- Haskell-Cafe - Haskell.org

In other words is there a good place to post stuff like the following?
by caseyh＠istar.ca 23 Jun '10

23 Jun '10

-- Algorithms From: "Selected Papers on Design of Algorithms", Donald Knuth, 2010 -- Chapter 10 Addition Machines -- Haskell version by Casey Hawthorne -- Note this is only a skeleton of the chapter, -- so as to wet your interest to buy the book. -- Addition Machine -- The only operations allowed are the following: -- read x -- x <- y -- x <- x + y -- x <- x - y -- if x >= y -- write x -- Can all functional versions be tail recursive? ----------------------------------------------------------------------- ----------------------------------------------------------------------- -- Modulus -- Assuming without loss of generality x>=0 and y>0, -- since one can use various identities for other cases. -- Performs floor(x/y) subtractions. modulusNaive x y | x >= y = modulusNaive (x-y) y | otherwise = x -- Can we do better? -- Uses a doubling procedure to subtract larger multiplies of y. -- Bounded by O(log(x/y))^2 modulusByDoubling x y | x >= y = helper y y | otherwise = x where helper w z | x < z = helper z z+z | otherwise = modulusByDoubling (x-w) y -- Can we do better? -- Want bounded by O(log(x/y)). -- Addition Machine, so cannot divide by 2. -- Implicitly use the Fibonacci representation of floor(x/y) -- instead of its the binary representation. -- F0 = 0; F1 = 1; Fn = F(n-1) + F(n-2), n >=2 -- Every nonnegative integer n can be uniquely represented in the form -- n = F(m1) + F(m2) + ... + F(mt), m1 >> m2 >> ... >> mt >> 0 -- where t >= 0 and m >> m' means that m - m' >= 2 -- If n > 0 this representation can be found by choosing m1 such that -- F(m1) <= n < F(m1+1) ----------------------------------------------------------------------- -- Furthermore Fibonacci numbers grow exponentially, -- about 69% as fast as powers of 2. -- They have been used as power-of-2 analogs in a variety of algorithms. ----------------------------------------------------------------------- modulusFib x y | x >= y = helper x y y | otherwise = x where helper x y z | x < z = helper x z y+z | otherwise = helper2 x y z helper2 x y z | x >= y = helper2 (x-y) y z | y > z = helper2 x (z-y) y | otherwise = x

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FastCGI, new maintainer
by Christopher Done 23 Jun '10

23 Jun '10

I'm the new maintainer of the FastCGI package[1]. Please upgrade to the latest version, 3001.0.2.3. It includes a fix for a special but real case in which the library throws an exception when the httpd server unexpectedly closes the connection[2], and the base version and exceptions have been upgraded to the latest versions.[3] Any problems, get in touch, or post an issue on this github project: http://github.com/chrisdone/fastcgi If you have patches preferably fork the fastcgi project on github and do a "Pull request". Alternatively just send me a diff. Cheers [1]: http://hackage.haskell.org/package/fastcgi [2]: http://github.com/chrisdone/fastcgi/commit/de8d303b30dec097a1d24a8991da85d3…

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Control.Alternative --- some and many?
by Gregory Crosswhite 23 Jun '10

23 Jun '10

Hey everyone, Could someone explain to me (or point me to a reference explaining) the purpose of the "some" and "many" methods of the Alternative class? Also, there is a link posted in the documentation for Control.Applicative to a paper which describes the motivation behind the Applicative class; is there similarly a paper explaining the motivation behind the Alternative class? (The problem with Googling for "Alternative" is that this word is used a whole lot of the time, and very rarely does it refer to the Alernative class. :-) ) Cheers, Greg

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TypeFamillies and UndecidableInstances - why?
by Maciej Piechotka 23 Jun '10

23 Jun '10

When I tried to do something like: > {-# LANGUAGE FlexibleContexts #-} > {-# LANGUAGE TypeFamilies #-} > > class Test a where > type TestMonad a :: * -> * > from :: a b -> TestMonad a b > to :: TestMonad a b -> a b > > data Testable a b = Testable (a b) > > instance (Test a, Functor (TestMonad a)) => Functor (Testable a) where > f `fmap` Testable v = Testable $! (to . fmap f . from) v > It asks for adding UndecidableInstances as: test.hs:11:0: Constraint is no smaller than the instance head in the constraint: Functor (TestMonad a) (Use -XUndecidableInstances to permit this) In the instance declaration for `Functor (Testable a)' What is undecidable? a is bound so TestMonad a should be bound so Functor (TestMonad a) should be valid. Is it a bug/missing feature in ghc or do I fail to see something? Regards

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ANN: srec-0.0.0
by Tom Hawkins 23 Jun '10

23 Jun '10

A little library for reading s-record files: http://hackage.haskell.org/package/srec -Tom

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TH instance code.
by Andy Stewart 23 Jun '10

23 Jun '10

Hi all, I have below duplicate code, but i don't know how to use TH instance code. ------------------------------> duplicate code start <------------------------------ instance Variable PageType where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: PageType) $ fromVariant x instance Variable Int where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: Int) $ fromVariant x instance Variable (Maybe Char) where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: Maybe Char) $ fromVariant x instance Variable (Maybe Int) where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: Maybe Int) $ fromVariant x instance Variable ProcessID where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: ProcessID) $ fromVariant x instance Variable GWindowId where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: GWindowId) $ fromVariant x ------------------------------> duplicate code end <------------------------------ Any TH expert help? Thanks, -- Andy

