June 2010 - Haskell-Cafe

new recursive do notation (ghc 6.12.x) spoils layout
by John Lask 21 Jun '10

21 Jun '10

Whilst I have nothing against the change in syntax for recursive do aka http://old.nabble.com/Update-on-GHC-6.12.1-td26103595.html Instead of writing mdo a <- getChar b <- f c c <- g b putChar c return b you would write do a <- getChar rec { b <- f c ; c <- g b } putChar c return b it does spoil the nice layout - it would be nice to just be able to write (which does not parse) do rec a <- getChar b <- f c c <- g b putChar c return b I don't particularly care that the only recursive statements are #2,#3 - I just want my nice neat layout back. I have just spent an inordinate amount of time updating code when if the parser recognised "do rec" as a recursive group it would have been a drop in replacement and taken me one tenth of the time. Why can't we have this?

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Re: Installing Haskell on OSX
by Giuseppe Luigi Punzi 21 Jun '10

21 Jun '10

I died in more dependencies. One, ige-mac-integration. To avoid possible problems I uninstall all ports last night to reinstall full gtk2 with quartz. For now, i'm solving all of this and I hope to get it working this night. Sudish Joseph <sudish(a)gmail.com> wrote: >Antoine Latter <aslatter(a)gmail.com> writes: >> On Sun, Jun 20, 2010 at 12:53 PM, Giuseppe Luigi Punzi Ruiz >> <glpunzi(a)lordzealon.com> wrote: >>> Hi again, >>> >>> Yes, you are right, but now, "cabal install leksah" I get: >>> > >[...] > >>> Undefined symbols: >>> "_iconv_close", referenced from: >>> _hs_iconv_close in libHSbase-4.2.0.0.a(iconv.o) >>> "_iconv_open", referenced from: >>> _hs_iconv_open in libHSbase-4.2.0.0.a(iconv.o) >>> "_iconv", referenced from: >>> _hs_iconv in libHSbase-4.2.0.0.a(iconv.o) >>> ld: symbol(s) not found > >> This one is a bummer, and I see it all the time when I try to build a >> package linked against macports. > >This is caused by the two libiconv's in the system being >ABI-incompatible, sadly. The ghc pkg available for download from >haskell.org is linked against the system /usr/lib/libiconv.dynlib, which >has iconv_open() as a function. Macports has GNU libiconv which >#defines iconv_open to libiconv_open() in /opt/local/include/iconv.h. > >This then blows up as above when linking against other libraries in >macports - the linker pulls in GNU libiconv which lacks the symbols >needed as you see above. > >One workaround is to link GHC itself against the macports version of >libiconv (and libgmp) and have cabal-install link all subsequent >libraries against macports. I did just that this weekend and now have a >working threadscope using the Quartz backend for gtk2, which is very >nice (no need to run the X server). > >Recipe for reproducing this build is included below (and I can also >provide the resulting ghc-6.12.3 pkg file if needed). > >Another possible option is to use Homebrew to install gtk2 and other >dependencies. Homebrew prefers to use the system-provided libraries and >will not pull in GNU libiconv, so this should work with the existing GHC >pkg in theory. I didn't pursue this since the homebrew 'gtk+' package >didn't seem to have the option to use Quartz instead of X11 as its >backend. > >Steps for linking the ghc runtime against macports: > >- Specify EXTRA_CABAL_CONFIGURE_FLAGS in mk/build.mk as mentioned at > the bottom of http://www.haskell.org/ghc/download_ghc_6_12_3.html > > EXTRA_CABAL_CONFIGURE_FLAGS = --extra-include-dirs=/opt/local/include \ > --extra-lib-dirs=/opt/local/lib > >- Use the --with-iconv-* and --with-gmp-* flags when configuring ghc. > > ./configure --with-iconv-includes=/opt/local/include \ > --with-iconv-libraries=/opt/local/lib \ > --with-gmp-includes=/opt/local/include \ > --with-gmp-libraries=/opt/local/lib > >- This produces a GHC runtime and libraries linked against macports: > > % otool -L ghc > ghc: > /opt/local/lib/libncurses.5.dylib (compatibility version 5.0.0, current version 5.0.0) > /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 125.2.0) > /opt/local/lib/libiconv.2.dylib (compatibility version 8.0.0, current version 8.0.0) > /opt/local/lib/libgmp.10.dylib (compatibility version 11.0.0, current version 11.1.0) > > % nm HSbase-4.2.0.2.o | fgrep iconv > 001ff7f0 T _hs_iconv > 001ff7e0 T _hs_iconv_close > 001ff800 T _hs_iconv_open > U _libiconv > U _libiconv_close > U _libiconv_open > > ghc and included libraries use the macports libiconv, so linking > against other libraries in macports (gtk2!) will work. > >- Have cabal-install use macports as well for packages it installs by > editing ~/.cabal/config and setting: > > extra-include-dirs: /opt/local/include > extra-lib-dirs: /opt/local/lib > >- This gives, for e.g., threadscope linked against macports: > > % otool -L ~/.cabal/bin/threadscope | egrep '(gtk|iconv)' > /opt/local/lib/libgtk-quartz-2.0.0.dylib (compatibility version 2001.0.0, current version 2001.1.0) > /opt/local/lib/libiconv.2.dylib (compatibility version 8.0.0, current version 8.0.0) > >- Note that since the ghc runtime is still in 32-bit i386 mode, we > need universal versions of most libraries in macports. Recent > versions of macports (1.9 for sure, maybe 1.8) make it simple to > switch from an x86_64 library to a universal one: > > % port install gtk2 +universal > > This will recompile gtk2 and *all* dependent libraries as universal > libraries which is exactly what you need. You can then eliminate any > inactive 64-bit libraries with: > > % port -f uninstall inactive > >> Here's the last thread about with, with more links and discussion: >> >> http://thread.gmane.org/gmane.comp.lang.haskell.general/18064/ >> >> The response by Jean-Marie Gaillourdet has worked for me in the past. >> >> Antoine > >Thanks for that link, I didn't think of overriding libiconv on a >per-package basis. > >-Sudish

