August 2011 - Haskell-Cafe

Haddock chokes on function arguments
by Arnaud Bailly 25 Aug '11

25 Aug '11

Hello, while trying to generate Haddock documentation (Haddock version 2.9.2, (c) Simon Marlow 2006), it chokes on the following fragment -- |Apply a given function over all elements of a list and select one of the -- results. selector :: (Arbitrary b) => [a] -> -- ^ Elements to select from (a -> b) -> -- ^ Constructor to apply to selected element Gen b selector list ctor = oneof (map (return . ctor) list) with the following error message: ' parse error on input `-- ^ Elements to select from I am puzzled as this is exactly how arguments are supposed to be documented, according to http://www.haskell.org/haddock/doc/html/ch03s02.html#id565220 Any clues ? Arnaud

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PhD/PostDoc opening in programming techniques and technologies
by Ralf Laemmel 25 Aug '11

25 Aug '11

PhD/PostDoc opening in programming techniques and technologies in the Software Languages Team [1] in Koblenz. Applicants can safely assume that functional programming plays a huge role in this. The application deadline is 31 Aug. A German description of formalities is available [2]. One essential component of the position is research, teaching as well as development efforts on 101companies [3]. Regards, Ralf [4] [1] http://softlang.wikidot.com/ [2] http://softlang.uni-koblenz.de/docs/Ausschreibungstext_WiMi_HPII-Laemmel.pdf [3] http://101companies.org/ [4] http://softlang.wikidot.com/rlaemmel:home

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ANNOUNCE: HakkuTaikai: Haskell Hackathon in Tokyo, 25th September 2011
by Liyang HU 25 Aug '11

25 Aug '11

You may have heard the rumours, and I can confirm that it's true: HakkuTaikai is happening! After spending a long week at ICFP 2011 talking about Haskell, why not relax by actually hacking some Haskell? Full details on the wiki page: http://www.haskell.org/haskellwiki/HakkuTaikai When: Sunday, 25th September 2011 Where: Room 1208 of the NII building; the same location as ICFP. Registration is free! See the wiki page above for details. Feel free to email or IRC or twit me with any questions. Many thanks to http://www.nii.ac.jp/ for hosting the event! Cheers, /Liyang

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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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