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Minix
by Henk-Jan van Tuyl 23 Jun '10

23 Jun '10

L.S., I just read about Minix and found a discussion [1] about Andrew Tanenbaum (the creator of Minix) wanting to try drivers written in Haskell on Minix. It has been four years since, is there currently a way to (cross)compile Haskell programs for Minix? Haskell well suited to write reliable software and Minix is all about reliability; they seem to match perfectly. It would increase visibility of Haskell if it can be used on Minix systems. Regards, Henk-Jan van Tuyl [1] http://groups.google.com/group/fa.haskell/browse_thread/thread/43f22847305b… -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

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Different choice operations in a continuation monad
by Sebastian Fischer 22 Jun '10

22 Jun '10

Dear Café, `MonadPlus` instances are usually required to satisfy certain laws, among them the monad laws and monoid laws for `mzero` and `mplus`. Additionally one may require that (>>=f) is a monoid morphism, that is: mzero >>= f = mzero (a `mplus` b) >>= f = (a >>= f) `mplus` (b >>= f) The list monad satisfies these additional laws, the `Maybe`-Monad does not satisfy the second, distributive, law: ghci> (return False `mplus` return True) >>= guard :: [()] [()] ghci> (return False `mplus` return True) >>= guard :: Maybe () Nothing Instead of the distributive law, the `Maybe` monad satisfies a different law: return x `mplus` a = return x that is, `return` annihilates the `Maybe`-Monad regarding `mplus`. This "cancellation law" is incompatible with the distributive law because (together with other laws) it implies that the result of the above example expression is `Nothing` whereas the distributive law implies that it is `Just ()`. We can lift the `Maybe` type into a continuation monad: > newtype CMaybe r a = CMaybe ((a -> Maybe r) -> Maybe r) > > instance Monad (CMaybe r) where > return x = CMaybe (\k -> k x) > CMaybe ca >>= f = CMaybe (\k -> ca (\x -> let CMaybe cb = f x in cb k)) > > instance MonadPlus (CMaybe r) where > mzero = CMaybe (\_ -> mzero) > CMaybe ca `mplus` CMaybe cb = CMaybe (\k -> ca k `mplus` cb k) Unlike the `Maybe`-monad, the `CMaybe`-monad satisfies the distributive law, not the cancellation law. Can you define an associative operation orElse :: CMaybe r a -> CMaybe r a -> CMaybe r a with identity `mzero` that satisfies the cancellation law? Cheers, Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)

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Is there good place to post Haskell alorithms/data structures that follow Steven Skiena's book on algorithm design and also Haskell code snippets that follow some of Knuth's books?
by Casey Hawthorne 22 Jun '10

22 Jun '10

Is there good place to post Haskell alorithms/data structures that follow Steven Skiena's book on algorithm design and also Haskell code snippets that follow some of Knuth's books? These code snippets don't seem to fit with Hackage. Is there a Hatorial? -- Regards, Casey

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Ph.D. positions on HW acceleration of Haskell
by Rami Mukhtar 22 Jun '10

22 Jun '10

========================================================================= Vacancies for Ph.D. positions at NICTA with the University of Sydney, Australia ========================================================================= Background ~~~~~~~~~~ NICTA has established a project with a goal of developing technology to map high level descriptions of computer vision algorithms to heterogeneous parallel hardware architectures. NICTA scholarships are available at the University of Sydney for outstanding students in the areas listed below. For further information, please contact either: Dr Rami Mukhtar (rami.mukhtar _AT_ nicta.com.au) or Dr Philip Leong (philip.leong _AT_ sydney.edu.au). Further information concerning these scholarships is available from: http://www.nicta.com.au/education/scholarships/sydney_research_lab/scholars… Reconfigurable Computing Architectures for Acceleration of Functional Languages ---------------------------------------------------------------------- This project would provide a candidate with the opportunity to develop new reconfigurable computing hardware architectures that are well suited for accelerating computations on collections of data. The work would be driven by recent advances on computations of collections made in the functional language community. The outcomes of this research would be to develop new hardware accelerator architectures, supporting tools chains, and undertake performance analysis. The candidate would be part of a larger team working to develop a technology for mapping computer vision algorithms to heterogeneous parallel hardware architectures. Hardware Acceleration of Functional Languages --------------------------------------------- This project would provide a candidate with the opportunity to develop new methods for mapping functional languages to implementations that target hardware accelerators, including FPGAs and other reconfigurable computing architectures. The outcomes of this research would be to develop algorithms, methodologies and related software frameworks that make this possible. The candidate would be part of a larger team working to develop a technology for mapping computer vision algorithms to heterogeneous parallel hardware architectures. The information in this e-mail may be confidential and subject to legal professional privilege and/or copyright. National ICT Australia Limited accepts no liability for any damage caused by this email or its attachments.

