- Haskell-Cafe - Haskell.org

Fixed point newtype confusion
by Sebastien Zany 10 May '12

10 May '12

Hi, Suppose I have the following types: > data Expr expr = Lit Nat | Add (expr, expr) > newtype Fix f = Fix {unFix :: f (Fix f)} I can construct a sample term: > term :: Expr (Expr (Expr expr)) > term = Add (Lit 1, Add (Lit 2, Lit 3)) But isn't quite what I need. What I really need is: > term' :: Fix Expr > term' = Fix . Add $ (Fix . Lit $ 1, Fix . Add $ (Fix . Lit $ 2, Fix . Lit $ 3)) I feel like there's a stupidly simple way to automatically produce term' from term, but I'm not seeing it. Any ideas? Best, Sebastien

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Handcranked install of ghc / cabal-install and warning from ghc-pkg check
by Graham Berks 09 May '12

09 May '12

Have installed 7.4.1 ghc and 0.14.0 cabal-install and then did alex, happy & haddock. When I do a ghc-pkg check I get $ ghc-pkg check Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/random-1.0.1.1/html/random.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/random-1.0.1.1/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/zlib-0.5.3.3/html/zlib.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/zlib-0.5.3.3/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/HTTP-4000.2.3/html/HTTP.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/HTTP-4000.2.3/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/network-2.3.0.11/html/network.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/network-2.3.0.11/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/parsec-3.1.2/html/parsec.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/parsec-3.1.2/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/text-0.11.2.0/html/text.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/text-0.11.2.0/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/mtl-2.1/html/mtl.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/mtl-2.1/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/transformers-0.3.0.0/html/transformers.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/transformers-0.3.0.0/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/haddock-2.10.0/html/haddock.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/haddock-2.10.0/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/xhtml-3000.2.0.5/html/xhtml.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/xhtml-3000.2.0.5/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/ghc-paths-0.1.0.8/html/ghc-paths.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/ghc-paths-0.1.0.8/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/mtl-2.1.1/html/mtl.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/mtl-2.1.1/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/transformers-0.3.0.0/html/transformers.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/transformers-0.3.0.0/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/QuickCheck-2.4.2/html/QuickCheck.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/QuickCheck-2.4.2/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/random-1.0.1.1/html/random.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/random-1.0.1.1/html doesn't exist or isn't a directory Any suggestions to fix this ? Thanks

1 2

Haskell Hackathon in Munich
by Heinrich Hördegen 09 May '12

09 May '12

Dear all, this is a reminder of our local haskell Hackathon in Munich, Germany, which will be held on Saturday, the 12th of May. If you plan to join, check out the details at: http://www.haskell-munich.de/dates or contact me. We hope to see you on Saturday, Heinrich

1 0

cabal issue
by Graham Berks 09 May '12

09 May '12

Given that I have the dependant packages already installed when I do $ cabal install --dry-run --global buildwrapper Resolving dependencies... In order, the following would be installed: mtl-2.1.1 (new version) aeson-0.6.0.2 (reinstall) changes: mtl-2.0.1.0 -> 2.1.1 parsec-3.1.2 (reinstall) changes: mtl-2.0.1.0 -> 2.1.1 regex-base-0.93.2 (reinstall) changes: mtl-2.0.1.0 -> 2.1.1 regex-tdfa-1.1.8 (reinstall) changes: mtl-2.0.1.0 -> 2.1.1 buildwrapper-0.5.2 (new package) And that build wrapper doesn't have a defined dependency version number for MTL can someone tell me why cabal is trying to install a new version of MTL ? Thanks

1 1

Annotaing abstract syntax trees
by José Romildo Malaquias 09 May '12

09 May '12

Hello. I am reading Martijn's MSc Thesis "Generic Selections of Subexpressions", where one can found some discussions about annotating abstract syntax trees (AST). In order to follow the discussion I wrote the attached Haskell program, which is an interpreter for an simple typed expression language. The Annotations package is used. The expression pattern is represented by a single recursive data type. Annotations are used for positions in the input source, and also for the type of expressions and subexpressions. I would like somebody to review the code and comment on it, as I am not sure I am using the concepts right. Also I would like the type checker to produce an expression annotated with both positions and calculated types. Currently it discards the position annotations. Any sugestions on how to modify it is welcome. Next step is adding a new form of expression to introduce local variable bindings. After that I want to start working with ASTs represented by mutually recursive data types. Then I will need multirec... Romildo

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Re: [Haskell-cafe] Un-memoization
by oleg＠okmij.org 09 May '12

09 May '12

Victor Miller wrote: > I was writing a Haskell program which builds a large labeled binary tree > and then does some processing of it, which is fold-like. In the actual > application that I have in mind the tree will be *huge*. If the whole tree > is kept in memory it would probably take up 100's of gigabytes. Because of > the pattern of processing the tree, it occurred to me that it might be > better (cause much less paging) if some large subtrees could be replaced by > thunks which can either recalculate the subtree as needed, or write out the > subtree, get rid of the references to it (so it can be garbage collected) > and then read back in (perhaps in pieces) as needed. This could be fairly > cleanly expressed monadically. So does anyone know if someone has created > something like this? Yes. I had faced essentially the same problem. It is indeed tricky to prevent memoization: GHC is very good at it. The following article explains the solution: ``Preventing memoization in (AI) search problems'' http://okmij.org/ftp/Haskell/index.html#memo-off

1 0

[Q] multiparam class undecidable types - how to get this example to typecheck?
by Matthias Fischmann 09 May '12

09 May '12

Hi everybody, I am torturing the ghc type inference extensions, and I think i managed to break something, either in ghc or (more likely) in my brain: | {-# language FlexibleInstances, MultiParamTypeClasses, UndecidableInstances #-} | | module Main | where | | class Table table cell where | toLists :: table -> [[cell]] | fromLists :: [[cell]] -> table | | instance Table [[cell]] cell where | toLists = id | fromLists = id | | -- why i am using a class (not relevant for my question) - | -- data FastTable a = FastTable ... | -- instance Table (FastTable a) a where ... | | instance (Table a c, Show c) => Show a where | showsPrec p t = showParen (p > 10) $ showString "fromLists " . shows (toLists t) ghc 7.0.3 sais: | Overlapping instances for Show [[cell0]] | arising from a use of `shows' | Matching instances: | instance Show a => Show [a] -- Defined in GHC.Show | instance (Table a c, Show c) => Show a | -- Defined at /home/mf/tmp/z.hs:18:10-38 | In the second argument of `(.)', namely `shows (toLists t)' I would have thought that there is on overlap: the instance in my code above defines how to show a table if the cell is showable; the (allegedly) overlapping instance defines how to show lists of alpha if alpha is showable. So I was expecting the reduction to go something like: . table ~> [[cell]] (via whichever instance of class Table applies) . [[cell]] ~> [cell] (via instance "Show a => Show [a]") . [cell] ~> cell (via instance "Show a => Show [a]") But it gets worse. I decided to ignore the problem for now and be happy with at least showing the first cell in the upper left corner of the table: | instance (Table a c, Show c) => Show a where | showsPrec p t = showParen (p > 10) $ showString "fromLists " . shows (head . head $ toLists t) to that, ghc 7.0.3 sais: | /home/mf/tmp/z.hs:19:66: | Context reduction stack overflow; size = 21 | Use -fcontext-stack=N to increase stack size to N | $dShow :: Show c19 | $dShow :: Show c18 | [...] shouldn't type inference be able to determine that | ((head . head $ toLists t) :: c and use the instance guaranteed by the instance declaration's class constraint (Show c)? I suspect that there should be a type annotation that would make this approach possible, but even then I am curious what this behavior means. Should I upgrade to a more grown-up release of ghc? Any hints or pointers appreciated. (: Thanks, Cheers, Matthias

1 0

LLVM M.D. source code?
by Greg Fitzgerald 09 May '12

09 May '12

A paper, LLVM M.D.: A Denotational Translation Validator, mentions the source code for LLVM M.D is available here: http://llvm-md.seas.harvard.edu/ I can't seem to spot it there or on Hackage. Anyone seen it? Thanks, Greg

1 0

Unit and pair
by MigMit 09 May '12

09 May '12

Hi café, a quick question. Is there a somewhat standard class like this: class Something c where unit :: c () () pair :: c x y -> c u v -> c (x, u) (y, v) ? I'm using it heavily in my current project, but I don't want to repeat somebody else's work, and it seems general enough to be defined somewhere; but my quick search on Hackage didn't reveal anything. I know about arrows; this, however, is something more general, and it's instances aren't always arrows.

