- Haskell-Cafe - Haskell.org

Equivalent of withForeignPtr for System.Mem.Weak?
by Matthew Steele 01 Nov '10

01 Nov '10

I have an object to which I have added one or more finalizers via addFinalizer from System.Mem.Weak. I would like to have a function that allows me to make use of the object within a block of IO code, and guarantee that the finalizer(s) will not be called during the code block -- sort of like withForeignPtr, but for arbitrary objects. Does such a function exist, and if not, how could I write one? I can imagine writing something like: \begin{code} withObject :: a -> IO b -> IO b withObject obj action = do result <- action touchObject obj -- touchObject :: a -> IO () return result \end{code} However, I don't know how I should implement touchObject and be confident that it won't ever be optimized out. Any suggestions? Cheers, -Matt

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Data records. Default values initialisation. Reading datatype from string.
by Siim Haugas 01 Nov '10

01 Nov '10

Hello, I need some help with data records. Normally I can use a `read' function to convert string to some data type e.g. `(read somestring) :: Type'. However, sometimes I'd like to read a record with some uninitialised fields. I have also set up a default values for my data record, for example: `defrec = {a=0,b=1, ..., z=0}'. Now I would like to override some values, so I have a string "defrec{l=100,r=88}", but it seems that it can't be done with read `(read "defRec{l=100,r=88}") :: defRec'. Any suggestions? Siim

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who's in charge?
by Günther Schmidt 31 Oct '10

31 Oct '10

Hi all, this may be an odd question to some, but I think it's actually quite an un-extraordinary one. Who's in charge? Of Haskell I mean. If there was some alien from Planet Java to land on Planet Haskell and demand to be taken to our leader, whom would we take him to? Günther

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Generic traversals for GADTs
by Ozgur Akgun 31 Oct '10

31 Oct '10

Café, SYB-style libraries (and especially Uniplate) make it very easy to run generic traversals (queries/transformations) on ADTs. > data Expr = ... > x :: Expr > f :: Expr -> Expr > transform :: (Expr -> Expr) -> Expr -> Expr > transform f x :: Expr -- applies f to x (and its children) in a bottom-up manner Uniplate is very powerful and doesn't require any hand written type-class instances to get going reasonably efficiently. I can simply re-implement the functionality of Uniplate myself, modulo requiring hand-written instances. (I didn't want to go into that road, since I don't need it for this argument.) I did this exercise, because I wanted to understand why we don't have similar generic traversal libraries for GADTs. And, it didn't take long :) I can post my trials but they are really baby steps and they wouldn't be helpful. My question is: Considering how useful generic traversal libraries and GADTs are, has anybody tried implementing a generic traversal library (like Uniplate or similar) for GADTs? Is it a hard (or impossible, or not efficient, or problematic in some other way) thing to implement, or is it just not a requirement for anyone else? Best, Ozgur

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Re: [Haskell-cafe] Parsing workflow
by Stephen Tetley 31 Oct '10

31 Oct '10

On 31 October 2010 16:53, Nils Schweinsberg <ml(a)n-sch.de> wrote: > Am 31.10.2010 17:27, schrieb Stephen Tetley: >> >> Left factoring! :-) > > Stupid question: Whats that? :) > Actually a good question... Its a standard grammar transformation - if you have two alternative productions that share a common prefix A -> a b | a c Left factoring produces: A -> A_part (b | c) A_part -> a Left factoring can quickly lead to the grammar getting quite convoluted, but using 'try' can lead to very fragile parsers that stop working in mysterious ways when you extend them. Because parser combinators are just functions, you can do questionable hacks to make left factoring a bit less tedious: parseA :: Parser A parseA = do a <- a_part (parseB a <|> parseC a) parseB :: a -> Parser A parseB a = do b <- b_part return $ B a b parseC :: b -> Parser A parseC a = do c <- c_part return $ C a c