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Functional programming techniques in small projects
by Markus Dönges 22 Jun '10

22 Jun '10

Hello Community, I am a student from the University of Duisburg-Essen, Germany, and at the moment I am writing my bachelor thesis about "Post-Mass-Prodcution Softwaresupply/ -development in case of an University administration". This approach deals with student-driven software development in which functional programming techniques (ie, Haskell) and agile development approaches matter. I am looking for opinions and statements on how small (agile) software development projects may benefit from functional programming techniques (perhaps in contrast to imperative programming techniques). Since I am at the very start, I would appreciate further literature advices. In addition, does anybody know particular people who are familiar with this topic? Any answer is appreciated :-) Regards, Markus

2 1

Generics and type families
by Louis Wasserman 22 Jun '10

22 Jun '10

What is the relationship between -XGenerics and -XTypeFamilies? Can I automatically create a data family instance based on its generic decomposition? Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

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HaskellWiki and Wikipedia
by Roman Beslik 21 Jun '10

21 Jun '10

Hi all. There are some notions which are not described in HaskellWiki but described in Wikipedia, e.g. "catamorphism". When clicking on a link [[catamorphism]] that leads to "create a new page" it would be nice to show link to a corresponding Wikipedia page. Also "search in Wikipedia" on the search results page. -- Best regards, Roman Beslik.

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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

2 1

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

3 3

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

1 0

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

6 7

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.

1 0

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 20 Jun '10

20 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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Re: Modules and their explicit export lists (are an annoyance)
by Antoine Latter 20 Jun '10

20 Jun '10

Hi Christian, Is there a specific library you're having trouble with? One reason library authors can prefer explicit export lists is that they can hide implementation details, which can then make it easier to change in the future without breaking the users of the library. Antoine Cc'ing the haskell-cafe On Jun 19, 2010 1:39 PM, "Christian Höner zu Siederdissen" < choener(a)tbi.univie.ac.at> wrote: Hi everybody, I'd like some input on other peoples' thoughts on this. Recently, I played around with a library that uses an explicit export list. While there are reasons for having one: - efficiencey (inlining in the module) - encapsulation in practice, it seems to me that they are more annoying than useful. For once, it would think that ghc should produce efficient good across modules with -O / -O2 anyway. But the more important thing is, that it makes extending module functionality a pain (eg. if a constructor is not exported using (..)). So, should I really fork a library just to be able to add a function? Btw. there are libraries, where an explicit export list is used, that export the right amount of information. For example, in 'vector' enough is exported to allow you to extend unboxed vectors. _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users(a)haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

2 1

Vague: Assembly line process
by Martin Drautzburg 20 Jun '10

20 Jun '10

Hello all, this is a problem which has haunted me for some time. If this is simply hillarious, please tell me so. Or it may be some well known unsolvable problem... An assembly process takes inputs and produces outputs. I could say a Process is a function canProduce :: [Input]->[Output]->Bool which tells me if the outputs can be produced from the inputs There may be a similar function which tells me if the inputs are completely consumed to procude the output. The inputs do not determine the exact outputs. Think of a Process which combines a List of Ints into pairs, such that the input ints are consumed and each input Int occurs in only one position in the output. There are many ways to do this. Still for any set of input Ints and output pairs I could decide if the output can be produced from the input. Likewise the Input is not determined by the output. There may be lots of choices from what I could build my output (buy from different vendors). When I know more about the inputs and outputs my choices get more and more limited. I would like to to pass inputs and/or outputs to "something" and I would like to get a "something" which is more restricted, but still essentially a thing which tells me if the outputs can be produced from the inputs. I just cannot find a way to even THINK about this problem in a reasonable general way. -- Martin

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Re: Troubles Building LLVM (Fixed)
by Keith Sheppard 19 Jun '10

19 Jun '10

Hello, in case someone else hits this problem I was able to fix it by installing the latest haskell platform via macports. - I uninstalled ghc "sudo /Library/Frameworks/GHC.framework/Tools/Uninstaller" - deleted ~/.ghc and ~/.cabal - did a "sudo port install haskell-platform" (be prepared for this to take a couple of hours). ghc --version gives 6.10.4 - cabal install llvm I still don't know the root cause of the build error but now the llvm examples build and run with no errors. -Keith On Thu, Jun 17, 2010 at 10:46 PM, Keith Sheppard <keithshep(a)gmail.com> wrote: > Hello All, > > I'm trying to get up and running with the very interesting llvm > package[1] but I'm running into problems during the linking stage. > > keith@sugarglider-2:~/projects/third-party/llvm-hs-bindings/examples/> > make clean all > rm -f HelloJIT Fibonacci BrainF Vector Array DotProd Arith Align > Struct Varargs *.o *.hi *.s *.bc Fib *.exe *.exe.manifest *~ > ghc -Wall -optl -w --make -o HelloJIT.exe -main-is HelloJIT.main HelloJIT.hs > [1 of 1] Compiling HelloJIT ( HelloJIT.hs, HelloJIT.o ) > Linking HelloJIT.exe ... > Undefined symbols: > "_LLVMAddCondPropagationPass", referenced from: > _ssCM_info in libHSllvm-0.7.1.1.a(Scalar.o) > ld: symbol(s) not found > collect2: ld returned 1 exit status > make: *** [HelloJIT.exe] Error 1 > > It's probably something very simple that I'm doing wrong since this is > my first Haskell build linking against foreign libs. Do you know what > I might be doing wrong? > > Here are the versions: > Mac OS X 10.5.8 > GHC 6.10.1 (waiting out a leopard installer bug [2]) > llvm-2.7 installed to default location > llvm package version 0.7.1.1 > > Thanks! > > BTW, the combination of this package plus the awesome Haskell parsers > seem to make for a great compiler construction kit. I'm looking > forward to playing around w/ llvm > > [1] http://hackage.haskell.org/package/llvm > [2] http://trac.haskell.org/haskell-platform/ticket/110 > > -- > keithsheppard.name > -- keithsheppard.name

