aug 2011 17:02:49 +0300 > From: Michael Snoyman <michael(a)snoyman.com> > Subject: [Haskell-cafe] Lifting an enumerator > To: Haskell Cafe <haskell-cafe(a)haskell.org> > Cc: John Millikin <jmillikin(a)gmail.com> > Message-ID: > <CAKA2JgKF0dN4N8Ge1_q-zemLZM93bWg_FjMtbaZGzrC2Gqnq4w(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > Hi all, > > Max asked earlier[1] how to create a new instance of a class in > Persistent using a monad transformer. Without getting into the > specific details of persistent, I wanted to pose a question based on a > much more general question: how can we lift the inner monad of an > enumerator? We can easily do so for an Iteratee[2], but there is > nothing to allow it for an Enumerator. > > At first glance, this problem looks very similar to the shortcomings > of MonadIO when dealing with callbacks. In that case, you cannot use > liftIO on a function that takes an `IO a` as a parameter. A solution > to this issue is monad-control[3], which can be used to allow > exception catching, memory allocation, etc. > > So I'm wondering: can we come up with a similar solution to this issue > with enumerators? I have a working solution for the specific case of > the ErrorT monad[4], but it would be great to be able to generalize > it. Bonus points if we could express this in terms of the typeclasses > already provided by monad-control. Based upon a similar problem I worked on in the past, I suspect this isn't possible in the general case, at least not safely. That is, any implementation (presuming it's possible at all) would violate the resource guarantees enumerators typically provide (an inner MonadCont is able to do this). Unfortunately I don't have a proof, so I'm open to counter-examples ;-) John L.

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haskell or lazy functional as procedure language
by Permjacov Evgeniy 25 Aug '11

25 Aug '11

Ok, I know, I want something strange. But consider situation, when one is starting a project and finds, that he need s 1) ACID relational storage 2) Power of good RDBMS system (postgresql for example) 3) Power of some very hight level language and compiled (haskell for example) for stored procedures 4) And all data processing MUST be performed inside RDBMS 5) And does not have enough money to by Oracle ore other commercial RDBMS. I already considered using ghc with postgresql. It could be very, very good pair, but.... ghc runtime cannot be re-initialized, and reqular way for stored procedures in postgresql is calling function from shared object (meaning, I have to shut down ghc runtime each time stored procedure ended). What other options do you see?

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Re: [Haskell-cafe] Existential question
by oleg＠okmij.org 25 Aug '11

25 Aug '11

> -- Is there a way to make this one work also? > data Kl i o = forall s. Kl (i -> s -> (s, o)) > iso :: (i -> Kl () o) -> Kl i o > iso f = Kl $ \i s -> (\(Kl kl) -> kl () s) (f i) Yes, if you move the quantifier: type Kl i o = i -> Kl1 o data Kl1 o = forall s. Kl1 (s -> (s,o)) iso :: (i -> Kl () o) -> Kl i o iso f = \i -> f i () iso1 :: Kl i o -> (i -> Kl () o) iso1 f = \i -> (\() -> f i) I'm not sure if it helps in the long run: the original Kl and mine Kl1 are useless. Suppose we have the value kl1 :: Kl Int Bool with the original declaration of Kl: data Kl i o = forall s. Kl (i -> s -> (s, o)) Now, what we can do with kl1? We can feed it an integer, say 1, and obtain function f of the type s -> (s,Bool) for an _unknown_ type s. Informally, that type is different from any concrete type. We can never find the Bool result produced by that function since we can never have any concrete value s. The only applications of f that will type check are \s -> f s f undefined both of which are useless to obtain f's result.

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Issue with lhs2TeX 1.17
by Arnaud Bailly 25 Aug '11

