- Haskell-Cafe - Haskell.org

tail recursion
by Daryoush Mehrtash 07 Apr '09

07 Apr '09

Is the call to "go" in the following code considered as tail recursion? data DList a = DLNode (DList a) a (DList a) mkDList :: [a] -> DList a mkDList [] = error <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:error> "must have at least one element" mkDList xs = let (first,last <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:last>) = go last <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:last> xs first in first where go :: DList a -> [a] -> DList a -> (DList a, DList a) go prev [] next = (next,prev) go prev (x:xs) next = let this = DLNode prev x rest (rest,last <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:last>) = *go this xs next* in (this,last <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:last>) Daryoush

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automatically inserting type declarations
by FFT 07 Apr '09

07 Apr '09

I remember hearing about a Haskell mode for Vim, Emacs, Yi or VisualHaskell that inserts type declarations automatically (it's lazier to just check the type than to write it manually), but I can't remember any details. What editor mode / IDE was it? What do most people use with GHC on Linux? I'm more used to Vim than to Emacs. Yi sounds like something I might like. Is it stable enough to solve more problems than it would create? (I hate buggy and broken stuff)

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replicateM should be called mreplicate?
by Sjoerd Visscher 07 Apr '09

07 Apr '09

Considering these naming conventions: http://haskell.org/ghc/docs/latest/html/libraries/base/Control-Monad.html#3 • A postfix 'M' always stands for a function in the Kleisli category: The monad type constructor m is added to function results (modulo currying) and nowhere else. So, for example, filter :: (a -> Bool) -> [a] -> [a] filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a] • A postfix '_' changes the result type from (m a) to (m ()). Thus, for example: sequence :: Monad m => [m a] -> m [a] sequence_ :: Monad m => [m a] -> m () • A prefix 'm' generalizes an existing function to a monadic form. Thus, for example: sum :: Num a => [a] -> a msum :: MonadPlus m => [m a] -> m a replicateM has the following type: replicateM :: Monad m => Int -> m a -> m [a] Am I missing something or should this have been called mreplicate? greetings, -- Sjoerd Visscher sjoerd(a)w3future.com

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Infix tuple comma query (,)
by Paul Keir 07 Apr '09

07 Apr '09

module Main where data (:%^&) a b = a :%^& b deriving (Show) main = do print $ 18 :%^& (Just 99) print $ (,) 9 10 print $ 9 , 10 The last line in the code above causes a compile error. Why does infix use of the comma (tuple constructor?) function fail without brackets? Thanks, Paul

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Lazy vs correct IO [Was: A round of golf]
by oleg＠okmij.org 07 Apr '09