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ANN: BlazeHtml 0.1
by Jasper Van der Jeugt 19 Jun '10

19 Jun '10

Hello all, In light of Google Summer of Code, we are proud to release the first version of BlazeHtml today. It's a 0.1 release, so beware of bugs! Nevertheless, we encourage you to try it out. You can find more information: - On the website: http://jaspervdj.be/blaze - On hackage: http://hackage.haskell.org/package/blaze-html-0.1 - In this blogpost: http://jaspervdj.be/posts/2010-06-20-blazehtml-0.1.html Looking forward to your feedback, Kind regards, Jasper Van der Jeugt Simon Meier

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ANN: text-json-qq - json quasiquatation
by Oscar Finnsson 19 Jun '10

19 Jun '10

Hi, I've just uploaded the first version of text-json-qq (http://hackage.haskell.org/package/text-json-qq) It's a quasiquatation library converting json into Haskell (Text.JSON). You can use it like: > user = [$jsonQQ| {name: "Pelle", age: 34.3, likes: ["mac", "Haskell"] } |] or you can use Haskell variables with the <<x>>-syntax like: > user = [$jsonQQ| {name: <<name>>, age: <<age>>} |] > name = "Pelle" > age = 34 :: Integer It seems to have some performance problems at the moment so I guess I'll have to learn how to profile my code. -- Oscar

1 0

The functional-object style seems to be gaining momentum.
by caseyh＠istar.ca 19 Jun '10

19 Jun '10

The functional-object style seems to be gaining momentum. Is there any way to convert monads into objects, so that beginners have an easier time with the syntax and thus we can attract more people to the language? :)

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Has anybody tried to upgrade Cabal on HP (Win 7)
by Daniel Kahlenberg 19 Jun '10

19 Jun '10

Hello, I recently installed HP 2010.1.0.0 within Windows 7 and wanted to upgrade the Cabal package to 1.8.0.6 today, sadly I got stuck on a failure building the "directory" package. So I tried to isolate the messages a bit and gave the following a try in a ghci session (1st time only with path to cygwin binaries/2nd time only with path to msys binaries added on my path variable, for both times I append the logging output): Prelude System.Cmd> > rawSystem "cabal" ["upgrade", "--constraint=base==4.*", "directory", > "-v3", "--user", > "--build-log=d:/temp/log/build-$$pkgid-$$compiler.log"] Maybe we can find the cause?! Greetz Daniel

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Re: [Haskell-cafe] What is Haskell unsuitable for?
by caseyh＠istar.ca 18 Jun '10

18 Jun '10

> If you want to use cool languages, you may have to get a cool job. I > know: it's easy to say and harder to accomplish. Most functional languages (e.g. Lisp, Haskell, ...) have a challenging time in industry since they require some savvy with multiple levels of higher abstractions and some savvy with mathematical ideas and some savvy with functional algorithms and data structures. Also, your manager has probably come up through the technical ranks on his/her OOA/OOD/OOP skills and sees you as a young (or not) upstart trying to show off. :)

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Fun Facts: A lazy bill
by Leonel Fonseca 18 Jun '10

18 Jun '10

I share this just for the sake of fun. In Costa Rica (Central America) a new series of bills are to be emitted and to celebrate bio-diversity some animal drawings are going to be used. Look at the new 10.000 colones bill: http://www.bccr.fi.cr/WebPages/PaginaInicio/NuevaFamilia/10.html Choose "reverso" in order to see the other side. -- Leonel Fonseca.

2 1

Troubles Building LLVM
by Keith Sheppard 18 Jun '10

18 Jun '10

Hello All, I'm trying to get up and running with the very interesting llvm package[1] but I'm running into problems during the linking stage. keith@sugarglider-2:~/projects/third-party/llvm-hs-bindings/examples/> make clean all rm -f HelloJIT Fibonacci BrainF Vector Array DotProd Arith Align Struct Varargs *.o *.hi *.s *.bc Fib *.exe *.exe.manifest *~ ghc -Wall -optl -w --make -o HelloJIT.exe -main-is HelloJIT.main HelloJIT.hs [1 of 1] Compiling HelloJIT ( HelloJIT.hs, HelloJIT.o ) Linking HelloJIT.exe ... Undefined symbols: "_LLVMAddCondPropagationPass", referenced from: _ssCM_info in libHSllvm-0.7.1.1.a(Scalar.o) ld: symbol(s) not found collect2: ld returned 1 exit status make: *** [HelloJIT.exe] Error 1 It's probably something very simple that I'm doing wrong since this is my first Haskell build linking against foreign libs. Do you know what I might be doing wrong? Here are the versions: Mac OS X 10.5.8 GHC 6.10.1 (waiting out a leopard installer bug [2]) llvm-2.7 installed to default location llvm package version 0.7.1.1 Thanks! BTW, the combination of this package plus the awesome Haskell parsers seem to make for a great compiler construction kit. I'm looking forward to playing around w/ llvm [1] http://hackage.haskell.org/package/llvm [2] http://trac.haskell.org/haskell-platform/ticket/110 -- keithsheppard.name

