- Haskell-Cafe - Haskell.org

Pattern matching error
by georg86 06 Dec '07

06 Dec '07

Hello! I need to write a function which should is supposed to merge multiple entries with the same key (out of a sorted key-value-list) into one single entry. However, I keep getting a pattern matching error. (For example, for input [('b',[5]),('b',[4]),('b',[1]),('b',[6])]: "Program error: pattern match failure: kgroup [('b',[5,4]),('b',[1]),('b',[6])]") Thank you very much for your help. kmerge::Eq a => [(a,[b])]->[(a,[b])] kmerge [] = [] kmerge ((x,[y]):[]) = [(x,[y])] kmerge ((x,[y]):(u,[v]):xs) | x == u = kmerge ((x,[y,v]):xs) | otherwise = (x,[y]):(u,[v]):kmerge xs -- View this message in context: http://www.nabble.com/Pattern-matching-error-tf4957268.html#a14196413 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Template Haskell examples?
by Brent Yorgey 06 Dec '07

06 Dec '07

Hi all, I'm looking for good examples of using Template Haskell to evaluate CAFs at compile time, e.g. if I have some expensive-to-calculate lookup table which I'd rather have evaluated at comile time and included in the executable as a constant value, rather than having it computed every time the executable is run. I know I've seen examples of this before, but now that I would actually like to study such an example, I can't seem to find any. Any suggestions? Or can anyone make up a simple example on the spot? =) thanks, -Brent

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Problems building and using ghc-6.9
by Daniel Fischer 06 Dec '07

06 Dec '07

Hi, so today I built ghc-6.9.20071124. First, make died because HsColour version >= 1.8 was needed, couldn't determine the version. I had HsColour 1.6, got myself 1.8, built and installed. make died again, same problem. I added ("v", Version) to the optionTable in HsColour.hs and it worked :) Then I tried to build zlib-0.4.0.1: $ runghc ./Setup.hs configure --user --prefix=$HOME Configuring zlib-0.4.0.1... Setup.hs: At least the following dependencies are missing: base >=2.0&&<2.2 ??? okay, there was something with flag bytestring-in-base, removed that, so that build-depends was base < 2.0 || >= 2.2, bytestring >= 0.9, then $ runghc ./Setup.hs configure --user --prefix=$HOME Configuring zlib-0.4.0.1... Setup.hs: At least the following dependencies are missing: base <2.0||>=2.2, bytestring >=0.9 but: $ ghc-pkg list /home/dafis/lib/ghc-6.9.20071124/package.conf: ALUT-2.1.0.0, Cabal-1.3, GLUT-2.1.1.1, HUnit-1.2.0.0, OpenAL-1.3.1.1, OpenGL-2.2.1.1, QuickCheck-1.1.0.0, array-0.1, base-3.0, bytestring-0.9, cgi-3001.1.5.1, containers-0.1, directory-1.0, fgl-5.4.1.1, filepath-1.1, (ghc-6.9.20071124), haskell-src-1.0.1.1, haskell98-1.0.1, hpc-0.5, html-1.0.1.1, mtl-1.1.0.0, network-2.1.0.0, old-locale-1.0, old-time-1.0, packedstring-0.1, parallel-1.0.0.0, parsec-2.1.0.0, pretty-1.0, process-1.0, random-1.0, readline-1.0.1, regex-base-0.72.0.1, regex-compat-0.71.0.1, regex-posix-0.72.0.2, rts-1.0, stm-2.1.1.0, template-haskell-2.2, time-1.1.2.0, unix-2.2, xhtml-3000.0.2.1 and in stringsearch-0.2: $ runghc ./Setup.lhs configure --user --prefix=$HOME Configuring stringsearch-0.2... Setup.lhs: At least the following dependencies are missing: base -any What's going on?? How can I build packages with ghc-6.9? Or do I have to go back to 6.8.1? Thanks, Daniel

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Clean Dynamics and serializing code to disk
by gwern0＠gmail.com 06 Dec '07