07 Apr '09

Lennart Augustsson wrote > main = do > name:_ <- getArgs > file <- readFile name > print $ length $ lines file Given the stance against top-level mutable variables, I have not expected to see this Lazy IO code. After all, what could be more against the spirit of Haskell than a `pure' function with observable side effects. With Lazy IO, one indeed has to choose between correctness and performance. The appearance of such code is especially strange after the evidence of deadlocks with Lazy IO, presented on this list less than a month ago. Let alone unpredictable resource usage and reliance on finalizers to close files (forgetting that GHC does not guarantee that finalizers will be run at all). Is there an alternative? -- Counting the lines in a file import IterateeM count_nl = liftI $ IE_cont (step 0) where step acc (Chunk str) = liftI $ IE_cont (step $! acc + count str) step acc stream = liftI $ IE_done acc stream count [] = 0 count ('\n':str) = succ $! count str count (_:str) = count str main = do name:_ <- getArgs IE_done counter _ <- unIM $ enum_file name >. enum_eof ==<< count_nl print counter The function count_nl could have been in the library, but I'm a minimalist. It is written in a declarative rather than imperative style, and one easily sees what it does. The above code as well as the IterateeM library is Haskell98. It does not use any unsafe Haskell functions whatsoever. time wc -l /usr/share/dict/words 235882 /usr/share/dict/words real 0m0.024s user 0m0.022s sys 0m0.000s time ~/Docs/papers/DEFUN08/Wc /usr/share/dict/words 235882 real 0m0.141s user 0m0.126s sys 0m0.008s To compare with lazy IO, the code using readFile gives time ~/Docs/papers/DEFUN08/Wc /usr/share/dict/words 235882 real 0m0.297s user 0m0.262s sys 0m0.023s So, choosing correctness does not mean losing in performance; in fact, one may even gain. Can enumerators compose? Well, we already seen the example above (enum_file name >. enum_eof) where the operation (>.) e1 >. e2 = (==<<) e2 . e1 is a flipped composition if monadic bind were considered a flipped application. Here is a more interesting example: count words in all the files whose names are given on the command line. There may be many files given, thousands of them. -- Count the stream. Again, could have been in the library stream_count :: Monad m => IterateeGM el m Int stream_count = liftI $ IE_cont (step 0) where step acc (Chunk []) = liftI $ IE_cont (step acc) step acc (Chunk [_]) = liftI $ IE_cont (step $! succ acc) step acc (Chunk ls) = liftI $ IE_cont (step $! acc + length ls) step acc stream = liftI $ IE_done acc stream main = do names <- getArgs let enumerators = foldr (\name -> (enum_file name >.)) enum_eof names IE_done (IE_done counter _) _ <- unIM $ enumerators ==<< enum_words stream_count print counter We notice that the composition of enumerators corresponds to the `concatenation' of their sources. Declaratively, the meaning of the above code is: -- all the given files are concatenated -- the resulting stream of characters is converted to a stream of words -- the stream of words is counted. Operationally, the code does not open more than one file at a time. More importantly, the code *never* reads more than 4096 characters at a time. A block of the file is read, split into words, counted, and only then another chunk is read. After one file is done, it is closed, and another file is processed. One can see that only one file is being opened at a time by enabling traces. The processing is fully incremental. /usr/local/share/doc/ghc6> find . -name \*.html -print | time xargs ~/Docs/papers/DEFUN08/Wc 3043421 16.99 real 15.83 user 0.71 sys BTW, the program has counted words in 1169 files. It is interesting to compare the above main function with the corresponding lazy IO: main'' = do names <- getArgs files <- mapM readFile names print $ length $ words (concat files) The number of lines is comparable. The execution is not. If we try to run the lazy IO code, we get: /usr/local/share/doc/ghc6> find . -name \*.html -print | time xargs ~/Docs/papers/DEFUN08/Wc Wc: ./libraries/Win32/Graphics-Win32-GDI-Path.html: openFile: resource exhausted (Too many open files)

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Volunteer
by Xiao-Yong Jin 07 Apr '09

07 Apr '09

Hi, I learned that no one is doing generic unix source tarball, right now, and thought I might help to get it start. I have decent skills on Makefile, but knows very little about auto-tools. I can only spent a few hours every week on it, because I don't have too much spare time as a PhD student. But I still want to help, since it is quite important for Haskell to distribute as widely as possible. So, how should I start? I learned that ghc supports quite a lot of platforms in its tier2 support list through community supporters. I'm not sure what is the most convenient way to combine these into a generic source tarball. And what do we use to bootstrap ghc? Some pre-compiled binary? I'm not sure how I should start. Any insight? Best, Xiao-Yong P.S. I tried to send to haskell-platform, but got rejected. What's going on? ---------- begin error message ---------- ... while talking to community.haskell.org.: >>> DATA <<< 550 Unrouteable address 550 5.1.1 <haskell-platform(a)projects.haskell.org>... User unknown <<< 503 valid RCPT command must precede DATA ---------- end error message ---------- -- c/* __o/* <\ * (__ */\ <

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Configuring cabal dependencies at install-time
by Jeff Heard 06 Apr '09

06 Apr '09

Is there a way to do something like autoconf and configure dependencies at install time? Building buster, I keep adding dependencies and I'd like to keep that down to a minimum without the annoyance of littering Hackage with dozens of packages. For instance, today I developed an HTTP behaviour and that of course requires network and http, which were previously not required. I'm about to put together a haxr XML-RPC behaviour as well, and that of course would add that much more to the dependency list. HaXml, haxr, and haxr-th most likely. so... any way to do that short of making a bunch of separate packages with one or two modules apiece? Otherwise I'm thinking of breaking things up into buster, buster-ui, buster-network, buster-console, and buster-graphics to start and adding more as I continue along. -- Jeff

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GHC accepts libraries compiled with Visual Studio?
by Peter Verswyvelen 06 Apr '09

06 Apr '09

To my surprise GHC seems to accept .lib files (produced by e.g. Visual Studio on Windows) directly; I don't need to convert these to .a files using e.g. reimp That's really cool. Is this by design or did I miss something? The manual does not seem to mention this.