1 0

Using Hint with a socket server
by Tom Jordan 18 Jun '10

18 Jun '10

I'm trying to receive small segments of Haskell code over a socket, and be able to evaluate them in real time in GHCI. I've already downloaded Hint and have run the test code, and it's working great. I'm also using the socket server code from Ch.27 of "Real World Haskell" and that is working well also. directly below is the function from the socket server code that handles the incoming messages. Instead of doing this: "putStrLn msg"... I want to send whatever is captured in "msg" to the GHC interpreter that is used in the Hint code, something like this: "eval msg". I'm not sure how to combine both of these functionalities to get them to work with each other.. -- A simple handler that prints incoming packets plainHandler :: HandlerFunc plainHandler addr msg = putStrLn msg Below is the full code for the socket server, then below that is "SomeModule" used in the Hint example test below that. -- file: ch27/syslogserver.hs import Data.Bits import Network.Socket import Network.BSD import Data.List type HandlerFunc = SockAddr -> String -> IO () serveLog :: String -- ^ Port number or name; 514 is default -> HandlerFunc -- ^ Function to handle incoming messages -> IO () serveLog port handlerfunc = withSocketsDo $ do -- Look up the port. Either raises an exception or returns -- a nonempty list. addrinfos <- getAddrInfo (Just (defaultHints {addrFlags = [AI_PASSIVE]})) Nothing (Just port) let serveraddr = head addrinfos -- Create a socket sock <- socket (addrFamily serveraddr) Datagram defaultProtocol -- Bind it to the address we're listening to bindSocket sock (addrAddress serveraddr) -- Loop forever processing incoming data. Ctrl-C to abort. procMessages sock where procMessages sock = do -- Receive one UDP packet, maximum length 1024 bytes, -- and save its content into msg and its source -- IP and port into addr (msg, _, addr) <- recvFrom sock 1024 -- Handle it handlerfunc addr msg -- And process more messages procMessages sock -- A simple handler that prints incoming packets plainHandler :: HandlerFunc plainHandler addr msg = putStrLn msg -- main = serveLog "8008" plainHandler ---------------------------------------------------------------------------------------------------------------- module SomeModule(g, h) where f = head g = f [f] h = f ---------------------------------------------------------------------------------------------------------------- import Control.Monad import Language.Haskell.Interpreter main :: IO () main = do r <- runInterpreter testHint case r of Left err -> printInterpreterError err Right () -> putStrLn "that's all folks" -- observe that Interpreter () is an alias for InterpreterT IO () testHint :: Interpreter () testHint = do say "Load SomeModule.hs" loadModules ["SomeModule.hs"] -- say "Put the Prelude, Data.Map and *SomeModule in scope" say "Data.Map is qualified as M!" setTopLevelModules ["SomeModule"] setImportsQ [("Prelude", Nothing), ("Data.Map", Just "M")] -- say "Now we can query the type of an expression" let expr1 = "M.singleton (f, g, h, 42)" say $ "e.g. typeOf " ++ expr1 say =<< typeOf expr1 -- say $ "Observe that f, g and h are defined in SomeModule.hs, " ++ "but f is not exported. Let's check it..." exports <- getModuleExports "SomeModule" say (show exports) -- say "We can also evaluate an expression; the result will be a string" let expr2 = "length $ concat [[f,g],[h]]" say $ concat ["e.g. eval ", show expr1] a <- eval expr2 say (show a) -- say "Or we can interpret it as a proper, say, int value!" a_int <- interpret expr2 (as :: Int) say (show a_int) -- say "This works for any monomorphic type, even for function types" let expr3 = "\\(Just x) -> succ x" say $ "e.g. we interpret " ++ expr3 ++ " with type Maybe Int -> Int and apply it on Just 7" fun <- interpret expr3 (as :: Maybe Int -> Int) say . show $ fun (Just 7) -- say "And sometimes we can even use the type system to infer the expected type (eg Maybe Bool -> Bool)!" bool_val <- (interpret expr3 infer `ap` (return $ Just False)) say (show $ not bool_val) -- say "Here we evaluate an expression of type string, that when evaluated (again) leads to a string" res <- interpret "head $ map show [\"Worked!\", \"Didn't work\"]" infer >>= flip interpret infer say res say :: String -> Interpreter () say = liftIO . putStrLn printInterpreterError :: InterpreterError -> IO () printInterpreterError e = putStrLn $ "Ups... " ++ (show e)

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coming soon: ICFP programming contest 2010
by J.Waldmann 18 Jun '10

18 Jun '10

http://icfpcontest.org/2010/ will start roughly 12 hours from now. 221022000022010112201010022001122011110220010

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Understanding GHC allocations
by Roman Cheplyaka 17 Jun '10

17 Jun '10

I'm trying to optimize the following program: http://github.com/feuerbach/particles/blob/303c8a17c9b732e22457b5409bdce4b7… Of course general suggestions are welcome (BTW I'm going to give a try to vector), but currently I'm concerned with two questions: 1. Heavy allocations in 'distance' function. Here is (part of) the profile: COST CENTRE MODULE %time %alloc ticks bytes d2 Main 9.0 22.0 290 600000000 d Main 8.6 65.9 278 1800000000 d1 Main 7.5 11.0 242 299700000 >From reading core I got the impression that everything is strict & unboxed. Perhaps this is related to creating some closures? How to get rid of those allocations? 2. Again from reading the core I learned that although 'l' and other constants are inlined, their type is boxed Double. This makes sense since CAFs are evaluated on demand, but obviously in this particular case it does not make sense, so can I somehow make them unboxed? -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain