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Current thinking on CompositionAsDot issue in haskell prime?
by Alexander Kjeldaas 31 Oct '10

31 Oct '10

Hi haskellers. Reading through the Haskell Prime suggestions, one that caught my eye is the CompositionAsDot issue. I'm especially thinking of the Pro issue: * Paves the way for using . as the selection operator in improved record or module systems I think I recognize this issue from common lisp. Basically the code becomes verbose because accessor functions usually need to redundantly encode the name of the module or struct in its name. The alternative is to require users to import qualified, and that doesn't deal with the occasional intra-module collision. This is a typical large-software-system issue and it annoys me a little that Haskell seems to have inherited the problems from common lisp. I worry that in large systems, a very conservative coding standard is needed with quite verbose naming conventions as that's the only safe way to avoid these conflicts in a large system while allowing refactoring code etc. What is the current thinking on this issue? Thanks, Alexander

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Haskell meeting in Berlin
by Sönke Hahn 31 Oct '10

31 Oct '10

Hi all! There will be an informal Haskell meeting in Berlin. Date: Thursday, October 28th Time: from 20:00 Location: c-base, Rungestrasse 20, 10179 Berlin The bi-monthly lisp meeting takes place at the same time and place [1]. This is the first of hopefully more meetings to come, so we will probably discuss the details of following meetings. If you're in Berlin and want to talk to other Haskell users (or want to yell at lisp users for being from the dark side of the force), please come by. Cheers, Sönke [1] http://www.c-base.org/calender/phpicalendar/month.php

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cabal mystery (#562?)
by Conor McBride 30 Oct '10

30 Oct '10

Hi I've just installed cabal-install (just as a user: I am nowhere near root) on our unix server at work. That went fine. Clearly, the sensible thing to do next is get hold of an up-to-date package list. So, I tried conor@cafe:~$ cabal update Downloading the latest package list from hackage.haskell.org cabal: Codec.Compression.Zlib: premature end of compressed stream Here's the verbose version. conor@cafe:~$ cabal -v3 update Downloading the latest package list from hackage.haskell.org Sending: GET http://hackage.haskell.org/packages/archive/00-index.tar.gz HTTP/1.1 User-Agent: cabal-install/0.8.2 Host: hackage.haskell.org proxy uri host: www-cache.strath.ac.uk, port: :8080 Creating new connection to www-cache.strath.ac.uk:8080 Received: HTTP/1.0 200 OK Date: Thu, 28 Oct 2010 12:59:17 GMT Server: Apache/2.2.9 (Debian) mod_python/3.3.1 Python/2.5.2 Last-Modified: Thu, 28 Oct 2010 12:38:17 GMT ETag: "1888003-20bb4e-493ac9dbe6040" Accept-Ranges: bytes Content-Length: 2145102 Content-Type: application/x-tar Content-Encoding: x-gzip X-Cache: MISS from rouble-1.cc.strath.ac.uk X-Cache-Lookup: MISS from rouble-1.cc.strath.ac.uk:8080 Via: 1.1 rouble-1.cc.strath.ac.uk:8080 (squid/2.7.STABLE7) Connection: close Downloaded to /home/s3/conor/.cabal/packages/hackage.haskell.org/00-index.tar.gz cabal: Codec.Compression.Zlib: premature end of compressed stream So I poked about a bit, and I see it's a known issue. As a user, I'm wondering what to do. Just now, wget http://hackage.haskell.org/packages/archive/00-index.tar.gz is chugging away quite nicely. Anyhow, is there (a) something I can do to avoid this problem? I couldn't quite grasp what to do from the TRAC (b) some way to persuade cabal to use the file I'm currently downloading? Any tips to keep the gremlins at bay gratefully appreciated. Thanks Conor

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Monads and Functions sequence and sequence_
by Mark Spezzano 30 Oct '10