4 6

bytestring, array and Safe Haskell
by Francesco Mazzoli 08 May '12

08 May '12

Why are http://hackage.haskell.org/packages/archive/bytestring/0.9.2.1/doc/html/Dat… and http://hackage.haskell.org/packages/archive/array/0.4.0.0/doc/html/Data-Arr… Safe-inferred? The first one uses inlinePerformIO, so it clearly shouldn't be marked as Safe. Maybe Safe Haskell doesn't check that function? The second is a bit messier since it uses unboxed types and primitive operations... But they clearly should be marked as Unsafe, and it surprises me that Safe Haskell is that relaxed when checking for safe functions. Francesco.

2 2

Can Haskell outperform C++?
by Isaac Gouy 08 May '12

08 May '12

2012/5/8 Silvio Frischknecht >Also I challenge anyone to improve one of the haskell programs there. >It'd be cool if we could make haskell get a higher rank. I recently >managed to improve the Fasta algorithm, but not by much. Also I think >the benchmarks don't use llvm flag. It says somewhere that they don't >measure llvm because they don't have time. But I think they are >refering to clang. So maybe someone should tell them to turn on the >llvm flag since it makes a lot of haskell programs faster. Several GHC versions have come and gone since the Haskell benchmarks game programs were written, so perhaps it is time that a dozen new programs were written to replace those old programs - new programs that take advantage of GHC 7.4.1 and -llvm.

1 0

ANN: hspec-1.1.0
by Simon Hengel 07 May '12

07 May '12

Hi, I'm glad to announce a new release of hspec[1]. Detailed release notes are at [2]. Introductory documentation is at [3]. Cheers, Simon [1] http://hackage.haskell.org/package/hspec-1.1.0 [2] https://github.com/hspec/hspec/blob/master/CHANGES.markdown [3] http://hspec.github.com/

1 0

correction
by Serge D. Mechveliani 07 May '12

07 May '12

Correction to my last letter: Run on the second terminal > ./iface --> Run on the second terminal > ./a.out ------ Sergei mechvel(a)botik.ru

1 0

Haskell-Type-Exts -- Typechecker as a Library
by Shayan Najd Javadipour 07 May '12

07 May '12

Hello Haskell Cafe! Recently, I got my proposal “Haskell-Type-Exts”--submitted to haskell community--accepted for Google SoC 2012! I have the luxury (thanks to Haskell.org GSoC committee) to develop a typechecker[1] for ASTs extracted by HSE[2] during this summer. HSE is one of the most widely used (if not the most popular) standalone parsers for Haskell 2010 plus a wide variety of extensions. HSE is written (and maintained) by Niklas Broberg and he is also mentoring this project. To be realistic, it is a rather challenging project and support of the community is vitally important to achieve the goals. I have a weblog accessible at http://cleantypecheck.wordpress.com/ dedicated to this topic. I plan to use it as the main media to communicate with people interested. Also, you can contact me directly through [shayan at chalmers d0t se]. If you see yourself as a potential user of the tool or by any reason you are interested , I'd highly appreciate if you would take a look at the following post and leave your comment/suggestions: http://cleantypecheck.wordpress.com/2012/05/03/inquiry/ Thanks! /Shayan [1] http://hackage.haskell.org/package/haskell-type-exts [2] http://hackage.haskell.org/package/haskell-src-exts

1 0

MonadError vs Control.Exception
by Станислав Черничкин 07 May '12

07 May '12

Hi, guys, I'm interested in best practices in using of each approach. Personally I like MonadError because it is more explicit and Control.Exception-s becomes really ugly in complex scenarios. Here an example to illustrate my idea: http://hackage.haskell.org/packages/archive/network-conduit/0.4.0/doc/html/…. All network-conduit functions can raise IOException because of Socket-related stuff (and actually entire network-conduit is written to deal with IOException correctry) + Bind-port can raise ErrorCall (came from error "bindPort: addrs is empty"). User has to deal with both, but have no evidence on none of them from type signatures or documentation. Moreover, ErrorCall in this case can be caused by IOException (we try to bind only available address, we get IOException exception, we have no more address to try and raise ErrorCall), and I really don’t understand, why it should be treated in a different way. So, I hate exceptions, I blame it, I think exceptions is junk came from OO-world and horrible languages like C# or even more horrible like Java, and it should be wiped out from Haskell with fire. But it is only my humble opinion. Reality shows that Haskell provides extensive support for exceptions, it included in base libraries and everyone uses it, but MonadError is modest part of mtl, and don’t even have “bracket” and other useful functions. That is why I’m really interested in any your ideas and experience on using these approaches. Cheers, Stasik.

3 3

Learn you
by Kazu Yamamoto 06 May '12

06 May '12

Hello cafe, Translating "Learn You a Haskell for Great Good" into Japanese was finished and will be published on 22 May. I guess it's worth watching its cover page: http://www.amazon.co.jp/%E3%81%99%E3%81%94%E3%81%84Haskell%E3%81%9F%E3%81%A… There are two translators: Tanaka is the author of "peggy" and Muranushi is the author of "Monadius". Regards, --Kazu

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Issues with installing packages
by Graham Berks 06 May '12