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What is Haskell unsuitable for? Dis-functional programming!
by caseyh＠istar.ca 17 Jun '10

17 Jun '10

:)

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Re: [Haskell-beginners] Re: Accounting Engine in Haskell
by Patrick LeBoutillier 17 Jun '10

17 Jun '10

Hi, > Keep in mind that you have to invest some time in learning Haskell > before you can reap the benefits. For an example of the latter, see also > > Paul Hudak, Mark P. Jones. > Haskell vs. Ada vs. C++ vs. Awk vs. ..., > An Experiment in Software Prototyping Productivity > http://haskell.org/papers/NSWC/jfp.ps This paper was very interesting to me. Does anyone know if the full source code for the Haskell prototype is available somewhere? Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

2 1

Using the ContT monads for early exits of IO ?
by Günther Schmidt 17 Jun '10

17 Jun '10

Hi everyone, I'm about to write a rather lengthy piece of IO code. Depending on the results of some of the IO actions I'd like the computation to stop right there and then. Now I know in general how to write this but I'm wondering if this is one of those occasions where I should make use of the Cont monad to make an early exit. Günther

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Announce: hs2dot 0.1.1 - generate graphviz code by analyzing Haskell source code files
by Oscar Finnsson 17 Jun '10

17 Jun '10

Hi, I've just released hs2dot on hackagedb. It's a small tool that lets you automatically generate graphviz/dot code that visualize the relations between data types, types and type classes. Example usage: > hs2dot Hack.hs | dot -T pdf -o Hack.pdf Multiple files can be analyzed together as in > hs2dot Foo.hs Bar.hs | dot -T png -o FooBar.png so their data types can point to each other. Example diagrams generated using h2dot can be found at http://github.com/finnsson/hs2graphviz/tree/master/Examples/ Examples of real code analyzed: http://github.com/finnsson/hs2graphviz/raw/master/Examples/Control.pdf (from hpage) http://github.com/finnsson/hs2graphviz/raw/master/Examples/Hack.png (from Hack) Example of mock code analyzed: http://github.com/finnsson/hs2graphviz/raw/master/Examples/TestCode.pdf (three files analyzed together) The tool is still far from complete. Better handling of algebraic data types and instance declarations are a must. GADTs, data families and type synonym families are not yet implemented so they wont turn up. -- Oscar

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Announce: hlcm 0.2.2 - Parallel closed frequent itemsets mining
by Alexandre Termier 16 Jun '10

16 Jun '10

Dear all, I'm pleased to announce the release of hlcm on Hackage : http://hackage.haskell.org/package/hlcm-0.2.2 hlcm is data mining tool for computing closed frequent itemsets. This problem is famous as "market basket analysis": - given a list of transactions : [["bread", "butter","chocolate","tomato"] ,["bread","butter"] ,["bread","pencil","butter","chocolate"] ,["bread","butter","book"]] - and a minimal frequency threshold in [1..4], let's say 2: we want items that are sold together in at least 2 transactions hlcm will tell you that ["bread","butter","chocolate"] appears in 3 transactions and that ["bread","butter"] appears in 4 transactions. You can many funnier applications with your own data, for example log analysis, mining words in web pages, etc. You can see details on getting started with the program here: http://membres-liglab.imag.fr/termier/HLCM/hlcm/hlcm/Main.html The library documentation is here : http://membres-liglab.imag.fr/termier/HLCM/hlcm/HLCM.html hlcm is based on the most efficient algorithm for closed frequent itemset mining, LCM, which is much, much faster than the well-known Apriori algorithm (more details when following the pointers from the homepage: http://membres-liglab.imag.fr/termier/HLCM/hlcm.html) hlcm can also exploit parallelism through Strategies, with promising speedups. We still have more work to do in order to beat existing C/C++ implementations, but you can have a look at the paper that we submitted at Haskell Symposium this year for a detailed experimental study: http://membres-liglab.imag.fr/termier/HLCM/hlcm.pdf Don't miss out the section about the influence of RTS parameters on parallel performance. Feel free to send me an e-mail if you have any question about hlcm. Alexandre -- _____________________________________________________________ Alexandre Termier LIG (Laboratoire d'Informatique de Grenoble) Université Joseph Fourier 681 rue de la Passerelle B.P. 72, 38402 Saint Martin d'Hères (FRANCE) Phone: +33 4 76 82 72 07 Fax: +33 4 76 82 72 87 http://membres-liglab.imag.fr/termier/ _____________________________________________________________

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Terminology
by Emmanuel Castro 16 Jun '10

16 Jun '10

I am looking for the name of the property linking two functions f and g when : [f(a),f(b),f(c)] = g([a,b,c]) Is there a standard name? In practice, g is an optimised version of f when working on large amount of elements. Thank you

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learning advanced haskell
by Aran Donohue 16 Jun '10

