February 2008 - Haskell-Cafe

Parsec3 stream properties
by Philippa Cowderoy 05 Feb '08

05 Feb '08

I'm having a little difficulty finding full properties for Parsec3's Stream class, largely because I don't want to overspecify it with regard to side-effects. Here's the class: > class Stream s m t | s -> t where > uncons :: s -> m (Maybe (t,s)) The idea is that: * unfoldM uncons gives the [t] corresponding to the stream * Assuming no relevant side-effects, unconsing the same value twice will yield the same result What's a relevant side-effect? Well, that's up to the stream and the monad it's built in - but unconsing once shouldn't affect the result of unconsing a second time unless you've found something fiendishly clever that I haven't thought of. Using stream mutability to implement a programming language from within the parsing monad is not a good idea, though no doubt someone'll do it anyway! -- flippa(a)flippac.org Society does not owe people jobs. Society owes it to itself to find people jobs.

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Re: [Haskell-cafe] Mutable arrays
by John Lato 05 Feb '08

05 Feb '08

At least for file processing, I don't think the lazy solution is as bad as some people on this list indicate. My solution was to define a function processAudioFile :: (Handle, Handle) -> (ASig -> ASig) -> IO (), similar to interact. The function reads from the first handle and writes to the second (the problem domain requires two separate files). Contents of the first are read into a lazy bytestring with hGetContents (from Data.ByteString.Lazy), decoded into an ASig (which in the current version is actually a tuple of a list and an Int of the total length, but I'm reworking this into a monad), and processed. The processed list is then encoded back into a bytestring and written with hPut. I then stick the whole thing in a bracket to open and close the filehandles, and call the bracketed function when I'm ready to do processing. I'm pretty happy with this solution, for several reasons: 1. The actual processing code remains purely functional. 2. I didn't have to write imperative-style looping constructs. 3. Handles get closed after use (even with exceptions, thanks to bracket). 4. Because all IO, processing, and writing is encapsulated in one function, everything happens sequentially as it's supposed to, so I don't get exceptions about lazy filehandles being closed. 5. Performance has been good. Memory usage is lower than expected, and it's fairly fast (at least when I remember to use a non-profiled version). I've tested this approach with wave files into the 100's of MB so far. Perhaps not quite as fast as optimized C, but good enough for me. I'm not quite sure how to get around the problem of getElems being strict, though. I do have one idea, but I don't know how it would work in practice: -- let ary_max = foldl1' max $ elems $ unsafeFreeze myArray If you use a boxed array type (IOArray or STArray) for myArray, and compiled with GHC, no copying is necessary (you may need to use type annotations to guarantee this). Then use the foldl' function to get array_max, and map it onto the original mutable array. I think it would be safe provided that you calculate ary_max before you start to modify the array, which is true for normalization. It's worth a try, anyway. John Lato > > Changing the subject slightly, I once wrote code in Concurrent Clean that > > filtered a file that was larger than the available memory on my PC. I did > > this by creating a function that returned the contents of the original file > > as a lazy list. Then, I created functions to process the list and write the > > processed list to a results file. The code was not imperative at all. The > > function that wrote the results file forced the evaluation of the lazy list. > > As the lazy list was consumed, the contents of the original file were read. > > Is this possible with Monads in Haskell? > > Yes, using hGetContents, which is considered bad practice by many > people here. The problem is that hGetContents breaks referential > transparency, and I suspect that whatever Clean does to lazily read > files also does (though I can't be sure, I haven't looked in any > detail at uniqueness types). That is, the contents of the returned > list depend on when you read it, which is not allowed in a > referentially transparent language. > > The same applies to your problem. getElems cannot return a lazy list > of elements*, because what if the array were changed between the point > that you did the getElems and the point you required the element. So > it seems that actually specifying the order of evaluation using an > imperative-style loop is the only pure way to do this. > > * Well, it could, but it would require some cleverness like > copy-on-write logic under the hood.

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Parameterisations of Monads
by Matthew Sackman 05 Feb '08