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How to browse code written by others
by Martin Drautzburg 21 Jun '10

21 Jun '10

Hello all, I need your advice about how to browse code which was written by someone else (Paul Hudak's Euterpea, to be precise, apx. 10000 LOC). I had set some hopes on leksah, and it indeed shows me the interfaces, but I have not yet convinced it to show me more than that. I ran haddock over the sources, and again I could not see more that just signatures. I would be very happy with something like a Smalltalk browser. Something that would let me zoom down to the source code, but with "search" and hyperlink capabilities ("senders" and "implementers" in Smalltalk). Anyways, how do you guys do it, i.e. how to you dive into non-trivial foreign code? -- Martin

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MonadCatchIO-transformers and ContT
by Michael Snoyman 21 Jun '10

21 Jun '10

Hi cafe, I ran into a segfault while working on some database code. I eventually traced it back to a double-finalizing of a statement (read: freeing memory twice), which ultimately led back to switching my code to use the ContT monad transformer. I was able to isolate this down to a minimal test case (catch.hs); when run, it prints the line "released" twice. In an attempt to understand what's going on, I rewrote the code to avoid the libraries entirely (catch-simplified.hs); it didn't give me any insight into the problem, but maybe it will help someone else. If someone sees an obvious mistake I'm making in my usage of the bracket_ function, please let me know. Otherwise, I'd really like to get a fix for this so I can use this library. Thanks, Michael

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installation problem with cabal
by Louis Plissonneau 21 Jun '10

21 Jun '10

Hi, when I install packages from cabal, the package manager installs already installed files (some of his favorites are process and haskell98) so these packages are both system and user installed (that can cause later dependency problems, but the cabal FAQ explains very well how to recover from it) but the funniest is that if I reinstall it, it again reinstall the same packages even they are already twice installed does anyone know how to solve this issue? Thanks Louis

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Installing Erlang on OSX
by Giuseppe Luigi Punzi Ruiz 21 Jun '10

21 Jun '10

Hi all, I would like to play a little with erlang. I downloaded Haskell platform, ang ghc and ghci seems to work. Trying to install leksah, Cabal, gives me the following error. http://trac.haskell.org/haskell-platform/ticket/110 I downloaded and installed Haskell Platform from source. Did.. cabal update cabal install cabal-install cabal install gtk2hs-buildtools cabal install leksah The last command didn't finish, with this output: ************************************************************ .. ... [22 of 23] Compiling Text.Regex.TDFA.Sequence ( Text/Regex/TDFA/ Sequence.hs, dist/build/Text/Regex/TDFA/Sequence.o ) Text/Regex/TDFA/Sequence.hs:36:9: Warning: orphan instance: instance RegexContext Regex (Seq Char) (Seq Char) Text/Regex/TDFA/Sequence.hs:40:9: Warning: orphan instance: instance RegexMaker Regex CompOption ExecOption (Seq Char) Text/Regex/TDFA/Sequence.hs:46:9: Warning: orphan instance: instance RegexLike Regex (Seq Char) [23 of 23] Compiling Text.Regex.TDFA ( Text/Regex/TDFA.hs, dist/build/ Text/Regex/TDFA.o ) Registering regex-tdfa-1.1.3... Installing library in /Users/glpunzi/.cabal/lib/regex-tdfa-1.1.3/ ghc-6.12.1 Registering regex-tdfa-1.1.3... cabal: Error: some packages failed to install: cairo-0.11.0 failed during the configure step. The exception was: ExitFailure 1 gio-0.11.0 depends on glib-0.11.0 which failed to install. glib-0.11.0 failed during the configure step. The exception was: ExitFailure 1 gtk-0.11.0 depends on glib-0.11.0 which failed to install. gtksourceview2-0.11.0 depends on glib-0.11.0 which failed to install. leksah-0.8.0.6 depends on glib-0.11.0 which failed to install. leksah-server-0.8.0.6 depends on glib-0.11.0 which failed to install. ltk-0.8.0.6 depends on glib-0.11.0 which failed to install. pango-0.11.0 depends on glib-0.11.0 which failed to install. ************************************ Possible usefull info: galilea-2:haskell-platform-2010.1.0.0 glpunzi$ cabal --version cabal-install version 0.8.0 using version 1.8.0.2 of the Cabal library galilea-2:haskell-platform-2010.1.0.0 glpunzi$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.1 Some idea how to get leskah, or/and full Haskell working under OSX? Thanks in advance. Giuseppe Luigi Punzi Ruiz Blog: http://www.lordzealon.com Twitter & Skype & GoogleTalk accounts: glpunzi