06 May '12

New to haskell and having issues resolving packages, have the following output below and can't seem to resolve it. Text seems to have diff hash dependancies when I try and instal a diff version of 0.11.2.0 etc Any ideas on what todo ?? Thanks $ ghc-pkg check There are problems in package scion-browser-0.2.8: dependency "text-0.11.2.0-2b6cea8526e93af24bb0154400fd64a6" doesn't exist There are problems in package persistent-template-0.9.0.1: dependency "text-0.11.2.0-2b6cea8526e93af24bb0154400fd64a6" doesn't exist There are problems in package persistent-sqlite-0.9.0.1: dependency "text-0.11.2.0-2b6cea8526e93af24bb0154400fd64a6" doesn't exist There are problems in package persistent-0.9.0.3: dependency "text-0.11.2.0-2b6cea8526e93af24bb0154400fd64a6" doesn't exist There are problems in package aeson-0.6.0.2: dependency "text-0.11.2.0-2b6cea8526e93af24bb0154400fd64a6" doesn't exist There are problems in package path-pieces-0.1.1: dependency "text-0.11.2.0-2b6cea8526e93af24bb0154400fd64a6" doesn't exist There are problems in package hashable-1.1.2.3: dependency "text-0.11.2.0-2b6cea8526e93af24bb0154400fd64a6" doesn't exist There are problems in package conduit-0.4.1.1: dependency "text-0.11.2.0-2b6cea8526e93af24bb0154400fd64a6" doesn't exist There are problems in package blaze-html-0.4.3.4: dependency "text-0.11.2.0-2b6cea8526e93af24bb0154400fd64a6" doesn't exist There are problems in package blaze-builder-0.3.1.0: dependency "text-0.11.2.0-2b6cea8526e93af24bb0154400fd64a6" doesn't exist There are problems in package attoparsec-0.10.1.1: dependency "text-0.11.2.0-2b6cea8526e93af24bb0154400fd64a6" doesn't exist There are problems in package buildwrapper-0.5.2: dependency "aeson-0.6.0.2-f5264d3642c44cd43d87b542486e49c9" doesn't exist dependency "attoparsec-0.10.1.1-d3ad0453e3127b6c51ccb14a5cb6f38d" doesn't exist dependency "unordered-containers-0.2.1.0-9b5d4d7fe2d79561a684ee5184ea5fd0" doesn't exist There are problems in package hoogle-4.2.11: dependency "blaze-builder-0.3.1.0-adf48413bb3bfeef99ef5e18de097dbd" doesn't exist dependency "conduit-0.4.1.1-6c1a48d4c94e05132c53b85e7ca48cd9" doesn't exist dependency "parsec-3.1.1-d3c0b4413115a5e82a1a5cf9b0b35fee" doesn't exist There are problems in package warp-1.2.0.1: dependency "blaze-builder-0.3.1.0-adf48413bb3bfeef99ef5e18de097dbd" doesn't exist dependency "conduit-0.4.1.1-6c1a48d4c94e05132c53b85e7ca48cd9" doesn't exist There are problems in package wai-1.2.0.1: dependency "blaze-builder-0.3.1.0-adf48413bb3bfeef99ef5e18de097dbd" doesn't exist dependency "conduit-0.4.1.1-6c1a48d4c94e05132c53b85e7ca48cd9" doesn't exist There are problems in package network-conduit-0.4.0: dependency "conduit-0.4.1.1-6c1a48d4c94e05132c53b85e7ca48cd9" doesn't exist There are problems in package blaze-builder-conduit-0.4.0.1: dependency "blaze-builder-0.3.1.0-adf48413bb3bfeef99ef5e18de097dbd" doesn't exist dependency "conduit-0.4.1.1-6c1a48d4c94e05132c53b85e7ca48cd9" doesn't exist There are problems in package network-2.3.0.5: dependency "parsec-3.1.1-d3c0b4413115a5e82a1a5cf9b0b35fee" doesn't exist There are problems in package http-types-0.6.10: dependency "blaze-builder-0.3.1.0-adf48413bb3bfeef99ef5e18de097dbd" doesn't exist There are problems in package case-insensitive-0.4.0.1: dependency "hashable-1.1.2.3-037c817e51d5d10d89306e9472e2e0bd" doesn't exist There are problems in package test-framework-th-0.2.2: dependency "test-framework-0.6-7d2482720de09487769f301ebc001970" doesn't exist There are problems in package test-framework-quickcheck2-0.2.12.1: dependency "test-framework-0.6-7d2482720de09487769f301ebc001970" doesn't exist There are problems in package test-framework-quickcheck-0.2.8: dependency "test-framework-0.6-7d2482720de09487769f301ebc001970" doesn't exist There are problems in package test-framework-hunit-0.2.7: dependency "test-framework-0.6-7d2482720de09487769f301ebc001970" doesn't exist The following packages are broken, either because they have a problem listed above, or because they depend on a broken package. scion-browser-0.2.8 persistent-template-0.9.0.1 persistent-sqlite-0.9.0.1 persistent-0.9.0.3 aeson-0.6.0.2 path-pieces-0.1.1 hashable-1.1.2.3 conduit-0.4.1.1 blaze-html-0.4.3.4 blaze-builder-0.3.1.0 attoparsec-0.10.1.1 buildwrapper-0.5.2 hoogle-4.2.11 warp-1.2.0.1 wai-1.2.0.1 network-conduit-0.4.0 blaze-builder-conduit-0.4.0.1 network-2.3.0.5 http-types-0.6.10 case-insensitive-0.4.0.1 test-framework-th-0.2.2 test-framework-quickcheck2-0.2.12.1 test-framework-quickcheck-0.2.8 test-framework-hunit-0.2.7 unordered-containers-0.2.1.0 pool-conduit-0.1.0.1 resource-pool-0.2.1.0 simple-sendfile-0.2.3 cgi-3001.1.7.4 haskell-platform-2011.4.0.0 HTTP-4000.1.2

2 1

unsafePerformIO usage
by Serge D. Mechveliani 06 May '12

06 May '12

People, can anybody explain what incorrect may happen with the following program which uses exchange through two Unix pipes and unsafePerformIO ? The pipes are created by the Unix commands mkfifo toA; mkfifo fromA gcc iface.c compiles the C program to a.out. ghc --make Main compiles the Haskell program by GHC to ./Main Run on the second terminal > ./iface Run on the first terminal > ./Main It prints the result to the screen. In Main.hs, the function iface :: String -> String is defined via hPutStr, hGetLine, hFlush, unsafePerformIO. It outputs str :: String to the pipe toA; the process iface (programmed in C) waits for delay and returns the respond string through the pipe fromA; iface inputs this respond string. So far, I put the respond string to be the same string, I think, this is not essential, but another map can be considered. Assume that the parallel computation is not allowed. In the below `main', I tried f ss where ss = [iface $ shows i "\n" | i <- [1 .. 4]], f [s1, s2, s3, s4] = [s3++s1, s3++s2, s1++s2], and it works correct. The question is: what a simple definition is possible for f which will evaluate incorrect? Is this true that unsafePerformIO is _safe_ for all f in this context? The `delay' constant can be edited in fifo.c. See both sources below. Please, copy the respond to mechvel(a)botik.ru Thanks, ------ Sergei mechvel(a)botik.ru ------------------------------------------------------------------------ import System.IO (IOMode(..), IO(..), Handle, openFile, hPutStr, hGetLine, hFlush) import System.IO.Unsafe (unsafePerformIO) dir = showString "/home/mechvel/haxiom/unsafeTest/" -- openFile :: FilePath -> IOMode -> IO Handle -- hPutStr :: Handle -> String -> IO () -- hGetLine :: Handle -> IO String toA_IO = openFile (dir "toA") WriteMode :: IO Handle fromA_IO = openFile (dir "fromA") ReadMode toA = unsafePerformIO toA_IO fromA = unsafePerformIO fromA_IO initIO :: IO (Handle, Handle) initIO = do to <- toA_IO from <- fromA_IO return (to, from) ifaceIO :: (Handle, Handle) -> String -> IO String ifaceIO (h1, h2) str = do hPutStr h1 str hFlush h1 str' <- hGetLine h2 return str' iface :: String -> String iface str = unsafePerformIO $ ifaceIO (toA, fromA) str main = putStr $ shows [s3++s1, s3++s2, s1++s2] "\n" where ss@[s1, s2, s3, s4] = [iface $ shows i "\n" | i <- [1 .. 4]] -- The C program fifo.c --------------------------------------------- #include <stdio.h> #include <string.h> #define BOUND 64 static char str[BOUND]; main() { int l, i; long j; FILE *toA, *fromA; toA = fopen("/home/mechvel/haxiom/unsafeTest/toA", "r"); if (toA == NULL) {perror("fopen(toA, r) failed: "); return;}; fromA = fopen("/home/mechvel/haxiom/unsafeTest/fromA", "w"); if (fromA == NULL) {perror("fopen(fromA, w) failed: "); return;}; for (;;) { if (fgets(str, BOUND, toA) == NULL) { perror("fgets(str, bound, toA) failed: "); return; }; j = 20000000; // delay. edit it while (j > 0) {j = j-1;}; fputs(str, fromA); fflush(fromA); }; return; }

1 0

library conflicts and how to resolve them
by Graham Berks 06 May '12

06 May '12

Attempting $ cabal install persistent Resolving dependencies... In order, the following would be installed: aeson-0.6.0.2 (reinstall) changes: mtl-2.1.1 -> 2.0.1.0 persistent-0.9.0.3 (new package) cabal: The following packages are likely to be broken by the reinstalls: buildwrapper-0.5.2 Use --force-reinstalls if you want to install anyway. Any ideas what is going on ? I believe aeson-0.6.0.2 is already installed. And as far as I can tell the build of aeson-0.6.0.2 doesn't require that specific version of mtl. Am I missing something ? Is there a way to force ghc to ignore this ?? Thanks

