April 2010 - Haskell-Cafe

Haskell statefull computations and co-algebras?
by Vasili I. Galchin 26 Apr '10

26 Apr '10

Hello, I am fairly conversant in category. In any case, I am re-reading Bart Jacobs co-algebra tutorial citeseer.ist.psu.edu/jacobs97tutorial.html and draft book www.cs.ru.nl/B.Jacobs/CLG/JacobsCoalgebraIntro.pdf. Have others read these? What role do co-algebras play in Haskell stateful computations (as opposed to or complementing monads)? Kind regards, Vasili

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and [] = True; or [] = False
by Bjorn Buckwalter 26 Apr '10

26 Apr '10

Dear all, Does it make good sense that 'and []' returns 'True' and 'or []' returns 'False'? The Haskell Road to Logic, Maths and Programming says so: "The function or takes a list of truth values and returns True if at least one member of the list equals True, while and takes a list of truth values and returns True if all members of the list equal True." "Should the conjunction of all elements of [] count as true or false? As true, for it is indeed (trivially) the case that all elements of [] are true. So the identity element for conjunction is True. Should the disjunction of all elements of [] count as true or false? As false, for it is false that [] contains an element which is true. Therefore, the identity element for disjunction is False." While the above reasoning is fine, and allows straight-forward implementations, it isn't extremely convincing. In particular, it isn't clear that, while simple, the definitions of the first paragraph are the most sensible. Perhaps one of the more mathematically versed readers on the Cafe could enlighten me? What got me thinking about this was the apparently incorrect intuition that 'and xs' would imply 'or xs'. Thanks, Bjorn

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Re: FRP for game programming / artifical life simulation
by Patai Gergely 26 Apr '10

26 Apr '10

> (in my own FRP experiments I have an update thread and a render thread). I wonder how to nicely deal with state that requires communication with the outer world, even though it is functional at heart. For instance, if you want to change a vertex buffer or a texture or whatever during the update, how do you organise your code? Do you maintain separate pure and impure state information blocks? > However, it is not clear if you want to "pull" or "push" such a network >From what I've seen so far, I think you really need both at the same time. Pull for continuous signals and push for events. The tricky part is that events can be derived from signals (cf. Fran's predicate events), and you really don't want the signals depending on these events to be reevaluated even when there are no occurrences. The best you can do with pull is to explicitly keep the previous output when there's no incoming event. However, if you fmap over this output, the function will be evaluated in every step regardless of the lack of change, since functions are not memoised. I believe this is a problem Grapefruit aims to solve with its notion of segmented signals [1]. Deciding between push and pull according to profiling results is a nice idea. :) It might be too expensive to do it adaptively during runtime (the overhead might easily distort the results and thus render them invalid), but treating it as just a flag could give us an interesting architecture to play with. Gergely [1] http://hackage.haskell.org/packages/archive/grapefruit-frp/0.0.0.0/doc/html… -- http://www.fastmail.fm - A fast, anti-spam email service.

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The Poor Man's PVP-Checking Tool
by Neil Brown 26 Apr '10