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Re: Linking hmatrix without LAPACK
by Duane Johnson 06 Apr '09

06 Apr '09

Hi Alberto! Thanks for your informative reply. I looked in to the versions of liblapack on my system... it turns out there is indeed a liblapack.dylib that (apparently) comes with Mac OS X. How to tell ghc to link to that instead is still in question. I can run simple matrix operations in ghci as you suggest (in fact, "runTests 20" succeeds wonderfully); however, I created the following simple module (test.hs) and it fails: > module Main where > import Numeric.LinearAlgebra.Tests > > main = do > runTests 20 I tried plain "--make", as well as the "-dynamic" flag, and then I tried the "-L" shell argument to point to the /usr/lib dir where the liblapack.dylib library is located: > $ ghc --make test.hs -o test > [1 of 1] Compiling Main ( test.hs, test.o ) > Linking test ... > ld: in /opt/local/lib/liblapack.a( > ), not a valid archive member > collect2: ld returned 1 exit status > > $ ghc --make -dynamic test.hs -o test > Linking test ... > ld: library not found for -lHShmatrix-0.5.0.1-ghc6.10.1 > collect2: ld returned 1 exit status > > $ ghc --make -L/usr/lib test.hs -o test > Linking test ... > Undefined symbols: > "_zgemm_", referenced from: > _multiplyC in libHShmatrix-0.5.0.1.a(lapack-aux.o) > "_zgesv_", referenced from: > _linearSolveC_l in libHShmatrix-0.5.0.1.a(lapack-aux.o) > "_zpotrf_", referenced from: > _chol_l_H in libHShmatrix-0.5.0.1.a(lapack-aux.o) > "_dpotrf_", referenced from: > _chol_l_S in libHShmatrix-0.5.0.1.a(lapack-aux.o) > "_dgemm_", referenced from: > _multiplyR in libHShmatrix-0.5.0.1.a(lapack-aux.o) > "_dgesv_", referenced from: > _linearSolveR_l in libHShmatrix-0.5.0.1.a(lapack-aux.o) > "_zgetrf_", referenced from: > _lu_l_C in libHShmatrix-0.5.0.1.a(lapack-aux.o) > "_zgetrs_", referenced from: > _luS_l_C in libHShmatrix-0.5.0.1.a(lapack-aux.o) > "_dgetrf_", referenced from: > _lu_l_R in libHShmatrix-0.5.0.1.a(lapack-aux.o) > "_dgetrs_", referenced from: > _luS_l_R in libHShmatrix-0.5.0.1.a(lapack-aux.o) > ld: symbol(s) not found > collect2: ld returned 1 exit status I attempted to install hmatrix with the "-faccelerate" option, but when using "ghc --make" for the above test code, I received the same error messages noted previously. Is there a way to tell if the "- faccelerate" was acknowledged and that the alternate library was used? One more clue: I took a look at the directions found on the page at mit.edu/harold... it turns out I had installed atlas/lapack unnecessarily. That explains the mysterious "not a valid archive member" message. Nevertheless, I am still befuddled by the "Undefined symbols" above. ghc --make is still unable to figure out where the liblapack.dylib file is in spite of ghci's success. Any suggestions from here? Regards, Duane Johnson P.S. I'm CC'ing the Haskell Cafe so that our journey so far can be archived. On Apr 6, 2009, at 12:22 PM, Alberto Ruiz wrote: > Hi Duane, > > I have seen your messages to Haskell Cafe but I am still thinking > about the problem... :) > > Can you run simple matrix operations in ghci? > > $ ghci > Prelude> import Numeric.LinearAlgebra > Prelude Numeric.LinearAlgebra> let m = (2><2) [1..4 :: Double] > > (...Loading packages...) > > Prelude Numeric.LinearAlgebra> m <> m > (2><2) > [ 7.0, 10.0 > , 15.0, 22.0 ] > > If so, some version of blas/lapack can be found in your system, it > is strange that ghc --make doesn't find them. > > Perhaps the problem is that dynamic libraries like liblapack.so are > required instead of static ones like liblapack.a. (In ubuntu they > are in the "devel" packages for blas/lapack.) > > I am not familiar with Mac OS, but if you can use the "accelerate > framework" in your system you may try the -faccelerate configuration > option for hmatrix: > > cabal install hmatrix -faccelerate > > See also the following page (steps 3-8). It explains how to install > hmatrix on Mac OS (required for other project). > > http://mit.edu/harold/Public/easyVisionNotes.html > > Please let me know if any of these methods works for you. > > Thanks for your message, > > Alberto > > > > Duane Johnson wrote: >> Hi Alberto, >> I've been very happy with hmatrix as I've used it in ghci, and I >> should first thank you for making such an excellent package. I've >> had trouble when linking it using "ghc --make" on Mac OS >> (Leopard). Below are two messages I sent to the Haskell Cafe >> mailing list. Do you have any insight into either question: >> 1. How to link without atlas/lapack >> 2. What the "not a valid archive member" message means? >> Thank you! >> Duane Johnson >> Brigham Young University >> Provo, UT >> Begin forwarded message: >>> From: Don Stewart <dons(a)galois.com> >>> Date: April 5, 2009 11:48:38 PM MDT >>> To: Duane Johnson <duane.johnson(a)gmail.com> >>> Subject: Re: [Haskell-cafe] Re: Linking hmatrix without LAPACK >>> >>> It would be best to contact the author of hmatrix directly. >>> >>> >>> duane.johnson: >>>> On a related note, I've installed Atlas, and I get the following >>>> error >>>> when linking: >>>> >>>>> Linking SilkwormGame ... >>>>> ld: in /opt/local/lib/liblapack.a(), not a valid archive member >>>>> collect2: ld returned 1 exit status >>>> >>>> How would I go about diagnosing this? I've never seen an ld >>>> error like >>>> that. >>>> >>>> Thanks again! >>>> Duane Johnson >>>> >>>> >>>> On Apr 5, 2009, at 11:02 PM, Duane Johnson wrote: >>>> >>>>> I'm curious if there is a quick fix to this. I installed GLS and >>>>> hmatrix, and it runs wonderfully together in ghci. When I run >>>>> ghc -- >>>>> make, however, I run into the following link dependency: >>>>> >>>>> Linking SilkwormGame ... >>>>> Undefined symbols: >>>>> "_dgemm_", referenced from: >>>>> _multiplyR in libHShmatrix-0.5.0.1.a(lapack-aux.o) >>>>> "_dgesv_", referenced from: >>>>> _linearSolveR_l in libHShmatrix-0.5.0.1.a(lapack-aux.o) >>>>> ... etc >>>>> >>>>> Is there a way to tell ghc to not link these? Or am I making a >>>>> poor >>>>> assumption that if my code runs in ghci, it does not need LAPACK? >>>>> >>>>> Thanks, >>>>> Duane Johnson >>>> >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> Haskell-Cafe(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Combining sequences
by michael rice 06 Apr '09

06 Apr '09

Thanks. It looks like mergeBy will do the job, but is it available in Hugs? Michael --- On Sun, 4/5/09, Henning Thielemann <lemming(a)henning-thielemann.de> wrote: From: Henning Thielemann <lemming(a)henning-thielemann.de> Subject: Re: [Haskell-cafe] Combining sequences To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Sunday, April 5, 2009, 9:09 PM On Sat, 4 Apr 2009, michael rice wrote: > Is there a simple way to combine two sequences that are in ascending order into a single > sequence that's also in ascending order? An example would be the squares [1,4,9,16,25..] > combined with the cubes [1,8,27,64,125..] to form [1,1,4,8,9,16,25,27..]. http://hackage.haskell.org/packages/archive/utility-ht/0.0.4/doc/html/Data-…

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