06 Dec '07

Hey everyone; recently I've been toying around with various methods of writing a shell and reading the academic literature on such things. The best prior art on the subject seems to be the ESTHER shell (see <http://citeseer.ist.psu.edu/689593.html>, <http://citeseer.ist.psu.edu/744494.html>, <ftp://ftp.cs.kun.nl/pub/Clean/papers/2003/vWeA2003-Esther.pdf>). Now, ESTHER is a really cool looking shell, but it has two main problems for me: 1) Source doesn't seem to be available anywhere online 2) It's written in Clean and not Haskell No problem. All the hard stuff is done, and there's like a good 50 pages of documentation, so how hard could it be? Clean is practically Haskell anyway. But immediately I ran into a road-block: "The shell is built on top of Clean's hybrid static/dynamic type system and its dynamic I/O run-time support. It allows programmers to save any Clean expression, i.e a graph that can contain data, references to functions, and closures to disk. Clean expressions can be written to disk as a _dynamic_, which contains a representation of their (polymorphic) static type, while preserving sharing. Clean programs can load dynamics from disk and use run-time type pattern matching to reintegrate it into the statically-typed program." The Data.Dynamic library seems to do everything as far as dynamic types and run-time pattern matching goes, but I haven't figured out how one could write Haskell expressions to disk, like Clean's system <http://www.st.cs.ru.nl/papers/2002/verm2002-LazyDynamicIO.ps.gz> apparently allows. Does anyone know if there are any neat or tricky ways this could be done? Projects, extensions, whatever? On #haskell, quicksilver did tell me of one neat way to serialize various stuff through Data.Binary by using ADTs along the lines of the following simple example: -- module Main (main) where import Data.Binary data Math = Add | Subtract | Multiply deriving Show eval :: (Num a) => Math -> a -> a -> a eval f = case f of Add -> (+) Subtract -> (-) Multiply -> (*) instance Binary Math where put Add = putWord8 0 put Subtract = putWord8 1 put Multiply = putWord8 2 get = do tag_ <- getWord8 case tag_ of 0 -> return Add 1 -> return Subtract 2 -> return Multiply main = do encodeFile "tmp.s" [Add, Subtract, Multiply] a <- decodeFile "tmp.s" putStr $ show (a :: [Math]) -- Since from my Lisp days I know that code is data, it strikes me that one could probably somehow smuggle Haskell expressions via this route although I am not sure this is a good way to go or even how one would do it (to turn, say, a list of the chosen ADT back into real functions, you need the 'eval' function, but apparently eval can only produce functions of the same type - so you'd need to either create as many adts and instances as there are varieties of type signatures in Haskell '98 and the libraries, I guess, or somehow encode in a lambda calculus). Is that a route worth pursuing? -- gwern

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HOgg 0.3.0 Released
by Conrad Parker 06 Dec '07