26 Apr '10

Hi, The issue of a tool to help with checking packages against the Package Versioning Policy (PVP: http://www.haskell.org/haskellwiki/Package_versioning_policy) has come up several times on this list, and it seems to be a generally wanted tool. One of the things desired in such a tool is the ability to check what has been added/changed/removed in the latest version of a package, to help see what kind of version bump is needed according to the PVP. This could also be used to check if past releases have obeyed the PVP. A proper way to build this tool is to parse the types of everything involved (functions, data types, and instances), and check them for being the same (given that the type parameters may have been renamed, or a definition substituted for a synonym and so on). I have instead hacked together a tool in an hour or so that roughly does the job. It uses Perl, which I haven't written for a long time, because that was quickest -- if this bothers you, pretend its a crazy Haskell EDSL or something :-). I've attached the Perl script. It combines the command-line tools diff and sort with a tiny bit of manual processing along with the Haskell tool cabal-install, and brings them to bear on the output of "haddock --hoogle", which already nicely spits out the types of all functions, data types and instances in your library. It looks for differences in the output between two package versions, and tells you what version bump it thinks you need. It errs on the conservative side, I believe -- it should never miss a bump when it's needed, but you may sometimes get a false alarm if you've made a harmless change. As an example, let's take the recent stm release. You run the Perl script with the name and version of both package versions (if you omit the version, it should use the latest): ==== neil@beast ~: perl cmp.perl stm-2.1.2.0 stm-2.1.1.2 Warning: The documentation for the following packages are not installed. No links will be generated to these packages: ffi-1.0, rts-1.0 Warning: The documentation for the following packages are not installed. No links will be generated to these packages: ffi-1.0, rts-1.0 25a26 > Control.Concurrent.STM.TVar:readTVarIO :: TVar a -> IO a 39c40 < @version 2.1.1.2 --- > @version 2.1.2.0 Given previous version number A.B.C.D: It seems you have added something. You must increase C (or A or B) ==== For those who can read diff, you get the diff. At the bottom, the tool works out the required bump; here something was added so C must be increased, and we can see that the package did just that. Let's take another example from my latest development version of my CHP library; here I run the command with one argument (chp, which means the latest version on Hackage): ==== neil@banshee ~/work/chp: perl cmp.perl chp Warning: The documentation for the following packages are not installed. No links will be generated to these packages: rts-1.0 Control/Concurrent/CHP/Alt.hs:116:9: Warning: orphan instance: instance Alternative CHP Warning: The documentation for the following packages are not installed. No links will be generated to these packages: rts-1.0 6a7 > Control.Concurrent.CHP.Alt:instance Alternative CHP 82d82 < Control.Concurrent.CHP.Channels.Creation:class ChannelTuple t 88,92d87 < Control.Concurrent.CHP.Channels.Creation:instance (Channel r w) => ChannelTuple (Chan r w a, Chan r w a) < Control.Concurrent.CHP.Channels.Creation:instance (Channel r w) => ChannelTuple (Chan r w a, Chan r w a, Chan r w a) < Control.Concurrent.CHP.Channels.Creation:instance (Channel r w) => ChannelTuple (Chan r w a, Chan r w a, Chan r w a, Chan r w a) < Control.Concurrent.CHP.Channels.Creation:instance (Channel r w) => ChannelTuple (Chan r w a, Chan r w a, Chan r w a, Chan r w a, Chan r w a) < Control.Concurrent.CHP.Channels.Creation:instance (Channel r w) => ChannelTuple (Chan r w a, Chan r w a, Chan r w a, Chan r w a, Chan r w a, Chan r w a) 102d96 < Control.Concurrent.CHP.Channels.Creation:newChannels :: (ChannelTuple t, MonadCHP m) => m t 149c143 < Control.Concurrent.CHP.Monad:class (MonadIO m) => MonadCHP m --- > Control.Concurrent.CHP.Monad:class (Monad m) => MonadCHP m 155a150 > Control.Concurrent.CHP.Monad:foreverP :: CHP a -> CHP b 156a152 > Control.Concurrent.CHP.Monad:liftIO_CHP :: IO a -> CHP a 160a157 > Control.Concurrent.CHP.Monad:process :: String -> a -> a 164a162 > Control.Concurrent.CHP.Monad:subProcess :: String -> a -> a 169c167 < Control.Concurrent.CHP.Parallel:data (Monad m, MonadCHP m) => ForkingT m a --- > Control.Concurrent.CHP.Parallel:data ForkingT m a 173d170 < Control.Concurrent.CHP.Parallel:instance (MonadIO m) => MonadIO (ForkingT m) 175c172 < Control.Concurrent.CHP.Parallel:instance MonadTrans ForkingT --- > Control.Concurrent.CHP.Parallel:liftForking :: (Monad m) => m a -> ForkingT m a 239c236 < @version 2.1.0.1 --- > @version 2.2.0 Given previous version number A.B.C.D: It seems you have added, removed or changed some instances. You must increase A or B It seems you have removed something, or changed its name or type. You must increase A or B It seems you have added something. You must increase C (or A or B) ==== As you can see, I've been changing a lot in CHP, and I'm going to need a major version bump (which I've already put in, as you can see: 2.1.0.1 to 2.2.0). It shouldn't be too hard to augment the tool to check the versions against the required bump. I think that covers everything. The script has been tested with GHC 6.10.4 (and tools of a similar vintage) and 6.12.1 (with near-enough the latest versions of most things), but it will probably go wrong if you sneeze at it. In particular, many newer Haskell features (e.g. GADTs) may throw it off, and I'm not sure if it will work if you use it on an OS that's too far from Linux. But hopefully it'll provide a useful stop-gap for some until a real tool is developed, and I think it's at least better than having nothing. Thanks, Neil.