16 Jun '10

Hi Cafe, I've been doing Haskell for a few months, and I've written some mid-sized programs and many small ones. I've read lots of documentation and many papers, but I'm having difficulty making the jump into some of the advanced concepts I've read about. How do people build intuitions for things like RankNTypes and arrows? (Is the answer "Get a PhD in type theory?") Normally I learn by coding up little exercise programs, but for these I don't have good intuitions about the kinds of toy problems I ought to try to solve that would lead me to understand these tools better. For systems like Template Haskell and SYB, I have difficulty judging when I should use Haskell's simpler built-in semantic abstractions like functions and typeclasses and when I should look to these other mechanisms. I understand the motivation for other concepts like iteratees, zippers and functional reactive programming, but there seem to be few entry-level resources. John Lato's recent Iteratee article is a notable exception*. Hints? Tips? "Here's how I did it"? _______ is a great program to write to get to learn ______? Thanks in advance, Aran * Even in this article, he busted out (>=>). And it appears that the iteratee library's actual design is more sophisticated than the design presented.

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Re: [Haskell-cafe] Problems with threading?
by Isaac Gouy 15 Jun '10

15 Jun '10

--- On Mon, 6/7/10, Don Stewart <dons(a)galois.com> wrote: > From: Don Stewart <dons(a)galois.com> > Subject: Re: [Haskell-cafe] Problems with threading? > To: "Isaac Gouy" <igouy2(a)yahoo.com> > Cc: "Louis Wasserman" <wasserman.louis(a)gmail.com>, "Haskell Café List" <haskell-cafe(a)haskell.org> > Date: Monday, June 7, 2010, 4:43 PM > igouy2: > > As Louis has already mentioned this to me, I'll take > the opportunity > > to sketch out a simple approach - > > > > > > 1) GHC programs compiled without -threaded and run > without +RTS -N are already shown for x86 and x64 > > > > http://shootout.alioth.debian.org/u32/compare.php?lang=ghc > > > > http://shootout.alioth.debian.org/u64/compare.php?lang=ghc > > > > > > 2) For quad-core, the GHC programs will all be > compiled with -threaded and all run with +RTS -N4 > > > > > > 3) That seems to match the approach taken with Erlang, > where all the > > programs on quad-core run with smp built into the vm, > and all the > > programs on one core run without smp built into the > vm. > > > > Yep, that's fine. All the GHC programs have been re-measured. http://shootout.alioth.debian.org/u64q/compare.php?lang=ghc http://shootout.alioth.debian.org/u32/compare.php?lang=ghc Now how do we get those regex-dna and binary-trees programs to compile? http://shootout.alioth.debian.org/u32/measurements.php?lang=ghc

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Re: [Haskell-cafe] learning advanced haskell
by John Lato 15 Jun '10

15 Jun '10

> Message: 7 > Date: Mon, 14 Jun 2010 20:33:30 +0100 > From: Andrew Coppin <andrewcoppin(a)btinternet.com> > Subject: Re: [Haskell-cafe] learning advanced haskell > To: haskell-cafe(a)haskell.org > Message-ID: <4C16840A.7060700(a)btinternet.com> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > John Lato wrote: > >> However, there's a lot to be said for both intellectual curiosity and >> learning for the sake of knowledge. Just because you may never need >> to use a feature doesn't mean you shouldn't be able to understand it. > > There is that. However, in my experience, most of the advanced > techniques tend to be described in language beyond my comprehension. > (And most examples seem overly complex - although maybe that's just a > reflection of the fact that simple problems don't require sophisticated > techniques in the first place.) Having a specific problem to solve can > be quite helpful. Unlike an example, you already understand what the > problem is, and why it can't easily be solved any other way. Yes, having a specific problem to solve can be very helpful. The OP requested examples of them. I'd like to see them myself, partially as a reference but mostly to satisfy my own curiosity. More examples please! John

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Re: [Haskell-cafe] learning advanced haskell
by John Lato 15 Jun '10

15 Jun '10

> From: Martijn van Steenbergen <martijn(a)van.steenbergen.nl> > > On 6/14/10 10:39, Ivan Lazar Miljenovic wrote: >> By being told that using them would solve some problem you're >> complaining about on #haskell or the mailing lists, you look at >> examples, read up on them, etc. >> >> Short version: don't worry about advanced concepts until you have to. >> If all else fails, it doesn't hurt to write out the low-level version >> yourself and then get told in a code review that it would be "easier" or >> more elegant with an advanced technique. > > Exactly this. It's happened a few times now that I ran into a problem > and then a bit later found out that feature XYZ was exactly what I > needed. A feature I never understood but now suddenly had a good > intuition for because it is a (or the) solution to a problem I had been > thinking about for a while. I sort of agree with this, with some very large caveats. I would agree that being faced with an actual problem provides a particularly good learning environment for several reasons. For the "pragmatic programmer", this is probably the way to go. However, there's a lot to be said for both intellectual curiosity and learning for the sake of knowledge. Nearly all of us specialize in certain problem domains or areas and will likely never work on certain problems (related to e.g. compilers, web servers, etc.) in anything like a commercial environment. Some of us may never work in a commercial code environment at all. Just because you may never need to use a feature doesn't mean you shouldn't be able to understand it. John

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Reactive and 'real live' problem
by Maciej Piechotka 15 Jun '10

