November 2006 - Haskell-Cafe

ghc 6.6 crash
by Tim Newsham 24 Nov '06

24 Nov '06

I just built GHC 6.6 on my linux Fedora Core box (athlon 700mhz). When I run it, it crashes. The 6.4 version works fine (which I also built). # ghc-6.6 Segmentation fault # gdb /usr/local/lib/ghc-6.6/ghc-6.6 GNU gdb Red Hat Linux (6.0post-0.20040223.19rh) Copyright 2004 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "i386-redhat-linux-gnu"...(no debugging symbols found)...Using host libthread_db library "/lib/libthread_db.so.1". (gdb) run -B/usr/local/lib/ghc-6.6 Starting program: /usr/local/lib/ghc-6.6/ghc-6.6 -B/usr/local/lib/ghc-6.6 Error while mapping shared library sections: : Success. Error while reading shared library symbols: : No such file or directory. (no debugging symbols found)...(no debugging symbols found)...[Thread debugging using libthread_db enabled] [New Thread 16384 (LWP 20880)] Error while reading shared library symbols: : No such file or directory. Error while reading shared library symbols: : No such file or directory. Program received signal SIGSEGV, Segmentation fault. [Switching to Thread 16384 (LWP 20880)] 0x00421d3b in __pthread_clock_gettime () from /lib/i686/libpthread.so.0 (gdb) where #0 0x00421d3b in __pthread_clock_gettime () from /lib/i686/libpthread.so.0 #1 0x00dc0331 in clock_gettime () from /lib/i686/librt.so.1 #2 0x0888fb4d in ?? () #3 0x00000003 in ?? () #4 0xfef22228 in ?? () #5 0xfef222e8 in ?? () #6 0x0888fe26 in ?? () #7 0x09462dd4 in ?? () #8 0x00000000 in ?? () (gdb) x/64 $esp 0xfef221a4: 0x13ea7d62 0x00000000 0x00000003 0x29a79300 0xfef221b4: 0x006ec840 0x00421cf0 0x00dc3ff4 0x00000003 0xfef221c4: 0x00000000 0xfef22208 0x00dc0331 0x00000003 0xfef221d4: 0x29a79300 0x00000000 0xfef22228 0x00dbc7ec 0xfef221e4: 0x09462d88 0x29a79300 0x00000000 0xffffffff 0xfef221f4: 0x006ec840 0x00000000 0x09462d88 0xfef222d0 0xfef22204: 0x00000002 0xfef222e8 0x0888fb4d 0x00000003 0xfef22214: 0xfef22228 0xfef222e8 0x0888fe26 0x09462dd4 0xfef22224: 0x00000000 0x0945c280 0x00000000 0x00000000 0xfef22234: 0x09462d88 0xfef222d0 0x0888f127 0x09462dd4 0xfef22244: 0x08962469 0xfef22268 0x0041d525 0xfef222d4 0xfef22254: 0xfef222d0 0x00000002 0x0888f23d 0xfef222d0 0xfef22264: 0x00000002 0xfef222d4 0x08888c73 0x089e2aa0 0xfef22274: 0x00000000 0x00000000 0x00000000 0x00000000 0xfef22284: 0x00000000 0xfef222d4 0x08890019 0xfef222d4 0xfef22294: 0xfef222d0 0xfef222d4 0x08889bde 0xfef222d0 # uname -a Linux box 2.6.5-1.358 #1 Sat May 8 09:04:50 EDT 2004 i686 athlon i386 GNU/Linux # ls -ld /lib/*pthread* -rwxr-xr-x 1 root root 95012 May 11 2004 /lib/libpthread-0.10.so* lrwxrwxrwx 1 root root 18 Jun 21 15:00 /lib/libpthread.so.0 -> libpthread-0.10.so* Tim Newsham http://www.thenewsh.com/~newsham/

1 1

Starting your own Haskell project: part 2
by dons＠cse.unsw.edu.au 24 Nov '06

24 Nov '06

We've expanded the wiki page on 'How to write a Haskell project', to include a complete walkthrough creating: * Darcs * Cabal * QuickCheck infrastructure. http://haskell.org/haskellwiki/How_to_write_a_Haskell_program Feedback welcome! -- Don P.S. It might even be useful to have a tool, haskell-project, which sets up all these files automatically.