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Halp: wacky FFI problems
by Louis Wasserman 26 Apr '10

26 Apr '10

Hey, I'm trying to do some funky things with the FFI, and I'm clueless enough with C that I could use some help. In particular, I'm trying to bind to the glpk library in a multithreaded style. It's not normally thread-safe, but somebody suggested how to make it safe here <http://www.mail-archive.com/help-glpk@gnu.org/msg04021.html>. I have no idea how to integrate this with FFI-style bindings. A single-threaded set of bindings that I've written is here: http://hackage.haskell.org/package/glpk-hs I'm trying to figure out how to integrate FFI bindings with pthreads, and yadda yadda, and I'm pretty clueless. Any suggestions? Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

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Fwd: Mancoosi International Solver Competition
by Johannes Waldmann 26 Apr '10

26 Apr '10

This looks interesting: an international competition of solvers for package/component installation and upgrade problems. http://www.mancoosi.org/misc-2010/ (deadline: June 10) Is the underlying package dependency model the same as cabal's? If (nearly) so, then cabal's solver could enter the competition - if it wins, then fine; if not, then it could be replaced by the winner :-)

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Bulk Synchronous Parallel
by Gregory Crosswhite 26 Apr '10

26 Apr '10

Hey everyone, Has anyone done any work with bulk synchronous parallel computing in Haskell? The idea behind the model is that you divide your computation into a series of computation and communication phases, and it has recently occurred to me that this might be an ideal setup for parallelizing a pure language like Haskell because you can think of it as alternating between a stage that independently applies a bunch of functions to a bunch of independent chunks of data and a stage that applies a big function to all of the chunks that recombines them into new chunks for the next parallel phase, so that all stages are conceptually pure even if eventually the second stage is turned into something involving communication and hence side-effectful under the hood. Experiences? Thoughts? Cheers, Greg

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Order of arguments to GCC when building SDL on Windows with Cabal
by JP Moresmau 26 Apr '10

26 Apr '10

Hello, I've been trying for a while to build the SDL haskell library on windows. People have told me it worked for them, but I can't get to work for the life of me. I have the latest Haskell Platform, I think more or less the latest MINGW and MSYS tools. I'm on Windows 7 64 bits. I installed the SDL library without hassle, and the samples work. It's only the haskell library that's bothering me. I've modified the .cabal file as indicated for Windows (extra-include-dirs, extra-lib-dirs), and runhaskell Setup configure. But then runhaskell Setup build fails with: Preprocessing library SDL-0.5.9... dist\build\Graphics\UI\SDL\General_hsc_make.o:General_hsc_make.c:(.text +0x29): m ultiple definition of `main' d:/dev/mingw/bin/../lib/gcc/mingw32/4.5.0/../../../ libmingw32.a(main.o):main.c:( .text+0x0): first defined here D:\dev\SDL-1.2.14\lib/libSDLmain.a(SDL_win32_main.o): In function `console_main' : /Users/hercules/trunk/SDL-1.2/./src/main/win32/SDL_win32_main.c:315: undefined r eference to `SDL_main' collect2: ld returned 1 exit status linking dist\build\Graphics\UI\SDL\General_hsc_make.o failed command was: d:\dev\MinGW\bin\gcc.exe -LD:\dev\SDL-1.2.14\lib -L/usr/ lib -lmingw 32 -lSDLmain -lSDL -mwindows -LD:\dev\haskell\HaskellPlatform \2010.1.0.0\base-4. 2.0.0 -lwsock32 -luser32 -lshell32 -LD:\dev\haskell\HaskellPlatform \2010.1.0.0\i nteger-gmp-0.2.0.0 -LD:\dev\haskell\HaskellPlatform\2010.1.0.0\ghc- prim-0.2.0.0 -LD:\dev\haskell\HaskellPlatform\2010.1.0.0 -LD:\dev\haskell \HaskellPlatform\201 0.1.0.0/gcc-lib -lm -lwsock32 -LD:\dev\haskell\HaskellPlatform \2010.1.0.0 dist\b uild\Graphics\UI\SDL\General_hsc_make.o -o dist\build\Graphics\UI\SDL \General_hs c_make.exe (I have the same error with the gcc inside the Haskell Platform, the same error running in a windows shell without MSYS on the path, in the MSYS bash with MSYS on the path). And I figured out what's wrong with that line after thinking for a while (as in, several days...): basically the line says something like gcc sdl-flags file.o -o file.exe. If I revert the order of the arguments and run gcc file.o -o file.exe sdl-flags, it works! The gcc documentation explains that the libraries are searched in the order they appear (http://gcc.gnu.org/onlinedocs/gcc/Link-Options.html) so to put the libraries you depend on after the code that has the dependencies. And I can see that building the SDL samples indeed puts the .c file before the sdl linker flags. So this seems to be my problem. What's the solution? 1. I've done something stupid or forgot some essential MINGW tool that would make my system not dependent on the order of the arguments to gcc? I've tried with gcc 3.4.5 and gcc 4.5.0, same result. 2. I can hack somewhere (where???) the command passed on to GCC when building? Maybe some voodoo in the cabal file? 3. I can get runhaskell Setup build to only dump the commands it's going to run, modify them in my favorite text editor and run them manually (as said above, I've managed to create the first exe required but build insists on rebuilding it even if present) 4. ...? The fact that some people managed seem to point with a problem with my config, but it's seems reasonnably clean to me, and I've even managed to install wxWidgets and wxHaskell... Any help appreciated! -- JP Moresmau http://jpmoresmau.blogspot.com/

