January 2008 - Haskell-Cafe

Data constructors versus types
by Peter Verswyvelen 17 Jan '08

17 Jan '08

I know nothing about theoretical computer science, but I was wondering if it possible to forget about types, and just keep the concept of data constructors, and have an analyzer determine correctness of the code and "staticness" of the data? Basically this is what SCHEME does no? Doesn't SCHEME have static whole program analyzers to remove the overhead of the "symbol tags" and check correctness of a program (Stalin, Petit-Scheme, ...)? What are to pros/contras? Thank you, Peter

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Compiling Blobs
by Peter Verswyvelen 17 Jan '08

17 Jan '08

I'm trying to build http://www.cs.york.ac.uk/fp/darcs/Blobs using GHC 6.8.2. It looks like a good Haskell program to learn from. So far I managed to modify the source code so it makes use of the new HaXML libraries, and after a lot of hacking I could build and link to wxHaskell, but my app crashes (I do get a window however, woohoo) Maybe someone else managed already? Thanks, Peter

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libmpd-haskell RFC
by Ben Sinclair 17 Jan '08

17 Jan '08

Hello all, If anybody has already used libmpd-haskell (the darcs repo version) or would like to look over it I would appreciate their comments. Thanks, Ben http://turing.une.edu.au/~bsinclai/code/libmpd-haskell/

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Filter by several predicates at once
by Dougal Stanton 17 Jan '08

17 Jan '08

Are the functions > passall, passany :: [a -> Bool] -> a -> Bool > passall ps v = and $ map ($v) ps > passany ps v = or $ map ($v) ps or something similar defined anywhere? Such that one can write > filter (passany tests) [0..10] > filter (passall tests) [0..10] where > tests = [>5, odd] Or is there a better way of filtering by several predicates for each value without using > filter p3 . filter p2 . filter p1 or > filter (\v -> p1 v && p2 v && p3 v) vs Cheers, D -- Dougal Stanton dougal(a)dougalstanton.net // http://www.dougalstanton.net

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HPC of several modules?
by Magnus Therning 17 Jan '08

17 Jan '08

How do I get reports on coverage of all modules in a program? The documentation I've found http://blog.unsafeperformio.com/?p=18 and http://www.haskell.org/ghc/docs/latest/html/users_guide/hpc.html both do coverage of a single module. Going the naive route of first making sure there are no compiled modules in my source tree (i.e. removing all .o and .hi files) then running 'ghc --make -fhpc MyTool.hs' succeeds in building the program, and I get a MyTool.tix and a .mix file in .hpc/ for each module after running it, but how do I get 'hpc' to produce reports containing more than just Main? 'hpc6 markup MyTool' includes only Main 'hpc6 markup MyTool Main My.Module' includes only Main 'hpc6 markup MyTool My.Module' results in an error: Writing: hpc_index.html hpc6: Prelude.foldr1: empty list None of the arguments shown by 'hpc6 help markup' stands out as a clear candidate either... 'hpc6 report --per-module MyTool' generates this: -----<module Main>----- 80% expressions used (386/479) 100% boolean coverage (0/0) 100% guards (0/0) 100% 'if' conditions (0/0) 100% qualifiers (0/0) 100% alternatives used (0/0) 100% local declarations used (0/0) 100% top-level declarations used (17/17) Where are my other modules??? Any and all help is appreciated. /M

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announcing darcs 2.0.0pre2
by David Roundy 17 Jan '08

17 Jan '08

I am pleased to announce the availability of the second prerelease of darcs two, darcs 2.0.0pre2. This release fixes several severe performance bugs that were present in the first prerelease. These issues were identified and fixed thanks to the helpful testing of Simon Marlow and Peter Rockai. We also added support for compilation under ghc 6.4, so even more users should be able to test this release. As before, some information about the prerelease is available at: http://wiki.darcs.net/index.html/DarcsTwo You can either use darcs to grab the latest darcs, or you can download a tarball at: http://darcs.net/darcs2.0.0pre2.tar.gz A few outstanding performance issues are: 1. darcs whatsnew performance has dropped for hashed repositories due to no longer tracking file-modification times. We need to reenable this feature, but it's not quite clear how best to do so. 2. darcs get on a hashed repository is not as fast as the older darcs get --partial. We could fix this by enabling the downloading of a single file _darcs/pristine.hashed.tar.gz. This eliminates the potential benefits of caching file downloads on darcs get (currently a second "get" of the same remote repository is almost free if you've enabled a global cache), so we may want to make this behavior optional. I hope we can get even more testing with this release, and look forward to finding and fixing any remaining performance regressions (or bugs, of course)! David