8 16

Foldl
by Marco Comini 24 Nov '06

24 Nov '06

Donald Bruce Stewart: > zacara: > > i'd like to write a function that given a list like [1,2,3,4...] > > returns a list of couple where the first element is the corresponding > > element of the string, and the second is the sum of the previous > > elements. > Looks like a scanl (a prefix fold) -- the "of the previous > elements" is a clue that you're describing a scan: > > Prelude> :t scanl > scanl :: (a -> b -> a) -> a -> [b] -> [a] > > Prelude> scanl (+) 0 [1..4] > [0,1,3,6,10] > > You should be able to work out the rest from here (or rewrite it as a > fold). I agree Don, certainly with scanl one can solve the problem elegantly. [For ClareZako: it could be something like f (x:xs) = scanl acc (x,0) xs where acc (x,s) y = (y, s+x) ] On the contrary a solution in terms of a fold is indeed more tricky. Probably ClareZako was actually looking for a solution in term of a scan instead of a fold... otherwise would be interesting to know why he's willing to solve this problem in such a twisted way... don't you agree? Cheers, -- Marco.

2 1

list monad and controlling the backtracking
by isto 24 Nov '06

24 Nov '06

Hi all, Weekly news had a link to article "Local and global side effects with monad transformers" and the following is from there (minor modification done): import Control.Monad.List import Control.Monad.State import Control.Monad.Writer test5 :: StateT Integer (ListT (Writer [Char])) Integer test5 = do a <- lift $ mlist aList b <- lift $ mlist bList lift $ lift $ tell ("trying "++show a++" "++show b++"\n") modify (+1) guard $ a+b<5 return $ a+b go5 = runWriter $ runListT $ runStateT test5 0 If the aList = [1..5] as well as bList, then there will be 25 tryings. If aList and bList are [1..10000000], there will be a lot of tryings... However, guard is saying that we are interested in only values whose sum is less than 5. Is it possible to control easily in the above example that when we have tried out pairs (1,1), (1,2), (1,3), (1,4), that now we are ready to stop trying from the bList? And then similarly when we finally arrive to a pair (4,1), that now we are ready to finish also with aList? This would give a nice way to build branch & bounding algorithms, even though it does not take much more lines to do it in some other way. br, Isto

3 2

strange type mismatch when trying to use takusen
by Seth Gordon 24 Nov '06

24 Nov '06

I have a simple test program for takusen and PostgreSQL: > import Database.Enumerator > import Database.PostgreSQL.Enumerator > import Control.Monad.Trans > > gazdbSession dbname = connect [CAdbname dbname] > > resultCollector :: (Monad m) => String -> IterAct m [String] > resultCollector str accum = result' (str:accum) > > main = do withSession (connect [CAdbname "template1"]) $ main' > > main' = do r <- doQuery (sql "select 'fred'") resultCollector [] > liftIO $ putStrLn $ show r But it won't compile: [1 of 1] Compiling Main ( takusen-test.hs, takusen-test.o ) takusen-test.hs:11:57: Couldn't match expected type `forall mark. DBM mark Session a' against inferred type `DBM mark sess ()' In the second argument of `($)', namely `main'' In the expression: (withSession (connect [CAdbname "template1"])) $ main' In the expression: do (withSession (connect [CAdbname "template1"])) $ main' Adding an explicit "main' :: DBM mark Session ()" type signature doesn't change the error message. The takusen source that I'm using was updated from darcs on November 14 and compiled with ghc 6.6.