05 Feb '08

So I was thinking how dull and uninspiring the current definiton of Monad really is and came up with some more interesting parameterisations. The only problem with this one is I'm a) not sure if it still is a Monad and b) very unsure if it's of any use. There's the possibility that chucking Cont in there or using newtype to simultate multiple arrows / type lambdas may lead to more interesting instances, but can anyone think of exciting use cases for this stuff? Feel free to fill in the instances! It's also not a parameterisation I've seen before. Matthew > class SuperMonad (m1 :: * -> * -> *) (m2 :: * -> *) where > (>>~) :: m1 (m2 a) (m1 (m2 b) (m2 b)) > (>>=~) :: m1 (m2 a) (m1 (m1 a (m2 b)) (m2 b)) > returns :: m1 a (m2 a) > > instance (Monad m) => SuperMonad ((->)) m where > (>>~) :: m a -> m b -> m b > (>>~) = (>>) > (>>=~) :: m a -> (a -> m b) -> m b > (>>=~) = (>>=) > returns :: a -> m a > returns = return > > instance (Monad m) => SuperMonad ((,)) m where > (>>~) :: (m a, (m b, m b)) > (>>=~) :: (m a, ((a, m b), m b)) > returns :: (a, m a) > > instance (Monad m) => SuperMonad Either m where > (>>~) :: Either (m a) (Either (m a) (m b)) > (>>=~) :: Either (m a) (Either (Either a (m b)) (m b)) > returns :: Either a (m a) > > instance (Monad m) => SuperMonad State m where > (>>~) :: State (m a) (State (m a) (m b)) > (>>=~) :: State (m a) (State (State a (m b)) (m b)) > returns :: State a (m a)

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Re: [Haskell-cafe] building the unix package and cabal
by Thomas Schilling 05 Feb '08

05 Feb '08

I don't know. Maybe someone on @cafe can help. (I CC'd) On Feb 4, 2008 5:22 PM, Galchin Vasili <vigalchin(a)gmail.com> wrote: > I have the gmp shared objects installed, i.e. .so's. Does ghc require static > linking with .a archive files? > > > > > On 2/4/08, Thomas Schilling <nominolo(a)googlemail.com> wrote: > > This is the GNU Multi-Precision library. You have to install that > > separately, Cabal can't do that for you. You should use the redhat > > package manager to install it. HTH > > > > On Feb 4, 2008 4:43 PM, Galchin Vasili <vigalchin(a)gmail.com> wrote: > > > Hi Thomas, > > > > > > Have you tried to build this package yourself? I get a linker error > .. > > > unresolved symbol "gmp". > > > > > > Vasili > > > > > > > > > > > > > > > On 2/4/08, Thomas Schilling <nominolo(a)googlemail.com> wrote: > > > > Build-type: Configure means that *Cabal uses ./configure * to build > > > > the package. Nothing changes for you. Just use the usual: > > > > > > > > runhaskell Setup.hs configure --prefix=... [--user] > > > > runhaskell Setup.hs build > > > > runhaskell Setup.hs install > > > > > > > > or with cabal-install: > > > > > > > > cabal install unix > > > > > > > > 2008/2/4 Galchin Vasili <vigalchin(a)gmail.com>: > > > > > Hello, > > > > > > > > > > I have been trying to build the unix package on RedHat RHEL 5. > > > Over > > > > > the weekend I read through the Cabal documentation. The unix-2.2.0.0 > > > > > unix.cabal specifies the "build-type" attribute as "Configure". > Based on > > > > > Cabal doc, I should run ./configure. After this step then what? > Bottom > > > line > > > > > is how do I build the unix-2.2.0.0 package on RHEL 5? > > > > > > > > > > Thank you, Vasili > > > > > _______________________________________________ > > > > > Haskell-Cafe mailing list > > > > > Haskell-Cafe(a)haskell.org > > > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > > > > > > > > > > > > > > > > > > >

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Minor issue with mapAccumR
by Cale Gibbard 05 Feb '08

05 Feb '08

I'm not sure whether this would be considered worth fixing right away, or if we should wait until some other major compatibility breaking language change to fix it, but it appears that somehow the parameters to the function passed to mapAccumR are flipped relative to their natural order. To show what I mean, here is some code: Prelude Data.List> mapAccumR (\x y -> (concat ["(f ",x," ",y,")"], concat ["(g ",x," ",y,")"])) "z" ["1","2","3"] ("(f (f (f z 3) 2) 1)",["(g (f (f z 3) 2) 1)","(g (f z 3) 2)","(g z 3)"]) You can see here that the list is flipped over in the process, even though the right fold structure is there, it ends up looking like a left fold over the reverse of the list. One would want the law: fst . mapAccumR f z = foldr (fst . f) z to be true, but instead we have: fst . mapAccumR (flip f) z = foldr (fst . f) z You can see that structurally if we flip the parameters in the above example: Prelude Data.List> mapAccumR (\y x -> (concat ["(f ",x," ",y,")"], concat ["(g ",x," ",y,")"])) "z" ["1","2","3"] ("(f 1 (f 2 (f 3 z)))",["(g 1 (f 2 (f 3 z)))","(g 2 (f 3 z))","(g 3 z)"]) I also have some diagrams on http://cale.yi.org/index.php/Fold_Diagrams (near the end) displaying the difference, and that's where I first noticed it. Are many people using mapAccumR? How much would it hurt to change it? - Cale