25 Aug '11

Hello, I recently reinstalled (using cabal) lhs2TeX v 1.17 (Haskell Platform installation 2011.2.0, Windows XP SP2, MiKTeX 2.8) and I cannot run latex anymore on the .tex files produced. Any clues about this ? Did I forgot to install some package ? This is not clear from the documentation which seems to imply cabal install does the right thing (TM). Thanks in advance for you help. Arnaud PS: Here is the beginning of the compilation output for latex Running `LaTeX' on `foo' with ``latex -interaction=nonstopmode "\input" "foo.tex"'' This is pdfTeX, Version 3.1415926-1.40.10 (MiKTeX 2.8) entering extended mode LaTeX2e <2009/09/24> Babel <v3.8l> and hyphenation patterns for english, dumylang, nohyphenation, german, ngerman, german-x-2009-06-19, ngerman-x-2009-06-19, french, loaded. (d:\projets\foo.tex ("D:\Program Files\MiKTeX 2.8\tex\latex\base\article.cls" Document Class: article 2007/10/19 v1.4h Standard LaTeX document class ("D:\Program Files\MiKTeX 2.8\tex\latex\base\size11.clo")) ("D:\Program Files\MiKTeX 2.8\tex\latex\ams\math\amstext.sty" ("D:\Program Files\MiKTeX 2.8\tex\latex\ams\math\amsgen.sty")) ("D:\Program Files\MiKTeX 2.8\tex\latex\amsfonts\amssymb.sty" ("D:\Program Files\MiKTeX 2.8\tex\latex\amsfonts\amsfonts.sty")) ("C:\Documents and Settings\abailly\Application Data\MiKTeX\2.8\tex\latex\stmaryrd\stmaryrd.sty") ("C:\Documents and Settings\abailly\Application Data\MiKTeX\2.8\tex\latex\polytable\polytable.sty" ("C:\Documents and Settings\abailly\Application Data\MiKTeX\2.8\tex\latex\lazylist\lazylist.sty") ("D:\Program Files\MiKTeX 2.8\tex\latex\tools\array.sty")) ("C:\Documents and Settings\abailly\Application Data\MiKTeX\2.8\tex\latex\a4wide\a4wide.sty" ("C:\Documents and Settings\abailly\Application Data\MiKTeX\2.8\tex\latex\ntgclass\a4.sty")) (d:\projets\foo.aux) ("D:\Program Files\MiKTeX 2.8\tex\latex\amsfonts\umsa.fd") ("D:\Program Files\MiKTeX 2.8\tex\latex\amsfonts\umsb.fd") ("C:\Documents and Settings\abailly\Application Data\MiKTeX\2.8\tex\latex\stmaryrd\ustmry.fd") Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 $[][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][][] Overfull \hbox (25.83325pt too wide) in paragraph at lines 116--186 [][][][][][]$ ("D:\Program Files\MiKTeX 2.8\tex\latex\base\omscmr.fd") ! Missing $ inserted. <inserted text> $ l.609 \end{pboxed} ! Extra }, or forgotten $. <recently read> \egroup l.609 \end{pboxed} ! Missing $ inserted. <inserted text> $ l.609 \end{pboxed} ! Extra }, or forgotten $. <recently read> \egroup l.609 \end{pboxed} ! Missing $ inserted. <inserted text> $ l.609 \end{pboxed} ! Extra }, or forgotten $. \mbox #1->\leavevmode \hbox {#1} l.609 \end{pboxed} ! Missing $ inserted. <inserted text> $ l.609 \end{pboxed} ! LaTeX Error: \mathit allowed only in math mode.

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(Repa) hFlush: illegal operation
by Michael Orlitzky 25 Aug '11

25 Aug '11

I'm using Repa to process a ton of MRI data. The basic process is, * Read in the data * Create a big 'ol data structure (grid) from it * Compute the output in parallel using 'traverse' * Write the output to file However, during the last step, I'm getting, $ ./bin/spline3 +RTS -N4 spline3: output.txt: hFlush: illegal operation (handle is closed) Here's the entirety of my Main.hs: > module Main > where > > import Data.Array.Repa ( > DIM3, > Z(..), > (:.)(..), > ) > > import Values > import Grid(make_grid, zoom) > > mri_shape :: DIM3 > mri_shape = (Z :. 256 :. 256 :. 109) > > main :: IO () > main = do > mridata <- read_values_3d mri_shape "./data/mridata.txt" > let g = make_grid 1 mridata > let output = zoom g 1 > write_values_1d output "output.txt" And the two functions used from the Values module, > type Values1D = Array DIM1 Double > type Values3D = Array DIM3 Double > > read_values_1d :: FilePath -> IO Values1D > read_values_1d path = readVectorFromTextFile path > > > read_values_3d :: DIM3 -> FilePath -> IO Values3D > read_values_3d sh path = do > one_d <- read_values_1d path > return $ reshape sh one_d > > > write_values_1d :: Values3D -> FilePath -> IO () > write_values_1d v3d path = do > let size3d = size $ extent v3d > let shape1d = (Z :. size3d) > let v1d = reshape shape1d v3d > writeVectorToTextFile v1d path Am I doing something obviously wrong here? It takes about 45 minutes to run up to the point of failure, so I haven't attempted much trial & error. Packages: * ghc-7.2.1 * repa-2.1.1.6 * repa-io-2.1.0.1

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Using -> as both type and value constructor
by Armando Blancas 25 Aug '11