15 Jun '10

I tried to experiment with reactive[1] and rewrite NeHe tutorials using reactive-glut. However I run into problems. I tried to write first tutorial and exit on escape. However there were 2 problems: 1. Optimized constant functions 2. Continuous behaviour In reactive the final step is Behaviour which is 'function' from time to something a. Here a = IO (). The problem is that it is not indifferent how many time the same IO () was returned (we don't want to write to file twice etc.). I tried to workaround it in various ways but all I came up to was to switch after switch with time + * where * is the difference between times (unfortunately there is no function which give the next float). As with every tick everything is executed -- unless everything gets optimized to constant function. Am I failing to find proper docs? How solve those problems? If they are not easily solvable is there any general-purpose FRP library that allows to use IO inside? Regards [1] http://hackage.haskell.org/package/reactive

1 0

Possible solution to the FGL naming/compatability issue
by Ivan Miljenovic 15 Jun '10

15 Jun '10

Last night (AEST), Edward Kmett semi-convinced (in the sense that I'm not sure whether his examples are really those that someone would want/need or just thinking of possible future problems) me that some users may have a need for FGL to keep having explicit graphs with kind * -> * -> *. With his prompting, I may have a possible solution: duplicate the classes. We would have an InductiveGraph class (call it the parametric version) that requires graphs to have kind * -> * -> *, and then another class (call it the extended version) that removes this restriction. The parametric version would be closer to a drop-in replacement to the current FGL setup (see below for more details) and the extended one would provide all of the functionality requested, with some kind of semi-automatic lifting/unlifting for graph types that are instances of both. Now, this parametric version will still not be a drop-in for the current FGL, for several reasons: * I want to have one overall InductiveGraph class rather than two separate Graph and DynGraph classes * Context will still be an actual data type rather than a 4-tuple and LEdge would be come (Edge, Label) rather than (Node, Node, Label) [I'm still debating whether to keep Edge as a datatype as needed for the Type Familiy version or if it should just be a tuple). * Some function names will be changed (e.g. fromContexts rather than buildGr). * The Data.Graph.Inductive.Query stuff is going to be split out into its own separate library, the Data.Graph.Inductive.Graphviz stuff is going to be removed, etc. * With the removal of the sub-libraries, I'm seriously considering moving the class definitions from Data.Graph.Inductive.Graph to just Data.Graph.Inductive. There are also some caveats/design decisions with this solution: * There will obviously be duplication. With that said, do we try to make the two classes semi-drop-ins (same class name, methods, etc. but with different types) or think up new method names for the extended version? * How transparently can/will we make the lifting/unlifting? * If we want the parametric version to still have the ability to specify a custom node type, then it will still need either MPTCs + fundeps or Type Families (i.e. extensions); otherwise we stick with the current "type Node = Int" (or maybe replace it with some opaque abstract type down the track as suggested by Heinrich Apfelmus). * If we want to provide the ability to restrict the types of the labels in the parametric version, then we would need MPTCs (Type Families won't work since the label types won't be part of the class definition then unless we use MPTCs, and in that case there's not really any point in using Type Families). Thus we once again need extensions. * My semi-end goal is to build a proper hierarchy of graph types/libraries, such that the only thing FGL provides is the extra aspect/concept of inductive graphs. If we extend this parametric vs extended split down to the set of base graph classes, then we're going to have even more duplication and this kind of setup (where "Foo depends on Bar" means "class (Bar a) => Foo a where ..."): - fgl-parametric will depend on graph-parametric - fgl-extended will depend on graph-extended - There will be lifting/unlifting between graph-parametric and graph-extended - There will be lifting/unlifting between fgl-parametric and fgl-extended As such, an FGL graph of kind * -> * -> * being treated as an extended graph will have two possible ways (depending on the function in question) will have two graph hierarchies two choose from, and two possible points at which it can be lifted/unlifted (at the graph level and the fgl level). This could get confusing. That said, there is one other benefit of this parametric vs extended split: the types of the mapping functions in http://code.haskell.org/FGL/fgl/Data/Graph/Inductive/Graph.hs won't be as weird... So, what are people's thoughts/opinions on this kind of setup? Note that I haven't started coding it yet, so I'm not sure how well it will work in practice... -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Heist template splice functions and custom monad
by Kevin Jardine 15 Jun '10

15 Jun '10

I'd like to run Heist template splice functions in my own custom monad. I can define a splice function as: mySplice :: Splice MyMonad However, when I try to call functions that return values in my monad in mySplice, I get a compile error: Couldn't match expected type TemplateMonad MyMonad a against inferred type MyMonad () I've seen an example in the Heist documentation where the inner MyMonad is accessed using lift. But when I try that, I get another compile error: No instance for (Monad Trans TemplateMonad) arising from the use of 'lift' What do I do to get this to work? Kevin

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Re: [Haskell-cafe] learning advanced haskell
by John Lato 14 Jun '10

14 Jun '10

> From: Neil Brown <nccb2(a)kent.ac.uk> > > For example, if you find yourself writing the Nth boilerplate function > that pattern-matches all cases in your ADT just to apply a function in > its sub-types, that's when you'll want some form of generic programming > like SYB. And by that point you'll understand one of the problems that > generic programming solves, which is halfway towards understanding the > techniques themselves. This is probably the most approachable description of generic programming I've seen. Every generic framework should have a paragraph like this on the front page of the docs. John

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