3 3

Re: [Haskell] Re: [Haskell-cafe] SimonPJ and Tim Harris explain STM - video
by C.M.Brown 24 Nov '06

24 Nov '06

Hi, I got this working on Mac OS X. I had to download media player 9: http://www.microsoft.com/windows/windowsmedia/software/Macintosh/osx/defaul… This contains the WMV3 codec. Cheers, Chris. On Fri, 24 Nov 2006, James William Pye wrote: > On Fri, Nov 24, 2006 at 10:26:38AM +0100, Tomasz Zielonka wrote: > > Does anybody know how to watch this on Linux? I would prefer to simply > > download the movie file and use MPlayer on that, but I failed. > > > > . or on Mac OS X (haven't tried yet) > > The latest mplayer works for me on FreeBSD/amd64 (1.0rc1, iirc). > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

1 0

Implicit params and typeclasses
by Grzegorz Chrupała 24 Nov '06

24 Nov '06

Hi all, When I try to compile the following in GHC (v 6.6) (with -fno-monomorphism-restriction -fimplicit-params) class Configuration a where thestring:: a -> String foo c = let { ?c = c } in bar bar = thestring ?c I get: Ambiguous type variable `a' in the constraint: `Configuration a' arising from use of `thestring' at /home/grzegorz/Foo.hs:9:6-17 Possible cause: the monomorphism restriction applied to the following: bar :: (?c::a) => String (bound at /home/grzegorz/Foo.hs:9:0) foo :: a -> String (bound at /home/grzegorz/Foo.hs:8:0) Probable fix: give these definition(s) an explicit type signature or use -fno-monomorphism-restriction Adding the type signature bar :: (Configuration a,?c :: a) => String the compiler says: Ambiguous constraint `Configuration a' At least one of the forall'd type variables mentioned by the constraint must be reachable from the type after the '=>' In the type signature for `bar': bar :: (Configuration a, ?c :: a) => String Similar code without use of implicit params compiles fine: class Configuration a where thestring:: a -> String foo c = thestring c Why do implicit params cause this error and is there a way to make it work? Thanks, -- Grzegorz

2 1

Foldl
by ClareZako 24 Nov '06

24 Nov '06

Hello, i'd like to write a function that given a list like [1,2,3,4...] returns a list of couple where the first element is the corresponding element of the string, and the second is the sum of the previous elements. An example: input: [1,2,3,4] output: [(1,0)(2,1)(3,3)(4,6)] The problem is that I'd like to use foldl and a solution like: buildCouples = snd . foldl op (0,[]) where op (v,xs) x = (v+x,xs++[(x,v)]) is not good since the operator ++ let the function take 2*n time and not n. Can anyone help me please? Clare -- View this message in context: http://www.nabble.com/Foldl-tf2698202.html#a7524410 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

2 1

Command line prompt templates
by Dougal Stanton 24 Nov '06

24 Nov '06

I was trying to write my own equivalent to Don Stewart's mkcabal but ended up getting sidetracked. I made some generalised prompts for use at the command line and wanted to get some feedback on them. The full code can be found at [1] but the basic summary is like this: > prompt :: String -> IO String Simple prompt - supply a question and receive the user's answer. > promptYesNo :: String -> Maybe String -> IO Bool A yes/no question, with an optional value to ask about. I suppose this could be simplified to just one string. > promptList :: String -> [String] -> Maybe Integer -> IO Integer Ask the user to choose from a list, where the result is the number chosen. The optional value is a default if the user doesn't pick a number. > promptListEdit :: String -> [String] -> Maybe Integer -> IO String As above, but the last option in the list is an invitation to provide your own answer. Again the optional value is the default choice if nothing else is chosen. The result here is necessarily the string rather than the integer cos I didn't want to complicate matters further with stuff like Either types. If you can see any flaws or have any suggestions please let me know! Cheers, D. [1]: <http://brokenhut.no-ip.org/~dougal/darcs/joincabal/CommandPrompts.hs>

4 3

Equivalent of if/then/else for IO Bool?
by Dougal Stanton 23 Nov '06

23 Nov '06

Is there some sort of equivalent of the if/then/else construct for use in the IO monad? For instance the following can get quite tedious: > do bool <- doesFileExist filename > if bool > then sth > else sth' Is there a more compact way of writing that? Something akin to: > condM (doesFileExist filename) (sth) (sth') Or maybe there's a more sensible way of doing the above that I've missed. I seem to use endless alternate conditions sometimes and there's bound to be a better way.

8 9