25 Aug '11

Studying the paper *A Simple Implementation for Priority Search Queues*, by Ralf Hinze, I came across the following syntax that I didn't understand and I couldn't use in GHCi 7.0.3 for defining a binding data type (page 3): Bindings are represented by the following data type: *data k -> p = k -> p* key :: (k -> p) -> k key (k -> p) = k prio :: (k -> p) -> p prio (k -> p) = p Note that we use '->' both as a type constructor and value constructor. [...] data PSQ k p The following page has these value constructors: *0 :: PSQ k p* *{.} :: (k -> p) -> PSQ k p* The paper says that's Haskell '98 code. I'll appreciate info on what kind of data declaration those are and any pointers to related documentation. Or maybe those are deprecated features. I'm stuck in this part. -armando www.cs.ox.ac.uk/people/ralf.hinze/talks/ICFP01.pdf

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Haskell Weekly News: Issue 196
by Daniel Santa Cruz 25 Aug '11

25 Aug '11

Welcome to issue 196 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of August 14 to 20, 2011. [1] http://goo.gl/8hDku New and Updated Projects * graphziz (Update - Ivan Lazar Miljenovic) Wraper around Graphviz. [2] http://goo.gl/gg30u * TKYProf (New - Mitsutoshi Aoe) A web-based interactive visualizer for GHC time and allocation profiling. [3] http://goo.gl/Inqze Quotes of the Week * accel: next time I'll read a few research papers before I start trolling hakell * jmcarthur: your opinions all suck. i'm going back to #haskell (...) oh shit, i'm already there Top Reddit Stories * Haskell For Kids: Week 1 Domain: cdsmith.wordpress.com, Score: 37, Comments: 13 On Reddit: [4] http://goo.gl/RY6li Original: [5] http://goo.gl/azAVz * New IBM CPU has transactional memory built in Domain: nextbigfuture.com, Score: 35, Comments: 4 On Reddit: [6] http://goo.gl/cbjvN Original: [7] http://goo.gl/JOpWx * [GSoC] Text/UTF-8: Aftermath Domain: jaspervdj.be, Score: 32, Comments: 7 On Reddit: [8] http://goo.gl/MTHNy Original: [9] http://goo.gl/esQNO * CamHac post-hackathon report Domain: haskell.org, Score: 31, Comments: 8 On Reddit: [10] http://goo.gl/RHpfF Original: [11] http://goo.gl/GkVaa * GHC 7.4 won't have top-level MonoPatBinds anymore! Domain: hackage.haskell.org, Score: 25, Comments: 5 On Reddit: [12] http://goo.gl/bS1VL Original: [13] http://goo.gl/SK2Tz * Ending GSoC (cabal repl) Domain: lambdasandwich.blogspot.com, Score: 25, Comments: 1 On Reddit: [14] http://goo.gl/Rrzmj Original: [15] http://goo.gl/chb8A * If one of Haskell's goals is concurrency, then why is it based on the λ-calculus and not on a process calculus? Domain: en.wikipedia.org, Score: 23, Comments: 39 On Reddit: [16] http://goo.gl/Qjcgc Original: [17] http://goo.gl/ZSwt1 * University of Reddit Class: "Fixing Imperative Programmers" Domain: reddit.com, Score: 22, Comments: 2 On Reddit: [18] http://goo.gl/1hYlf Original: [19] http://goo.gl/26Qxd * A Command Line Reddit Viewer written in Haskell : programming (xpost : /programming ) Domain: reddit.com, Score: 22, Comments: 0 On Reddit: [20] http://goo.gl/bT9q8 Original: [21] http://goo.gl/VmVfA * Web-Based Environment for Gloss (Early Alpha: READ WARNINGS!) Domain: cdsmith.wordpress.com, Score: 21, Comments: 1 On Reddit: [22] http://goo.gl/e4Tjo Original: [23] http://goo.gl/wVA9H Top StackOverflow Questions * What are practical uses of applicative style? votes: 21, answers: 9 Read on SO: [24] http://goo.gl/Ms6fs * Functional equivalent of decorator pattern? votes: 20, answers: 7 Read on SO: [25] http://goo.gl/o8FhO * Why do 3 and x (which was assigned 3) have different inferred types in Haskell? votes: 18, answers: 2 Read on SO: [26] http://goo.gl/cm9av * Haskell tuple constructor (GHC) and the separation between a language and its implementation votes: 15, answers: 1 Read on SO: [27] http://goo.gl/tFvtk * Polymorphism within higher-order functions? votes: 13, answers: 1 Read on SO: [28] http://goo.gl/hNvkE * What happens to missing type variables in Church-style core? votes: 12, answers: 2 Read on SO: [29] http://goo.gl/kb2TM * Trying to implement Data.Either votes: 12, answers: 1 Read on SO: [30] http://goo.gl/lguDh * How can I make file I/O more transactional? votes: 11, answers: 2 Read on SO: [31] http://goo.gl/BjGFV * Trying to understand the types produced by monad transformers votes: 11, answers: 2 Read on SO: [32] http://goo.gl/YHsRQ * Why doesn't Safe Haskell support Template Haskell? votes: 11, answers: 1 Read on SO: [33] http://goo.gl/E1NCS 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://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18888 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/91743 4. http://cdsmith.wordpress.com/2011/08/16/haskell-for-kids-week-1/ 5. http://www.reddit.com/r/haskell/comments/jlrva/haskell_for_kids_week_1/ 6. http://nextbigfuture.com/2011/08/ibm-plants-transactional-memory-in-cpu.htm… 7. http://www.reddit.com/r/haskell/comments/jperf/new_ibm_cpu_has_transactiona… 8. http://jaspervdj.be/posts/2011-08-19-text-utf8-the-aftermath.html 9. http://www.reddit.com/r/haskell/comments/jo6cd/gsoc_textutf8_aftermath/ 10. http://www.haskell.org/haskellwiki/CamHac/PostHackathonReport 11. http://www.reddit.com/r/haskell/comments/jj5lw/camhac_posthackathon_report/ 12. http://hackage.haskell.org/trac/ghc/ticket/2357#comment:11 13. http://www.reddit.com/r/haskell/comments/jkeqp/ghc_74_wont_have_toplevel_mo… 14. http://lambdasandwich.blogspot.com/2011/08/ending-gsoc.html 15. http://www.reddit.com/r/haskell/comments/jn8nl/ending_gsoc_cabal_repl/ 16. http://en.wikipedia.org/wiki/Process_calculus 17. http://www.reddit.com/r/haskell/comments/jlipx/if_one_of_haskells_goals_is_… 18. http://www.reddit.com/r/uorfip/ 19. http://www.reddit.com/r/haskell/comments/jok49/university_of_reddit_class_f… 20. http://www.reddit.com/r/programming/comments/jorky/a_command_line_reddit_vi… 21. http://www.reddit.com/r/haskell/comments/jouql/a_command_line_reddit_viewer… 22. http://cdsmith.wordpress.com/2011/08/14/haskell-for-kids-a-web-based-enviro… 23. http://www.reddit.com/r/haskell/comments/jis8g/webbased_environment_for_glo… 24. http://stackoverflow.com/questions/7103864/what-are-practical-uses-of-appli… 25. http://stackoverflow.com/questions/7064389/functional-equivalent-of-decorat… 26. http://stackoverflow.com/questions/7055146/why-do-3-and-x-which-was-assigne… 27. http://stackoverflow.com/questions/7086668/haskell-tuple-constructor-ghc-an… 28. http://stackoverflow.com/questions/7061538/polymorphism-within-higher-order… 29. http://stackoverflow.com/questions/7076517/what-happens-to-missing-type-var… 30. http://stackoverflow.com/questions/7121257/trying-to-implement-data-either 31. http://stackoverflow.com/questions/7059223/how-can-i-make-file-i-o-more-tra… 32. http://stackoverflow.com/questions/7088085/trying-to-understand-the-types-p… 33. http://stackoverflow.com/questions/7107308/why-doesnt-safe-haskell-support-…