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Haskell Indonesia
by vipex.id 21 Jun '10

21 Jun '10

Hi, I'm new in Haskell & wondering is there Indonesian people using Haskell here. Nice meet* you all :) Regards, vipex

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Third Ghent FPG Meeting on Tuesday, June 29: Final Program
by Jeroen Janssen 20 Jun '10

20 Jun '10

(apologies if you receive multiple copies) Dear all, We have finalized the program for the third Ghent FPG Meeting on Tuesday, June 29 in the Technicum building of Ghent University (Sint-Pietersnieuwstraat 41, 9000 Gent) at 19h. As before, to enter the building, you should go to the automatic sliding door on the far left of the building and dial the phone number provided on the note that is taped to the door. Someone will then open the door for you. The program is as follows: 1. Pierre Carbonnelle - Declarative programming for Business Logic - a new Open-source project Declarative programming has made major inroads in the Presentation and Persistence layers of applications, with HTML, CSS and SQL. Why has it not been popular in the Business Logic layer of applications ? Pierre Carbonnelle, an engineer with 20 years of experience in the IT industry, will present a new open source project, "Yoopf", that aims to fix this gap. It uses the "formula- based" models of spreadsheets for the development of business applications, with a particular emphasis on the Google App Engine platform. Research opportunities will be discussed. 2. Tom Schrijvers - Monadic Constraint Programming A constraint programming system combines two essential components: a constraint solver and a search engine. The constraint solver reasons about satisfiability of conjunctions of constraints, and the search engine controls the search for solutions by iteratively exploring a disjunctive search tree defined by the constraint program. We give a monadic definition of constraint programming where the solver is defined as a monad threaded through the monadic search tree. We are then able to define search and search strategies as first class objects that can themselves be built or extended by composable search transformers. Search transformers give a powerful and unifying approach to viewing search in constraint programming, and the resulting constraint programming system is first class and extremely flexible. This is joint work with Peter Stuckey (University of Melbourne) and Philip Wadler (University of Edinburgh). 3. Wouter Kampmann, Lieven Lemiengre - Lightning Talk on Scala Hope to see you there! Bart Coppens (bart.coppens(a)elis.ugent.be) Jeroen Janssen (jejansse(a)gmail.com) Jasper Van der Jeugt (jaspervdj(a)gmail.com) The Ghent FPG organizing committee.

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Mapping a list of functions
by Martin Drautzburg 20 Jun '10

20 Jun '10

Hello all The standard map function applies a single function to a list of arguments. But what if I want to apply a list of functions to a single argument. I can of course write such a function, but I wonder if there is a standard way of doing this, Related to that is the problem, that the function in map may require more than one argument, so I need to apply it partially to some value(s) first. But what if I want to fix its second and third argument and not its first argument? Can flip help? Thanks -- Martin

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lecture notes for Finally Tagless - benefit of explicit fix combinator
by Günther Schmidt 19 Jun '10

19 Jun '10

Hi all, I'm working my way through the lecture notes to the "Finally Tagless" course, available from: http://okmij.org/ftp/tagless-final/course/index.html As usual at every round of doing so, I think it's my 4th, I discover something new, something that hadn't occurred to me previously. This time it's a comment in the code of SerializeExt.hs -- * Tie up the knot fromTree = S.fix fromTreeExt -- One does use fix in real programs -- Now we can see the real benefit of using fix in real programs. -- The fixpoint combinator is NOT a mere curiosity about the fix-point combinator. I don't think I can deduce what he meant by that from the context alone, could somebody please enlighten me? I'm refering to the benefit of using the fix-point combinator in this particular scenario. Günther

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