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ANNOUNCE: graphviz 2999.9.0.0
by Ivan Lazar Miljenovic 26 Apr '10

26 Apr '10

I'm pleased to announce the latest version of my graphviz library that provides Haskell bindings to the Graphviz graph visualisation suite. There are numerous changes in this release, the most important of which are: * graphviz now has an FAQ and an improved README as well as its own homepage: http://projects.haskell.org/graphviz/ (as prompted by Eric Kow). * Add support for record labels; values are automatically escaped/unescaped. The `Record` and `MRecord` shapes have been added for use with these labels. Requested by Minh Thu and Eric Kow. * Add support for HTML-like values (this replaces the wrong and completely broken URL datatype). Strings are automatically escaped/unescaped. * Various parsing improvements (including a slight parsing speed increase!). In particular, graphviz is now able to parse almost all Dot graphs found on my system (including samples shipped with Graphviz, Linux kernel documentation and various other package documentations). A list of the breakages and why: * /usr/share/sgml/docbook/xsl-stylesheets/roundtrip/template.dot seems to be a binary file and thus can't be read. * /usr/share/graphviz/graphs/directed/Latin1.gv uses Latin1 encoding; at the moment graphviz uses the system's locale encoding (or whatever GHC < 6.10 defaults to). * /usr/share/doc/boost-*/html/libs/graph/example/graphviz_test.dot (various boost versions) has subgraphs in edges; graphviz currently can't cope with these. * /usr/src/linux-2.6.33-gentoo-r1/Documentation/blockdev/drbd/drbd-connection-state-overview.dot uses incorrect syntax for the "minlen" attribute (it is meant to be an integer but actually contains a floating point value). The plans for the next release (which I don't plan on even starting for a while) are to focus on improving printing and parsing performance, using a state-based printer and parser (as part of Dot syntax is state-based) and force usage of UTF-8 (via text or utf8-string). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Why does cabal select base-3.0.3.2 when base-4.2.0.0 is available?
by Bjorn Buckwalter 26 Apr '10

26 Apr '10

Dear all, Why does cabal seem to prefer base-3.0.3.2 over base-4.2.0.0 when installing packages with an unqualified base requirement? Example: $ cabal install -v fad --reinstall [snip] Resolving dependencies... selecting fad-1.0 (hackage) selecting base-3.0.3.2 (installed) and 4.2.0.0 (installed) and discarding syb-0.1.0.0, 0.1.0.1, 0.1.0.2, 0.1.0.3, 0.2 and 0.2.1 [snip] Configuring fad-1.0... Dependency base ==3.0.3.2: using base-3.0.3.2 [snip] [1 of 1] Compiling Numeric.FAD ( Numeric/FAD.hs, dist/build/Numeric/FAD.o ) Numeric/FAD.hs:1:0: Warning: Module `Prelude' is deprecated: You are using the old package `base' version 3.x. Future GHC versions will not support base version 3.x. You should update your code to use the new base version 4.x. [snip] Thanks, Bjorn

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