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Lifting an enumerator
by Michael Snoyman 24 Aug '11

24 Aug '11

Hi all, Max asked earlier[1] how to create a new instance of a class in Persistent using a monad transformer. Without getting into the specific details of persistent, I wanted to pose a question based on a much more general question: how can we lift the inner monad of an enumerator? We can easily do so for an Iteratee[2], but there is nothing to allow it for an Enumerator. At first glance, this problem looks very similar to the shortcomings of MonadIO when dealing with callbacks. In that case, you cannot use liftIO on a function that takes an `IO a` as a parameter. A solution to this issue is monad-control[3], which can be used to allow exception catching, memory allocation, etc. So I'm wondering: can we come up with a similar solution to this issue with enumerators? I have a working solution for the specific case of the ErrorT monad[4], but it would be great to be able to generalize it. Bonus points if we could express this in terms of the typeclasses already provided by monad-control. Michael [1] http://groups.google.com/group/yesodweb/browse_thread/thread/be2a77217a7f33… [2] http://hackage.haskell.org/packages/archive/enumerator/0.4.14/doc/html/Data… [3] http://hackage.haskell.org/package/monad-control [4] https://gist.github.com/1168128

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why the name lambda calculus?
by C K Kashyap 24 Aug '11

24 Aug '11

Hi, Can someone please tell me what is the root of the name lambda calculus? Is it just because of the symbol lambda that is used? Why not alpha or beta calculus? Regards, Kashyap

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