3 2

Haskell and arrays
by Morten Olsen lysgaard 05 May '12

05 May '12

I'm a bit confused on what the standard is when it comes to arrays and performance in Haskell. Say I have a function that takes an n dimensional array. The array only contains some primitive type. The function mutates the array in an deterministic but chaotic pattern. The new value of an element might depend on other values in the array, and the order of update might depend on what values are found. How would I implement this in an efficient fashion in Haskell. Is the array package the way to go? Would vector with some custom indexing scheme work well? The array package provides n dimensional indexing, vector doesn't. Does vector perform better than array? Is it realistic to approach the performance of a naive c implementation? What steps would need to be taken in order to achieve that? I know there are many question here, but I don't expect perfect answers to them. I just need to bootstrap my map of numerics in Haskell. PS: An example of the above function could be Gaussian elimination of a matrix. -- Morten Olsen Lysgaard NTNU-IT - Orakeltjenesten NTNU - Physics and Mathematics

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haskellonline.org, a web frontend for ghc
by Gintautas Miliauskas 05 May '12

05 May '12

Hello, check out http://haskellonline.org, an online Haskell typechecker. It is essentially a thin web wrapper over ghc, but it takes some friction out of learning Haskell, if only because no local setup is required. It is a weekend's work, so do not be too harsh, but feedback is welcome! I have also written up an accompanying blog post [1]. [1] http://blog.miliauskas.lt/2012/05/online-haskell-typechecker.html Best regards, -- Gintautas Miliauskas

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Generalizing (++) for monoids instead of using (<>)
by aditya bhargava 05 May '12

05 May '12

After asking this question: http://stackoverflow.com/questions/9963050/standard-way-of-joining-two-data… I found out that the new infix operator for `mappend` is (<>). I'm wondering why ghc 7.4 didn't generalize (++) to work on monoids instead. To me, (++) is much more clear. (<>) means "not equal to" for me. Can anyone shed light on this decision? Adit -- adit.io

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ANN: meta-par, meta-par-accelerate and friends -- heterogeneous parallel programming
by Ryan Newton 04 May '12

04 May '12

Hi all, I'm happy to announce a release of meta-par, a parallel programming library that is a successor to monad-par. - http://hackage.haskell.org/package/abstract-par - http://hackage.haskell.org/package/monad-par - http://hackage.haskell.org/package/meta-par - http://hackage.haskell.org/package/abstract-par-accelerate - http://hackage.haskell.org/package/meta-par-accelerate The distributed programming components aren't released yet, but this blog posts describes how to use the above packages to do hybrid CPU/GPU programming: http://parfunk.blogspot.com/2012/05/how-to-write-hybrid-cpugpu-programs.html Cheers, -Ryan

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conduit: Finalize field in PipeM
by Paolo Capriotti 04 May '12

04 May '12

Hi, I'm trying to write a function to convert a conduit to a Pipe (from pipes-core), and I'm having trouble understanding why the `Finalize` field in a `PipeM` constructor returns `r`. This makes it impossible to create it from the corresponding `M` constructor in pipes-core, since `M` includes an exception handler which is not guaranteed to run fully, hence may not provide a return value. `HaveOutput` presents a similar problem. >From a cursory look at conduit's code, it doesn't look like the return value of `Finalize` is ever used, and it seems that the conduits that actually manage to define it either have `()` as return type or just throw an exception. The definition of `lift` for the `MonadTrans` instance duplicates the base monad action there, which doesn't look quite right to me (isn't that supposed to contain a _cleanup_ action?). Shouldn't the constructor be changed to something like `PipeM (m (Pipe i o m r)) (Finalize m ())` or am I completely off base here? Thanks. BR, Paolo

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Fwd: Problem with forall type in type declaration
by Chris Smith 04 May '12

04 May '12

Oops, forgot to reply-all again... ---------- Forwarded message ---------- From: Chris Smith <cdsmith(a)gmail.com> Date: Fri, May 4, 2012 at 8:46 AM Subject: Re: [Haskell-cafe] Problem with forall type in type declaration To: Magicloud Magiclouds <magicloud.magiclouds(a)gmail.com> On Fri, May 4, 2012 at 2:34 AM, Magicloud Magiclouds <magicloud.magiclouds(a)gmail.com> wrote: > Sorry, it was just a persudo code. This might be more clear: > > run :: (Monad m) => m IO a -> IO a Unfortunately, that's not more clear. For the constraint (Monad m) to hold, m must have the kind (* -> *), so then (m IO a) is meaningless. I assume you meant one of the following: run :: MonadTrans m => m IO a -> IO a or run :: MonadIO m => m a -> IO a (Note that MonadIO is the class from the mtl package; there is no space there). Can you clarify which was meant? Or perhaps you meant something else entirely? -- Chris Smith

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Annoyed at System.Random
by Thomas DuBuisson 04 May '12

04 May '12

Ryan, I've grown annoyed at System.Random enough (specifically, StdGen). How much, if any, pushback would there be if I put together a FFI binding to a C AES-CTR based RNG. There are many advantages: 0) The API wouldn't have to change (though some parts should, and some change is already planned) 1) It can be fast (maybe not MT fast, but really quite good) 2) It's secure and the splitting properties can be related to cryptographic problems. 3) It could use Intel NI instructions when available. 4) It's NIST standardized, so there exist known answer tests. 5) We could expose a method to fill an arbitrary buffer :: Storable a => Ptr a -> Int -> IO () 6) The rest of the community doesn't have to do any of the work. I'd be tempted to pull in the 'entropy' package for seeding, but will make that a separate proposal. Cheers, Thomas

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Indent aware parser
by Lyndon Maydwell 04 May '12

04 May '12

Hi all. What's the best indentation-aware parser at the moment? I see three when I look in cabal: lyndon@pugno:~ » cabal list indent * IndentParser Synopsis: Combinators for parsing indentation based syntatic structures Default available version: 0.2.1 Installed versions: [ Not installed ] Homepage: http://www.cse.iitk.ac.in/~ppk License: GPL * indentparser Synopsis: A parser for indentation based structures Default available version: 0.1 Installed versions: [ Not installed ] Homepage: http://www.cse.iitk.ac.in/users/ppk/code/HASKELL/indentparser License: PublicDomain * indents Synopsis: indentation sensitive parser-combinators for parsec Default available version: 0.3.2 Installed versions: [ Not installed ] Homepage: http://patch-tag.com/r/salazar/indents License: BSD3

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unable to run ghci (7.4.1)?
by Baojun Wang 04 May '12

04 May '12

Dear list, I build haskell-platform 2011.04 by gentoo ~amd64 haskell overlay. But I can't run ghci even I rebuilt ghc or run haskell-updater, error message: ghci GHCi, version 7.4.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... ghc: /usr/lib64/ghc-7.4.1/base-4.5.0.0/HSbase-4.5.0.0.o: unknown symbol `stat' ghc: unable to load package `base' Do you have some ideas how to avoid this error? Many thanks. Best Regards Baojun

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ANN: containers-0.5.0.0
by Johan Tibell 04 May '12

04 May '12

Hi all, I'm proud to announce a new major release of the containers package : http://hackage.haskell.org/package/containers-0.5.0.0 There has been a whopping 160 commits since the last release. The major improvements are: * a clearer distinction between value-lazy and value-strict containers, * performance improvements across the board, * a big internal clean-up, and * new functions for e.g. merging, updating, and searching containers. The first item is perhaps the most visible. While the old Data.Map and Data.IntMap modules will continue to exist for the foreseeable future, we've abandoned the practice of having the strict and lazy versions of each function distinguished by an apostrophe. The distinction is instead made at the module level, by introducing four new modules: * Data.Map.Strict * Data.Map.Lazy * Data.IntMap.Strict * Data.IntMap.Lazy This split has three benefits: * It makes the choice between value-strict and value-lazy containers more declarative; you pick once at import time, instead of having to remember to use the strict or lazy versions of a function every time you modify the container. * It alleviates a common source of performance issues, by forcing the user to think about the strictness properties upfront. For example, using insertWith instead of insertWith' is a common source of containers-related performance bugs. * There are fewer functions per module, making it easier to get an overview of each module. Note that the types used in the strict and lazy APIs are the same, so you can still use the same container in a "mixed" manner, if needed. Contributors to this release: Milan Straka Johan Tibell Joachim Breitner Edward Z. Yang Herbert Valerio Riedel Matt West Max Bolingbroke Cheers, The containers maintainers

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Haskell source AST zipper with state
by Tony Morris 04 May '12