06 Dec '07

HOgg 0.3.0 Released ------------------- The HOgg package provides a commandline tool for manipulating Ogg files, and a corresponding Haskell library. HOgg is in hackage, or on the web at: http://www.kfish.org/~conrad/software/hogg/ This is the second public release. The focus is on correctness of Ogg parsing, production and editing. The capabilities of the hogg commandline tool are roughly on par with those of the oggz* [0] tools. Although hogg does not yet provide an equivalent to oggz-validate, it has subcommands for chopping out a section of a file, and for adding skeleton metadata. HOgg supports chained and multiplexed Ogg bitstreams conformant with RFC3533[1]. HOgg can parse headers for CMML, FLAC, OggPCM, Speex, Theora and Vorbis media codecs, and can read and write Ogg Skeleton bitstreams. [0] Oggz: http://www.annodex.net/software/liboggz/index.html [1] RFC3533: http://www.ietf.org/rfc/rfc3533.txt New in this release ------------------- The hogg tool contains new subcommands: chop, addskel and list-codecs. Additionally, subcommands for inspecting streams (hogg dump, hogg pagedump) can now take start and end time options. $ hogg help chop chop: Extract a section (specify start and/or end time) Usage: hogg chop [options] filename ... Examples: Extract the first minute of file.ogg: hogg chop -e 1:00 file.ogg Extract from the second to the fifth minute of file.ogg: hogg chop -s 2:00 -e 5:00 -o output.ogg file.ogg Extract only the Theora video stream, from 02:00 to 05:00, of file.ogg: hogg chop -c theora -s 2:00 -e 5:00 -o output.ogg file.ogg Extract, specifying SMPTE-25 frame offsets: hogg chop -c theora -s smpte-25:00:02:03::12 -e smpte-25:00:05:02::04 -o output.ogg file.ogg Options: -h, -? --help Display this help and exit -V --version Output version information and exit -c Content-Type --content-type=Content-Type Select the logical bitstreams for a specified content type -s Timestamp --start=Timestamp Specify a start time -e Timestamp --end=Timestamp Specify an end time -o filename --output=filename Specify output filename Additionally, the HOgg package now contains support for building with GHC version 6.8, and the Codec.Container.Ogg library contains various internal improvements. Installation ------------ I am very interested in hearing about problems with building or installing the package, particularly from people who are not yet familiar with building from Haskell source. You need ghc instead of gcc; it compiles to a binary: $ ./Setup.hs configure $ ./Setup.hs build $ sudo ./Setup.hs install Building of this release has been tested with: * GHC versions 6.4, 6.6 and 6.8.1 [2] * The Haskell build system "Cabal" versions 1.1.3, 1.1.4, 1.1.6, and the current development trunk. [3] The GHC and Cabal versions listed above span the packages available in most current distributions. I've tested on Debian unstable and Ubuntu Feisty. I'm particularly interested to hear reports of build success or failure on other distributions or operating systems. [2] GHC: http://www.haskell.org/ghc/ [3] Cabal: http://www.haskell.org/cabal/ Usage ----- $ hogg help Usage: hogg <subcommand> [options] filename ... Commands: help Display help for a specific subcommand (eg. "hogg help chop") Reporting: info Display information about the file and its bitstreams dump Hexdump packets of an Ogg file pagedump Display page structure of an Ogg file dumpraw Dump raw (unparsed) page data Extraction: rip Rip selected logical bistreams from an Ogg file (default: all) reconstruct Reconstruct an Ogg file by doing a full packet demux Editing: chop Extract a section (specify start and/or end time) merge Merge, interleaving pages in order of presentation time addskel Write a Skeleton logical bitstream Miscellaneous: known-codecs List codecs known by this version of hogg Please report bugs to <ogg-dev(a)xiph.org> Source ------ Source code is available from the darcs repository at: darcs get http://www.kfish.org/~conrad/software/hogg/ cheers, Conrad.

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Re: About -Wall option
by Yitzchak Gale 06 Dec '07

06 Dec '07

Luis Cabellos wrote: >> I have a question, what's the best way to program? >> - put all the signatures in the Haskell Code? >> - Only put the type signatures needed to compile (like monomorphism errors >> or ambiguous signature)? Wolfgang Jeltsch wrote: > Inserting all type signatures is definitely best practice. (Moving to the cafe from ghc-users...) My personal style is: o Put signatures on everything at the top-level o Inside let and where, only where necessary, BUT: Inside let and where, I try to make sure that every function I define there is such that I would never want to call it separately in ghci/hugs while debugging, or in a unit test. Besides making testing and debugging easier, that rule forces the overall structure of every function to be simple and easy to read. In that case, type signatures just clutter the code and make it less readable. Conversely, if the relationship between the type of the let/where function and the type of the outer function is complex and not obvious, I will likely need to test that function. So it will go into a separate top-level function with a type signature. (I don't think this style creates any problems in Cabal.) -Yitz

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Re: C Preprocessor
by Reinier Lamers 06 Dec '07

06 Dec '07

Bernd Brassel wrote: >Is it already a known problem that the preprocessor cannot cope with the >whole set of possible string declarations? > Yes, it is: http://hackage.haskell.org/trac/hackage/ticket/146 I ran into it lately. I was totally unaware of what caused GHC's parse error on a valid line until someone on the IRC channel pointed it out. According to the ticket, cpphs (a Haskell-oriented CPP replacement) does get it right. Regards, Reinier

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Re: [Haskell-cafe] Looking for largest power of 2 <= Integer
by Dan Piponi 06 Dec '07