30 Oct '10

Hi, Can somebody please explain exactly how the monad functions "sequence" and "sequence_" are meant to work? I have almost every Haskell textbook, but there's surprisingly little information in them about the two functions. From what I can gather, "sequence" and "sequence_" behave differently depending on the types of the Monads that they are processing. Is this correct? Some concrete examples would be really helpful. Even references to some research papers that explain the rationale behind these (and all the other..?) monad functions would be great. Thanks in advance, Mark Spezzano

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Package regex-posix-0.94.2 suddenly failed to link
by Arnaud Bailly 30 Oct '10

30 Oct '10

Hello, All of a sudden, the package regex-posix-0.94.2 failed to link after i installed a couple of other packages (http, json). When I try to reinstall it, I got the folowing errors: D:\projets\crete1941>cabal install --global --reinstall --enable-documentation regex-posix-0.94.2 Resolving dependencies... Configuring regex-posix-0.94.2... Preprocessing library regex-posix-0.94.2... Text\Regex\Posix\Wrap.hsc:107:19: regex.h: No such file or directory Text\Regex\Posix\Wrap.hsc: In function `main': Text\Regex\Posix\Wrap.hsc:149: error: `regoff_t' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:149: error: (Each undeclared identifier is reported only once Text\Regex\Posix\Wrap.hsc:149: error: for each function it appears in.) Text\Regex\Posix\Wrap.hsc:149: error: syntax error before "int" Text\Regex\Posix\Wrap.hsc:384: error: `REG_NOTBOL' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:385: error: `REG_NOTEOL' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:389: error: `REG_EXTENDED' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:390: error: `REG_ICASE' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:391: error: `REG_NOSUB' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:392: error: `REG_NEWLINE' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:398: error: `REG_NOMATCH' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:399: error: `REG_BADBR' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:400: error: `REG_BADPAT' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:401: error: `REG_BADRPT' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:402: error: `REG_ECOLLATE' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:403: error: `REG_ECTYPE' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:404: error: `REG_EESCAPE' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:405: error: `REG_ESUBREG' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:406: error: `REG_EBRACK' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:407: error: `REG_EPAREN' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:408: error: `REG_EBRACE' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:409: error: `REG_ERANGE' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:410: error: `REG_ESPACE' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:447: error: `regex_t' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:481: error: syntax error before ')' token Text\Regex\Posix\Wrap.hsc:484: error: `regmatch_t' undeclared (first use in this function) Text\Regex\Posix\Wrap.hsc:510: error: syntax error before ')' token Text\Regex\Posix\Wrap.hsc:510: error: syntax error before "int" Text\Regex\Posix\Wrap.hsc:511: error: syntax error before ')' token Text\Regex\Posix\Wrap.hsc:511: error: syntax error before "int" Text\Regex\Posix\Wrap.hsc:525: error: syntax error before ')' token compiling dist\build\Text\Regex\Posix\Wrap_hsc_make.c failed command was: D:\Program Files\Haskell Platform\2010.2.0.0\mingw\bin\gcc.exe -c -D__GLASGOW_HASKELL__=612 -DHAVE_REGEX_H -DSPLIT_BASE =1 -ID:\Program Files\Haskell Platform\2010.2.0.0\lib\bytestring-0.9.1.7\include -ID:\Program Files\Haskell Platform\2010.2.0.0\lib\ base-4.2.0.2\include -ID:\Program Files\Haskell Platform\2010.2.0.0\lib/include -ID:\Program Files\Haskell Platform\2010.2.0.0\lib/i nclude -ID:/Program Files/Haskell Platform/2010.2.0.0/lib/include/ dist\build\Text\Regex\Posix\Wrap_hsc_make.c -o dist\build\Text\Re gex\Posix\Wrap_hsc_make.o cabal: Error: some packages failed to install: regex-posix-0.94.2 failed during the building phase. The exception was: ExitFailure 1 I am really clueless about the failure and what caused it. Thanks for your help, Arnaud Bailly

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