04 May '12

Is there a library to traverse a source AST keeping state? -- Tony Morris http://tmorris.net/

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Tutorial on how to use multirec
by j.romildo＠gmail.com 03 May '12

03 May '12

Hello. I am writing a compiler in Haskell and need some tools to work with abstract syntax trees (like annotating them with source positions and other things). I have been pointed out to the multirec package. I have found some information about the package, but it is too dense to read and understand, as the ideas are presented from a theoretical point of view. So I am looking for something more light, as tutorials or simple applications that illustrate the use of the package. Any help? Romildo

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Specify compile error
by Ismael Figueroa Palet 03 May '12

03 May '12

Hi, I'm writing a program like this: data B = B Int data A = Safe Int | Unsafe Int createB :: A -> B createB (Safe i) = B i createB (Unsafe i) = error "This is not allowed" Unfortunately, the situation when createB is called with an Unsafe value is only checked at runtime. If I omit the second case, it is not an error to not be exhaustive :-( Is there a way to make it a compile time error?? Thanks! -- Ismael

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twitter election on favorite programming language
by Kazu Yamamoto 03 May '12

03 May '12

Hello, A twitter election on favorite programming language was held in Japan and it appeared that Heskell is No. 10 loved language in Japan. :-) http://twisen.com/election/index/654 Regards, --Kazu

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OpenGL: No instance for Random GLfloat
by Mark Spezzano 03 May '12

03 May '12

Hi Haskellers, I'm trying to generate a random vertex in OpenGL as follows. genPosition :: IO (Vertex3 GLfloat) genPosition = do x <- getStdRandom $ randomR (-1.6,1.6) y <- getStdRandom $ randomR (-1.0,1.0) return (Vertex3 x y (-1)) Unfortunately the compiler complains about me having to implement an instance of Random for GLfloat. How do I do this (or avoid having to do this)? Cheers, Mark

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Haskell Weekly News: Issue 225
by Daniel Santa Cruz 03 May '12

03 May '12

Welcome to issue 225 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of April 22 to 28, 2012. Announcements Matthias Fischmann made a call for participation to the Hong Kong Haskell hackathon, with tentative dates of May 11-13 or 25-27. Follow to link for more information. [1] http://goo.gl/9sRLF HacPhi, the hackathon in Philadelphia, Pennsylvania, USA, will be held between August 3-5, 2012. [2] http://goo.gl/Rm2qH Quotes of the Week * lambdabot was last seen on #haskell 1 hour, 37 minutes and 46 seconds ago, saying: <IO ()> * elliott: oh, it's not haskell-cafe? that explains the reasonable responses ["it" = GHC mailing list] * Morandat: "The astute reader will have noticed that the above example clashes with our claim that R is lexically scoped. As is often the case, R is lexically scoped up to the point it is not." * quicksilver: lisp doesn't have equational reasoning so there is nothing to lose :) * JoeyA: One thing I like about Haskell versus C++ is that abusing language features tends to give more satisfying results. monochrom: that seems to be a criterion for drugs rather than languages :) Top Reddit Stories * Darcs 2.8: faster, better annotate; patch stash (obliterate -O); revert hunk split; and more! Domain: wiki.darcs.net, Score: 63, Comments: 21 On Reddit: [3] http://goo.gl/4tj4i Original: [4] http://goo.gl/R7TpP * The Haskell / Snap ecosystem is as productive (or more) than Ruby/Rails. Domain: blog.dbpatterson.com, Score: 63, Comments: 46 On Reddit: [5] http://goo.gl/cP3NC Original: [6] http://goo.gl/23alh * Cabal is now hosted on GitHub Domain: github.com, Score: 44, Comments: 20 On Reddit: [7] http://goo.gl/yBqCC Original: [8] http://goo.gl/0ze9y * Eight Haskell Google Summer of Code projects accepted Domain: google-melange.com, Score: 40, Comments: 25 On Reddit: [9] http://goo.gl/c2KXg Original: [10] http://goo.gl/vu681 * “Live” code update for Haskell Domain: self.haskell, Score: 31, Comments: 17 On Reddit: [11] http://goo.gl/ZZDb6 Original: [12] http://goo.gl/ZZDb6 * Functional Programming in C++ Domain: altdevblogaday.com, Score: 30, Comments: 12 On Reddit: [13] http://goo.gl/pFuHk Original: [14] http://goo.gl/aG6iP * Why do we not define IO as a Free monad? Domain: self.haskell, Score: 28, Comments: 47 On Reddit: [15] http://goo.gl/jiSdb Original: [16] http://goo.gl/jiSdb * Elm: Source code and Compiler/Server now available. Write FRP code for the web! Domain: self.haskell, Score: 27, Comments: 11 On Reddit: [17] http://goo.gl/9hE6X Original: [18] http://goo.gl/9hE6X * An experiment with Bayesian Networks in Haskell Domain: alpheccar.org, Score: 27, Comments: On Reddit: [19] http://goo.gl/21VDw Original: [20] http://goo.gl/2piKL * More Client Side Yesod: todo sample Domain: yesodweb.com, Score: 25, Comments: 10 On Reddit: [21] http://goo.gl/LGO75 Original: [22] http://goo.gl/QGsrZ * C Style Haskell Domain: kormacode.blogspot.com, Score: 24, Comments: 21 On Reddit: [23] http://goo.gl/VsShj Original: [24] http://goo.gl/o75jn Top StackOverflow Questions * Avoiding IORefs in pure code votes: 19, answers: 1 Read on SO: [25] http://goo.gl/327fO * How can I get JHC to cooperate with the android ndk? votes: 11, answers: 1 Read on SO: [26] http://goo.gl/oGMQ9 * Is there a name for this function or pattern? votes: 11, answers: 2 Read on SO: [27] http://goo.gl/HJixe * Haskell rank two polymorphism compile error votes: 11, answers: 2 Read on SO: [28] http://goo.gl/hYdRX * Python faster than compiled Haskell? votes: 11, answers: 5 Read on SO: [29] http://goo.gl/sUXMa * How does one release a *nix project? votes: 10, answers: 2 Read on SO: [30] http://goo.gl/JTgCz * Optimizing partial computation in Haskell votes: 8, answers: 1 Read on SO: [31] http://goo.gl/Dncn7 * Proper way to treat global flags in Haskell votes: 8, answers: 2 Read on SO: [32] http://goo.gl/di2Ld * Generating code for another language in Haskell votes: 7, answers: 1 Read on SO: [33] http://goo.gl/OlRCI * Why does haskell enumerator based IO call sigprocmask so often? votes: 7, answers: 4 Read on SO: [34] http://goo.gl/kfTSy * confused about function as instance of Functor in haskell votes: 7, answers: 5 Read on SO: [35] http://goo.gl/aX9JA Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/98038 2. http://www.haskell.org/haskellwiki/Hac_%CF%86 3. http://wiki.darcs.net/Releases/2.8 4. http://www.reddit.com/r/haskell/comments/snyvh/darcs_28_faster_better_annot… 5. http://blog.dbpatterson.com/post/21885034168 6. http://www.reddit.com/r/haskell/comments/suj5s/the_haskell_snap_ecosystem_i… 7. https://github.com/haskell/cabal 8. http://www.reddit.com/r/haskell/comments/smwft/cabal_is_now_hosted_on_githu… 9. http://www.google-melange.com/gsoc/org/google/gsoc2012/haskell 10. http://www.reddit.com/r/haskell/comments/sqr4r/eight_haskell_google_summer_… 11. http://www.reddit.com/r/haskell/comments/stcwz/live_code_update_for_haskell/ 12. http://www.reddit.com/r/haskell/comments/stcwz/live_code_update_for_haskell/ 13. http://www.altdevblogaday.com/2012/04/26/functional-programming-in-c/ 14. http://www.reddit.com/r/haskell/comments/suxv8/functional_programming_in_c/ 15. http://www.reddit.com/r/haskell/comments/swffy/why_do_we_not_define_io_as_a… 16. http://www.reddit.com/r/haskell/comments/swffy/why_do_we_not_define_io_as_a… 17. http://www.reddit.com/r/haskell/comments/subvk/elm_source_code_and_compiler… 18. http://www.reddit.com/r/haskell/comments/subvk/elm_source_code_and_compiler… 19. http://www.alpheccar.org/content/98.html 20. http://www.reddit.com/r/haskell/comments/swfud/an_experiment_with_bayesian_… 21. http://www.yesodweb.com/blog/2012/04/yesod-js-todo 22. http://www.reddit.com/r/haskell/comments/snzsa/more_client_side_yesod_todo_… 23. http://kormacode.blogspot.com/2011/11/c-style-haskell_10.html 24. http://www.reddit.com/r/haskell/comments/sxoof/c_style_haskell/ 25. http://stackoverflow.com/questions/10298664/avoiding-iorefs-in-pure-code 26. http://stackoverflow.com/questions/10305909/how-can-i-get-jhc-to-cooperate-… 27. http://stackoverflow.com/questions/10319659/is-there-a-name-for-this-functi… 28. http://stackoverflow.com/questions/10345809/haskell-rank-two-polymorphism-c… 29. http://stackoverflow.com/questions/10357663/python-faster-than-compiled-has… 30. http://stackoverflow.com/questions/10279508/how-does-one-release-a-nix-proj… 31. http://stackoverflow.com/questions/10266954/optimizing-partial-computation-… 32. http://stackoverflow.com/questions/10285454/proper-way-to-treat-global-flag… 33. http://stackoverflow.com/questions/10268052/generating-code-for-another-lan… 34. http://stackoverflow.com/questions/10273696/why-does-haskell-enumerator-bas… 35. http://stackoverflow.com/questions/10294272/confused-about-function-as-inst…