06 Dec '07

On Dec 3, 2007 10:05 PM, David Benbennick <dbenbenn(a)gmail.com> wrote: > Could you please post your code here when you're done? I'd be > interested to see the final result. This is just experimental code I'm playing with in order to implement exact real arithmetic, so there'll never be a "final" result :-) But this is what I'm currently playing with. It's hard-coded for a platform with 64 bit Ints and 64 bit "limbs" of Integers. This is my first ever foray into the binary underbelly of Haskell, using information from http://www.haskell.org/ghc/docs/4.06/users_guide/ghc-libs-ghc.html, and I've probably written this code really stupidly. import GHC.Exts import Data.Bits fastTestBit :: Integer -> Int -> Bool fastTestBit n i = case n of S# m -> testBit (I# m) i J# l d -> let I# w = shiftR i 6 b = i .&. 63 in testBit (I# (indexIntArray# d w)) b -- Assumes n/=0 topBit :: Integer -> Int topBit n = case n of S# m -> topBit' n 63 J# l _ -> topBit' n (64*I# l-1) where topBit' n i = if fastTestBit n i then i else topBit' n (i-1) I don't need something super-fast (ie. clever bit twiddling tricks), just something not stupidly slow. Despite what dons says, testBit is stupidly slow :-) fastTestbit takes microseconds instead of seconds on 2^10000000. Maybe a portable tidied up version of fastTestBit ought to go into Data.Bits. These kinds of operations are ubiquitous in numerical algorithms. And note the fixed title :-) -- Dan

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Re: [Haskell] IVar
by Simon Marlow 06 Dec '07

06 Dec '07

Jan-Willem Maessen wrote: >> Consider this: >> >> do >> x <- newIVar >> let y = readIVar x >> writeIVar x 3 >> print y >> >> (I wrote the let to better illustrate the problem, of course you can >> inline y if you want). Now suppose the compiler decided to evaluate y >> before the writeIVar. What's to prevent it doing that? Nothing in >> the Haskell spec, only implementation convention. > > Nope, semantics. If we have a cyclic dependency, we have to respect > it---it's just like thunk evaluation order in that respect. Ah, so I was thinking of the following readIVar: readIVar = unsafePerformIO . readIVarIO But clearly there's a better one. Fair enough. Cheers, Simon

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Re: [Haskell-cafe] ghc overlapping instances -> solved
by Steffen Mazanek 06 Dec '07

06 Dec '07

Hello, Isaac, this works for me. Thx a lot, Steffen 2007/12/5, Isaac Dupree <isaacdupree(a)charter.net>: > > Steffen Mazanek wrote: > > Hi, > > > > Stefan and Isaac, thx for providing quick advice. > > > > @Stefan: Unfortunately I have to use a list. > > @Isaac: I do not get it. Could you please provide a short example of > your > > approach? > > > > The question still remains. Which arguments do I have ghc to start with > to > > get the same behavior than hugs with -98 +o (here it works). > > > > I provide my example for testing purposes: > > > > module Test where > > import Test.QuickCheck > > import Monad(liftM,liftM2) > > > > type Program = [Stmt] > > data Stmt = Text | IfElse Program Program | While Program deriving (Eq, > > Show) > > > > instance Arbitrary [Stmt] where > > arbitrary = sized genProg > > instance Arbitrary Stmt where > > arbitrary = sized genStmt > > > > genStmt::Int->Gen Stmt > > genStmt 0 = return Text > > genStmt 1 = return Text > > genStmt 2 = oneof [return Text, return (While [Text])] > > genStmt n | n>2 = oneof ([return Text, > > liftM While (genProg (n-1))]++ > > [liftM2 IfElse (genProg k) (genProg > > (n-k-1))|k<-[1..n-2]]) > > > > genProg::Int->Gen Program > > genProg 0 = return [] > > genProg 1 = return [Text] > > genProg n | n>1 = oneof ((liftM (\x->[x]) (genStmt n)):[liftM2 (:) > (genStmt > > k) (genProg (n-k))|k<-[1..n-1]]) > > > > prop_ConstructParse progr = True > > where types = progr::Program > > > > main = mapM_ (\(s,a) -> putStrLn s >> a) [("flowchart construct and > parse", > > test prop_ConstructParse)] > > is prop_ConstructParse the only thing that breaks when you remove the > instance Arbitrary [Stmt] where arbitrary = sized genProg, or have I > missed something? If that's all, try this (untested) : > > prop_ConstructParse = forAll (sized genProg) (\progr -> True) > > and similarly for other properties. > > Or, you _can_ use a newtype for quickcheck-only, something like this: > > newtype P = P { unP :: Program } > instance Show P where show = show . unP > instance Arbitrary P where arbitrary = sized genProg . unP > prop_ConstructParse (P progr) = True > > > Isaac > -- Dipl.-Inform. Steffen Mazanek Institut für Softwaretechnologie Fakultät Informatik Universität der Bundeswehr München 85577 Neubiberg Tel: +49 (0)89 6004-2505 Fax: +49 (0)89 6004-4447 E-Mail: steffen.mazanek(a)unibw.de

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