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Is Haskore dead?
by Maurício 05 Feb '08

05 Feb '08

Hi, I've just tried using Haskore (I use Ubuntu and GHC), with no success. Since Haskore was started a long time ago, but it's not yet cabalized, and the author's page can not be reached, I can't say for sure if it's still maintained. Does anybody know? Thanks, Maurício

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Adding gcc type options with cabal (e.g. -mno-cygwin)
by bbrown 05 Feb '08

05 Feb '08

Is there a way to pass misc options to the cabal, ghc process. I tried the following: extra-libraries: sqlite3 extra-lib-dirs: C:\cygwin\lib include-dirs: C:\cygwin\usr\include ghc-options: -mno-cygwin runhaskell Setup.lhs build --ghc-options=-mno-cygwin -v No dice, doesn't show up. -- Berlin Brown email: berlin-dot-brown-AT-gmail-dot-com http://botspiritcompany.com/botlist/

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RE: highlighting-kate - syntax highlighting library
by mgsloan 04 Feb '08

04 Feb '08

Awesome! It looks quite nice, though it takes ages to compile. Would this be suitable for a text editor? One issue with text editors is you don't want to reprocess the entire file for each highlight. This, combined with multi-line syntactic elements, such as multiline strings or comments, makes for a rather awkward combination. It becomes a bit harder to predict the scope, highlighting wise, of editing actions. It may not really be necessary to do this very well. I for one find it annoying when you introduce an opening/closing symbol, it freezes for an instant, and the highlighting of the rest of the document alternates... So perhaps it could check if you're introducting a multiline element at the end when navigating off, re-evaluate highlighting of the rest of the document. Regardless of if the library can handle the requirements of text-editing in its current state, it looks like it wouldn't be too hard to add. Most of the work has been done :) Thanks! -Michael Sloan http://mgsloan.nfshost.com > Hello all, > > I've been working on a source code syntax highlighting library. It is > now somewhat usable, and help would be welcome in testing it further, so > I'm making it publicly available: > > darcs get http://johnmacfarlane.net/repos/highlighting-kate > > Currently, the following languages are supported: > > Ada Asp Awk Bash Bibtex > C Cmake Coldfusion Commonlisp Cpp > Css D Diff Djangotemplate Doxygen > Dtd Erlang Fortran Haskell Html > Java Javadoc Javascript Json Latex > Lex LiterateHaskell Lua Makefile Matlab > Mediawiki Nasm Objectivec Objectivecpp Ocaml > Pascal Perl Php Postscript Prolog > Python Ruby Scala Scheme Sgml > Sql SqlMysql SqlPostgresql Tcl Texinfo > Xml Xslt Yacc > > The parsers for individual languages are automatically generated > from Kate syntax definitions. The xml files have been included > in the repository, together with the program that converts them to > Haskell modules. There's also a standalone program, Highlight, that > you can use to test the highlighting. (See README for instructions.) > > Bug reports and patches are welcome. Note that I don't want to make > any manual modifications to the generated modules in > Text/Highlighting/Kate/Syntax. Changes should be made either in the > source xml files or in the program ParseSyntaxFiles.hs that generates > these modules. > > John

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A handy little consequence of the Cont monad
by Cale Gibbard 04 Feb '08

04 Feb '08

Hello, Today on #haskell, resiak was asking about a clean way to write the function which allocates an array of CStrings using withCString and withArray0 to produce a new with* style function. I came up with the following: nest :: [(r -> a) -> a] -> ([r] -> a) -> a nest xs = runCont (sequence (map Cont xs)) withCStringArray0 :: [String] -> (Ptr CString -> IO a) -> IO a withCStringArray0 strings act = nest (map withCString strings) (\rs -> withArray0 nullPtr rs act) Originally, I'd written nest without using the Cont monad, which was a bit of a mess by comparison, then noticed that its type was quite suggestive. Clearly, it would be more generally useful whenever you have a bunch of with-style functions for managing the allocation of resources, and would like to turn them into a single with-style function providing a list of the acquired resources. - Cale

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Bug in System.Environment - EOT inserted into argument string
by David Fox 04 Feb '08

04 Feb '08

I'm seeing the character ^D inserted into argument strings that are about 256 characters long with GHC 6.8.2. Anyone else? Test.hs: module Main where import System.Environment import System.IO main = do args <- getArgs hPutStrLn stderr ("args: " ++ show args) Output: $ ghc6 --make Test.hs -o test [1 of 1] Compiling Main ( Test.hs, Test.o ) Linking test ... $ ./test "012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789" args: ["01234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234\EOT5678901234567890123456789"]

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