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Re: [Haskell-cafe] heterogeneous environment
by oleg＠okmij.org 03 May '12

03 May '12

Ben wrote: > - use Data.Unique to identify Refs, and use existential quantification > or Data.Dynamic to create a heterogenous Map from uid to log. for > example, to represent a log of compare-and-swaps we might do something > like > data Ref a = Ref (IORef a) Unique > data OpaqueCAS = forall a . > OpaqueCAS { casRef :: Ref a, oldVal :: a, newVal :: a } > type CASLog = Map Unique OpaqueCAS > logCAS r@(Ref _ uid) o n log = insert uid (OpaqueCAS r o n) log... > but what if the transaction wants to perform a second CAS on a > reference we've already CAS'd? then we should create a combined > OpaqueCAS record which expects t he first oldVal and swaps in the > second newVal. unfortunately the type checker balks at this, as it > can't prove that the type variable 'a from the first record is the > same as the 'a in the new record; i suppose there might be some fancy > type shenanigans which might solve this... It seems you actually prefer this solution, if it worked. This solution will entail some run-time check one way or another, because we `erase' types when we store them in the log and we have to recover them later. If the problem is of merging two CASLog entries into one, your code above already solves it. There are no type shenanigans are necessary. I will make slight modification though by introducing an extra IORef for `ephemeral' communication. Values written to that IORef almost immediately retrieved. It is not strictly necessary since your Ref already has one IORef; I assume that it can't be garbled, even for a brief moment. The complete code is enclosed at the end of the message. If one wants to live a bit dangerously but efficiently, one may wish to create a module of reference cells indexed by unique numbers, like your Ref. The data constructor Ref is not exported. If the only way to create Refs is to use the function newRef of the module, and that function assuredly creates a Ref with a unique label, one is justified in writing the function cast :: Ref a -> Ref b -> Maybe (Ref b) cast r1@(Ref _ u1) (Ref _ u2) | u1 == u2 -> Just (unsafeCoerce r1) cast _ _ = Nothing because the same labels correspond to references of the same type. The _safe_ solution, similar to that in the enclosed code below, was used in HANSEI, which is the embedded DSL for probabilistic programming. Here is the relevant code describing first-class memory (it is OCaml). (* We often use mutable variables as `communication channel', to appease the type-checker. The variable stores the `option' value -- most of the time, None. One function writes a Some x value, and another function almost immediately reads the value -- exactly once. The protocol of using such a variable is a sequence of one write almost immediately followed by one read. We use the following helpers to access our `communication channel'. *) let from_option = function Some x -> x | None -> failwith "fromoption";; let read_answer r = let v = from_option !r in r := None; v (* for safety *) (* First-class storage: for the implementation of `thread-local' memory *) module Memory = struct type 'a loc = int * 'a option ref type cell = unit -> unit module M = Map.Make(struct type t = int let compare x y = x - y end) type mem = cell M.t let newm = M.empty let genid : unit -> int = (* generating fresh locations *) let counter = ref 0 in fun () -> let v = !counter in counter := succ v; v let new_loc () = (genid (), ref None) let new_cell (_,chan) x = fun () -> chan := Some x let mref (id,chan) m = let cell = M.find id m in cell (); read_answer chan let mset ((id,chan) as loc) x m = M.add id (new_cell loc x) m end;; Enclosed Haskell code: {-# LANGUAGE ExistentialQuantification #-} module Ref where import Data.IORef import Data.Unique import Data.Map as M data Ref a = Ref (IORef a) (IORef a) Unique data OpaqueCAS = forall a . OpaqueCAS { casRef :: Ref a, oldVal :: a, newVal :: a } type CASLog = M.Map Unique OpaqueCAS logCAS r@(Ref _ _ uid) o n log = M.insert uid (OpaqueCAS r o n) log -- If one is bothered with undefined below, use Nothing newRef :: a -> IO (Ref a) newRef x = do r1 <- newIORef x r2 <- newIORef undefined -- auxiliary u <- newUnique return (Ref r1 r2 u) writeOld :: OpaqueCAS -> IO () writeOld (OpaqueCAS (Ref _ ra _) o n) = writeIORef ra o readOld :: OpaqueCAS -> IO OpaqueCAS readOld (OpaqueCAS r@(Ref _ ra _) _ n) = do o <- readIORef ra -- guard against errors and memory leaks writeIORef ra undefined return (OpaqueCAS r o n) writeNew :: OpaqueCAS -> IO () writeNew (OpaqueCAS (Ref _ ra _) o n) = writeIORef ra n readNew :: OpaqueCAS -> IO OpaqueCAS readNew (OpaqueCAS r@(Ref _ ra _) o _) = do n <- readIORef ra -- guard against errors and memory leaks writeIORef ra undefined return (OpaqueCAS r o n) -- Create CAS record with the old value from the first CAS and -- the new value from the new CAS. -- First we assume that the two OpaqueCAS correspond to the same -- variable. A run-time error occurs otherwise. -- Second, we make no checks for the relation between the new -- value of the first CAS and the old value of the new CAS. Probably -- they are meant to be the same. Anyway, we don't even look at them. combineCAS:: OpaqueCAS -> OpaqueCAS -> IO OpaqueCAS combineCAS cas1 cas2 = do writeNew cas2 readNew cas1

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12th GhentFPG meeting: Tuesday, May 8th, 2012: final program
by Andy Georges 02 May '12

02 May '12

Dear GhentFPG members and functional programming enthousiasts, After consulting the doodle, we have decided on holding the 12th GhentFPG meeting at Tuesday, May 8th, 2012 starting at 19:30. The venue will be the same as for the previous meeting. We will convene at Ghent University, campus De Sterre, Krijgslaan 281 (main entrance), building S9, see also the attached image. The program will be as follows: 1. Tom Schrijvers - A functional approach to solving challenging programming problems - Flemish programming contest solutions. You can find the contest challenges here (in Dutch): http://www.vlaamseprogrammeerwedstrijd.be/current/opgaves.php 2. Jasper Van der Jeugt - A short tutorial on using semi-implicit parallelism in Haskell programs using the available annotations. We hope to see you all there. Regards, The GhentFPG Organizing Committee.