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Poisx select support
by Galchin Vasili 17 Jan '08

17 Jan '08

Hello, In the ghc libraries directory I can't find the Haskell .hs/.lhsthat implements Posix select. ?? I found Select.c. Regards, Vasili

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haskelldb basic documentation needed
by Immanuel Normann 16 Jan '08

16 Jan '08

with the release update haskelldb-0.9 -> haskelldb-0.10 several things have changed. Unfortunately the API documentation does not give enough information in generall. Is there any additional uptodate documentation around? In particular the fundamental function "connect" hast a new signature: *connect* :: (forall m a . MonadIO<http://hackage.haskell.org/packages/archive/mtl/1.1.0.0/doc/html/Control-Mo…>m => [( String<http://hackage.haskell.org/packages/archive/base/3.0.0.0/doc/html/Data-Char…>, String<http://hackage.haskell.org/packages/archive/base/3.0.0.0/doc/html/Data-Char…>)] -> (Database<file:///usr/share/doc/haskelldb-0.10/Database-HaskellDB-Database.html#t%3ADatabase>-> m a) -> m a) I don't know what pairs of strings this function needs. The API description is to unspecific: The connect<file:///usr/share/doc/haskelldb-0.10/Database-HaskellDB-DriverAPI.html#v%3Aconnect>function takes some driver specific name, value pairs use to setup the > database connection, and a database action to run. What are the specific name value pairs needed (for a connection to a mysql db )? Immanuel

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Re: [Haskell] Simulating client server communication with recursive monads
by Ryan Ingram 16 Jan '08

16 Jan '08

(redirected to haskell-cafe) mfix is "value" recursion, not "effect" recursion. It allows you to "tie-the-knot" with data being constructed recursively even in a monadic context. When you are using the Writer monad like this, the bind operation between statements in a "do" construct is just ++. "simulation" in your message is > simulation:: Writer [String] () > simulation = mdo > a <- server cr > cr <- client $ take 10 a > return () This is really just the following: > simulation :: Writer [String] () > simulation = Writer result where > ( a, a_out ) = runWriter (server cr) > ( cr, cr_out ) = runWriter (client $ take 10 a) > result = ( (), a_out ++ cr_out ) With "mdo" you are allowed to have the values refer to each other; the right hand side of "( a, a_out ) = ..." can refer to "cr" and vice versa. But there's no way to follow the thread of computation between the server and the client with this style. -- ryan

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ANNOUNCE: Haddock version 2.0.0.0
by David Waern 16 Jan '08

16 Jan '08

Dear Haskell community, I'm proud to announce the release of Haddock 2.0.0.0! http://www.haskell.org/haddock http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haddock-2.0.0.0 Changes in version 2.0.0.0: * The GHC API is used as the front-end * Haddock now understands all syntax understood by GHC 6.8.2 * Haddock can generate documentation for some of the language extensions in GHC 6.8.2 * Format of module attributes has changed. The only way of specifiying module attributes is via a new OPTIONS_HADDOCK pragma. Example: {-# OPTIONS_HADDOCK hide, prune #-} * Haddock understands literate source files * Add a small library to read Haddock's interface files * Add a flag -B for passing the path to the GHC lib dir * Add a flag --optghc for passing options to GHC * Add a flag --ghc-version for printing the GHC version * Remove --use-package, --allow-missing-html, --ghc-pkg, in favour of only supporting --read-interface * Remove --package flag, the GHC flag -package-name can be used instead * Remove --no-implicit-prelude flag, the GHC flag -XNoImplicitPrelude can be used instead * Hoogle support temporarily removed It should be noted that this version of Haddock is not compatible with interface files from previous versions. Also, currently, when Haddock comes across a GHC language extension that it doesn't know how to render as HTML, it will bail out. If this happens to you, bug me about it, or help out by sending a patch. On the development side, Haddock has been slightly refactored for easier code navigation, and has the beginnings of a test suite. Haddock also ships with (as noted in the list of changes) a small, experimental library that can be used to read its interface files. Tools and IDE-type applications can use this library to e.g. look up the documentation for identifiers. The code repository is at http://code.haskell.org/haddock. David

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