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Edmonton Functional Programming Users Group (EFPUG)
by Kevin Charter 02 May '12

02 May '12

Hi Cafe, If you live in the Edmonton, Canada area, and you're interested in functional programming, EFPUG is an informal discussion group you might find interesting. Most of our talks have been on Haskell topics, but we're interested in functional languages in general. Our Google Group is https://groups.google.com/forum/?fromgroups&hl=en#!forum/edmonton-functiona… We meet on the first Tuesday of (almost) each month at the Computing Science building on the University of Alberta north campus. New participants, and new ideas for talks, hack nights, or other FP-related activities, are always welcome. Cheers, Kevin -- Kevin Charter kevin.charter(a)acm.org

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Threads and hGetLine
by H. M. 02 May '12

02 May '12

Hello, The simplified problem: There are two threads, one which is waits on input via hGetLine and another, which should terminate this thread or close this handle. hClose as well as killThread doesn't seem to work, caused by the fact, that the thread is blocked until input is availiable. Hopefully you have some solution how to kill the thread and/or close the handle and/or have some other idea to get the lined input in the other thread. -- Thanks in advance H.

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ANN: HacPhi 2012
by Chris Casinghino 02 May '12

02 May '12

Greetings, I am very pleased to officially announce Hac Phi 2012, a Haskell hackathon/get-together to be held August 3-5 at the University of Pennsylvania in Philadelphia. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday (with breaks for sleep, of course). Last year's Hac Phi was a lot of fun, drawing more than 30 Haskellers, and many people have already expressed interest in coming back this year. Come meet your Haskelly comrades-in-arms! I want to stress that everyone is welcome - you do not have to be a Haskell guru. Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, please officially register [1]. Registration, travel, lodging and many other details can be found on the Hac Phi wiki [2]. This year, we are making a public list of attendees' names and nicks - please let us know if you'd prefer not to be listed when you register. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game - a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hac Phi 2012 is supported by contributions from Amgen and Jane Street. Hope to see you in Philadelphia! - The Hac Phi team Brent Yorgey (byorgey) Daniel Wagner (dmwit) Chris Casinghino (ccasin) [1] http://www.haskell.org/haskellwiki/Hac_%CF%86/Register [2] http://www.haskell.org/haskellwiki/Hac_%CF%86 [3] http://www.haskell.org/haskellwiki/Hac_%CF%86/Talks

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heterogeneous environment
by Ben 02 May '12

02 May '12

dear static typing masters -- while working on an STM-like library, i ran into the issue of trying to create a transaction log for reads / writes of heterogeneous reference types. i don't have a good answer to this problem. the problem is twofold : first, the general heterogeneous collection problem, and second, connecting a reference to it's log. there are some solutions i've tried but each has drawbacks : - store the transaction log inside of the reference itself. essentially, each reference has an IORef (Map ThreadId a) associated to it. this is the approach used by [1]. unfortunately this creates a point of concurrency contention at each reference for each read / write, and a lot of repeated computation (the transaction log is spread out in a lot of pieces.) - use Data.Unique to identify Refs, and use existential quantification or Data.Dynamic to create a heterogenous Map from uid to log. for example, to represent a log of compare-and-swaps we might do something like data Ref a = Ref (IORef a) Unique data OpaqueCAS = forall a . OpaqueCAS { casRef :: Ref a, oldVal :: a, newVal :: a } type CASLog = Map Unique OpaqueCAS logCAS r@(Ref _ uid) o n log = insert uid (OpaqueCAS r o n) log... but what if the transaction wants to perform a second CAS on a reference we've already CAS'd? then we should create a combined OpaqueCAS record which expects the first oldVal and swaps in the second newVal. unfortunately the type checker balks at this, as it can't prove that the type variable 'a from the first record is the same as the 'a in the new record; i suppose there might be some fancy type shenanigans which might solve this... Data.Dynamic works but puts a Typeable constraint on the Ref type, so requires the user to modify their data types, and requires a run-time type check (and while performance isn't paramount now it will become important to me later.) also it doesn't feel right... - tupling and functional representations. a monadic function that does a read on an reference can be thought of as a pure function with an extra argument. a monadic function that does a write can be thought of as a pure function with an extra return value. combining all the reads and writes into a transaction log is a big network / switchboard connecting problem, e.g. creating the extra inputs / outputs to the various functions and then stitching them together. running the monad then just is connecting up the final composed function to the actual input / output references. in a sense this amounts to tupling (or currying) the heterogeneous types. (is this is a kind of "final" representation, in the "finally tagless" sense?) i looked at this there were two problems : 1 - the representation is very difficult to manipulate, at least the way i was trying (using the arrow operations); the switchboard problem is extremely verbose, and 2 - it is hard to reconcile identity and position in the tuples -- the repeated read / write problem again. i also experimented with HList but got stuck on similar issues. it strikes me this is just the problem of keeping an environment for an interpreter of a language with mutable heterogeneous reference types. this must be a problem that has either been solved or else there is a haskell point of view on it i'm not grasping which avoids the need for this data structure. maybe there is a way of writing this as an interpreter or using some existing monad, like the ST monad? best, ben [1] - Frank Huch and Frank Kupke, A High-Level Implementation of Composable Memory Transactions in Concurrent Haskell

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using ResourceT with MVars
by Warren Harris 02 May '12

02 May '12

I would like to use LevelDB in my code, and the HEAD version of leveldb-haskell now uses runResourceT to manage open db connections and iterators. I would also like to run multiple threads, and coordinate them for transaction-like behavior, i.e. read and writing data without interruption. LevelDB doesn't support transactions itself, so the alternative would seem to be using MVars for mutual exclusion. With the 0.0.3 version of leveldb-haskell, the get and put operations were in the IO monad, which made it trivial to write something like this: withMVar state $ \db -> do maybeValue <- get db rdOpts key put db wrOpts key $ maybe init incr maybeValue Now with the HEAD version, the get and put operations have types: get :: MonadResource m => DB -> ReadOptions -> ByteString -> m (Maybe ByteString) put :: MonadResource m => DB -> WriteOptions -> ByteString -> ByteString -> m () I don't think it makes sense in general to lift the withMVar into MonadResource and use runResourceT to run its body in IO. This would not satisfy the intention behind using ResourceT -- resource allocated inside the withMVar body would be deallocated when the withMVar scope is exited, rather than being managed by the outermost ResourceT. (Although, in this case we aren't allocating any resources, so maybe it's a moot point for my simple example.) I also don't think it makes sense to use takeMVar and putMVar instead of withMVars as this would subvert the exception handling that withMVars provides. And my attempts to define a withMVarR (in the ResourceT IO monad) also seem to require runResourceT: withMVarR :: MVar a -> (a -> ResourceT IO b) -> ResourceT IO b withMVarR m io = do -- mask $ \restore -> do a <- lift $ takeMVar m b <- {-restore-} (io a) `onExceptionR` (lift $ putMVar m a) lift $ putMVar m a return b onExceptionR :: ResourceT IO a -> ResourceT IO b -> ResourceT IO a onExceptionR a b = do runResourceT $ transResourceT (\a -> transResourceT (onException a) b) a Maybe I need to create a "HasMVar" class analogous to HasRef? (http://hackage.haskell.org/packages/archive/conduit/0.0.0/doc/html/Control-…) I'd appreciate any suggestions on how to make these fit together. Warren

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Haskell Hackathon in Munich
by Heinrich Hördegen 02 May '12

02 May '12

Dear all, the Munich Haskell Meeting is glad to announce a local one day Hackathon in Munich. It will take place the 12th of May 2012 from 10am to 6pm. Please checkout the details at: http://www.haskell-munich.de/dates Cordially yours, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

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Reader monad, refactoring and missing the point all at once
by Eugene Dzhurinsky 02 May '12

02 May '12

Hi all! Last day I was trying to fix idiii library, because it uses utf8 for parsing non-unicode content. I found the functions > -- | Parses one value and returns it as a 'String' > parseString :: CharEncoding -> TagParser String > parseString enc = do > v <- case enc of > 0x01 -> parseUntilWord16Null -- UTF-16 > 0x02 -> parseUntilWord16Null -- UTF-16 BOM > _ -> parseUntilWord8Null -- ISO-8859-1 or UTF-8 > return $ encPack enc v > > encPack :: CharEncoding -> [Token] -> String > encPack 0x00 s = Text.unpack $ decodeASCII $ BS.pack s > encPack 0x01 (0xFF:0xFE:s) = Text.unpack $ decodeUtf16LE $ BS.pack s > encPack 0x01 (0xFE:0xFF:s) = Text.unpack $ decodeUtf16BE $ BS.pack s > encPack 0x02 s = Text.unpack $ decodeUtf16BE $ BS.pack s > encPack _ s = Text.unpack $ decodeUtf8 $ BS.pack s updated the dependency from > import Data.Text.Encoding (decodeASCII, decodeUtf16LE, decodeUtf16BE, decodeUtf8) to > import Data.Text.ICU.Convert and added implementation for decoding functions: > decodeAny :: String -> BS.ByteString -> Text.Text > decodeAny charset src = unsafePerformIO $ ((flip toUnicode) src) `fmap` open charset (Just True) > > decodeASCII :: BS.ByteString -> Text.Text > decodeASCII = decodeAny "latin1" > > decodeUtf16LE = decodeAny "utf-16le" > > decodeUtf16BE = decodeAny "utf-16be" > > decodeUtf8 = decodeAny "utf-8" Now I want to add possibility to specify encoding to yse with decodeASCII. I was thinking of adding Reader monad and providing some sort of charset configuration there - but it will lead up to complicating the code, which uses this parseString function. And this code is used inside Parser of Text.ParserCombinators.Poly.State - so I will need to update all usages of this parser. Another approach might be to use IORef with encoding stored there, but I don't really like this solution. What would be the best way of refactoring of such kind? Thanks! -- Eugene N Dzhurinsky

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Creating a dependency graph (in the hope it may show some structure)
by Bram Neijt 02 May '12

02 May '12

Dear Haskelllers, I've reacently started with Haskell and during the Dutch Hackaton[3], I decided to generate graphs of dependencies. I was thinking about doing this for Java earlier, and that weekend I decided to write the tooling in Haskell. I've written two programs: hs2dot[1] and dotimate[2]. The first generates a dot file (Graphviz) from all .hs files below the current working directory. The second will take one or more dot files and call neato (from Graphviz) multiple times to generate frames of neato output from one to the next. Trying hs2dot and dotimate on two sequencial commits in the cabal/cabal-install/frames directory already gives me a way to complex and full graph. See attached. ==> My question is: What dependencies, if any, would be nice to graph, and/or which dependencies could I filter out to keep this from becoming a mess? Greetings, Bram Neijt [1] https://gist.github.com/bneijt/hs2dot [2] https://gist.github.com/bneijt/dotimate [3] http://www.haskell.org/haskellwiki/DHD_UHac#Utrecht_Hackathon

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ANN: bitwise - fast multi-dimensional unboxed bit packed Bool arrays
by Claude Heiland-Allen 01 May '12

01 May '12

Hi all, I'm pleased to announce bitwise-0.1: http://hackage.haskell.org/package/bitwise --8<-- excerpt from the hackage page Unboxed multidimensional bit packed Bool arrays with fast aggregate operations based on lifting Bool operations to bitwise operations. There are many other bit packed structures out there, but none met all of these requirements: 1. unboxed bit packed Bool array, 2. multi-dimensional indexing, 3. fast (de)serialization, or interoperable with foreign code, 4. fast aggregate operations (fold, map, zip). --8<-- end excerpt bitwise has been tested in hugs -98, ghc-7.0.4, ghc-7.4.1. Bool data structures comparison (features numbered as excerpt above): package features additional notes 1 2 3 4 array Y Y n n bitarray Y n n ? based on array bitstream n n ? ? based on vector bitstring Y n ? ? based on bytestring bit-vector n n Y ? based on vector bitwise Y Y Y Y repa n Y Y ? based on vector uvector Y n ? ? deprecated vector n n Y ? uses a Word8 for each Bool Known shortcomings of this first release of bitwise: * missing common instances (Eq, Ord, Read, Show, ...); * missing additional functions (counting, searching, ...); * missing documentation on ByteString format: the bits are taken from the array and packed into Word8s, with the first bit (in array order) becoming the least significant bit of the first Word8; the last Word8 is padded with 0 at the most-significant-bit end; * PBM reading is not compliant to the official specification; * misleading benchmark (bitwise zipWith is fast, but there may be a faster way to zipWith UArray ix Bool than the one I used). What I'm using bitwise for at the moment: A building block for fractal imaging: an image is made up of cells, each cell can be in one of 4 states (represented with 2 bits): * cell is 100% interior to a set of points in the plane * cell is 100% exterior * partially interior and partially exterior (ie, on the boundary) * unknown with recursive subdivision of cells into images taking place only when the cell is on the boundary of the set being imaged. Thanks, Claude

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Re: [Haskell-cafe] Darcs home page updated
by Paul R 01 May '12

01 May '12

On Mon, 30 Apr 2012 12:27:57 -0700, Simon Michael <simon(a)joyful.com> said: Simon> With 2.8 released, I felt Darcs deserves better presentation. After Simon> surveying other VCS sites I worked on an update to our home page Simon> layout and content over the last few days, with review and input from Simon> #darcs, and it went live last night. It's far from perfect but I hope Simon> it's a good step forward. Thanks for the input, and have at it: Simon> http://darcs.net Hi, that looks good, thanks. Here are some ideas : - The purple color of titles is not very attractive, changing it to something more saturated would make it look fresher. - the purple color rounded box is not very attractive either :) - there is a video right on the first page, good ! Unfortunatly, it is not showing darcs but its cousin. Also, the title is "why do we continue to develop ... [camp]", which isn't the most positive approach one can expect on a start page - the compact tutorial is great, I love it. It deserves more poish. For inspiration, one can have a look at this website for example : http://gembundler.com/ - underlined links looks a bit 90's. You can add a simple css rule to choose an other color and remove the underline (though it is good practice to set it on mouseover) cheers, -- Paul

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ANN: HandsomeSoup-0.3: CSS selectors for HXT
by aditya bhargava 01 May '12

01 May '12

*Homepage:* http://egonschiele.github.com/HandsomeSoup *On Hackage:* http://hackage.haskell.org/package/HandsomeSoup *Blurb:* HandsomeSoup is the library I wish I had when I started parsing HTML in Haskell. It is built on top of HXT and adds a few functions that make is easier to work with HTML. Most importantly, it adds CSS selectors to HXT. The goal of HandsomeSoup is to be a complete CSS2 parser for HXT (it is very close to this right now). -- adit.io

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Darcs home page updated
by Simon Michael 01 May '12

01 May '12

With 2.8 released, I felt Darcs deserves better presentation. After surveying other VCS sites I worked on an update to our home page layout and content over the last few days, with review and input from #darcs, and it went live last night. It's far from perfect but I hope it's a good step forward. Thanks for the input, and have at it: http://darcs.net -Simon

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Re: [Haskell-cafe] [Haskell] ANN: Portackage - a hackage portal
by Andrew Seniuk 30 Apr '12

30 Apr '12

> On Apr 27, 2012 6:18 PM, "Andrew Seniuk" <rasfar(a)gmail.com> wrote: > Oh, the idea wasn't that you would make the app, but that by providing a web > service interface you would make it easier for me or anyone else to make an > app or other alternative interface. It would also reduce bandwidth usage > over screen scraping. Yes, well as I mentioned in the part of my response you didn't quote, I agree it would be desireable, and am working on it, but my motivation was to provide people with something immediately useful. Once I get something reasonably efficient worked out, I'll look at uploading the Haskell code to Hackage, providing JSON data, etc.. Meanwhile if someone wanted to work on their own app or interface, it's not too hard to get CSV from the table. I made a mistake and the link was down for about 12 hours. It's back now, with a number of improvements and up-to-date hackage info. It is an expensive page to run, you may get "Script not responding" pop-ups depending on your hardware, but I think it's worth it. http://fremissant.net/portackage Kind Reg'ds, Andrew

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