February 2008 - Haskell-Cafe

Circular enums
by Ben Butler-Cole 02 Feb '08

02 Feb '08

Hello I'm trying to define functions that allow you to traverse a bounded enumeration, "wrapping" at the start and the end. My implementation looks like this: next, prev :: (Enum a, Bounded a) => a -> a next = turn 1 prev = turn (-1) turn :: (Enum a, Bounded a) => Int -> a -> a turn n e = toEnum (add (fromEnum (maxBound::a) + 1) (fromEnum e) n) where add mod x y = (x + y + mod) `rem` mod Which fails to type check under GHC with this error: No instance for (Bounded a) arising from use of `maxBound' at Hbot.hs:6:34-41 Probable fix: add (Bounded a) to the expected type of an expression In the expression: maxBound :: a In the first argument of `fromEnum', namely `(maxBound :: a)' In the first argument of `(+)', namely `fromEnum (maxBound :: a)' My (clearly flawed) understanding of the signature I've specified for 'turn' means *exactly* that a is Bounded. Can anyone enlighten me as to where my understanding is going awry and how (whether) I can achieve what I'm trying to do. Thank you Ben

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Circular enums - resend
by Ben Butler-Cole 02 Feb '08

02 Feb '08

[Resending with formatting fixed.] Hello I'm trying to define functions that allow you to traverse a bounded enumeration, "wrapping" at the start and the end. My implementation looks like this: next, prev :: (Enum a, Bounded a) => a -> a next = turn 1 prev = turn (-1) turn :: (Enum a, Bounded a) => Int -> a -> a turn n e = toEnum (add (fromEnum (maxBound::a) + 1) (fromEnum e) n) where add mod x y = (x + y + mod) `rem` mod Which fails to type check under GHC with this error: No instance for (Bounded a) arising from use of `maxBound' at Hbot.hs:6:34-41 Probable fix: add (Bounded a) to the expected type of an expression In the expression: maxBound :: a In the first argument of `fromEnum', namely `(maxBound :: a)' In the first argument of `(+)', namely `fromEnum (maxBound :: a)' My (clearly flawed) understanding of the signature I've specified for 'turn' means *exactly* that a is Bounded. Can anyone enlighten me as to where my understanding is going awry and how (whether) I can achieve what I'm trying to do. Thank you Ben

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weird behavior with FFI
by Tim Newsham 02 Feb '08

02 Feb '08

I am working on haskell bindings to C functions using FFI. I have a callback function that returns "IO ()". When I pass in the callback function: foo = printf "foo" it crashes (is there an easy way to debug this exception using ghc6.8.2?) bot: SilcClient_d1al: uncaught exception however, if I pass in the callback function: foo = do printf "foo" return () it does not crash. Is this a bug in my bindings? In ghc/ffi? Full code is at: http://www.thenewsh.com/%7Enewsham/silcbot.tgz Tim Newsham http://www.thenewsh.com/~newsham/

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highlighting-kate - syntax highlighting library
by John MacFarlane 02 Feb '08

02 Feb '08

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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Issues with hsql-sqllite build; errors from the hackage download
by bbrown 02 Feb '08

02 Feb '08

There seems to be an issue with the hsql-sqlite3. Anyone have a fix. Should I use what is from darcs? Index of /packages/archive/hsql-sqlite3/1.7/logs/failure The Glorious Glasgow Haskell Compilation System, version 6.8.1 $ runhaskell Setup.lhs configure Setup.lhs:7:33: Module `Distribution.Simple.Utils' does not export `rawSystemVerbose' -- Berlin Brown [berlin dot brown at gmail dot com] http://botspiritcompany.com/botlist/?

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Re: HList error with hFoldr
by oleg＠okmij.org 02 Feb '08

02 Feb '08

Denis Bueno wrote: > For the moment my HLists only contain Ints. ... > -- Why should the following generate an error? > testApplyDistSum = hFoldr ApplyDistSum 0 ((4,4) .*. hNil) Here is the problem: in Haskell, 4 is not an Int. (4::Int) is an Int, but just 4 is a _polymorphic_ constant of the type Num a => a. Because it is polymorphic and so does not have any defined type, the type checker cannot chose an appropriate instance for ApplyDistSum. The error message says this directly: > No instance for (Apply ApplyDistSum ((t, t1), r1) r) Note the appearance of lower-case identifiers t, t1, r1, and r in the error message. They stand for the types that the type checker cannot figure out. When you use hFoldr in a larger expression, the types often become constrained by other components of the expression and so type variables become instantiated. In the simple cases as above, there is no information available to instantiate the type variables, hence the error. You have to give the type-checker hints in the form of type annotations: testApplyDistSum = hFoldr ApplyDistSum (0::Int) ((4::Int,4::Int) .*. hNil) In general, it is possible to avoid the type annotation on the accumulator (in this case, 0). The type checker could figure out the type of the result. But that requires specifying functional dependencies. The type annotations might be easier to start with. Incidentally, it is generally a good idea in Haskell to write Int constants as (1::Int), explicitly specifying their type. Here's a bit elaborated example: {-# OPTIONS -fglasgow-exts #-} module B where import HList class (Num i) => MetricSpace e i where dist :: e -> e -> i instance Num i => MetricSpace Int i where x `dist` y = fromIntegral $ abs (y - x) instance Floating i => MetricSpace Float i where x `dist` y = realToFrac $ abs (y - x) data ApplyDistSum = ApplyDistSum instance (MetricSpace e r) => Apply ApplyDistSum ((e, e), r) r where apply _ (p, v) = v + uncurry dist p testApplyDistSum = hFoldr ApplyDistSum (0::Double) ((4::Int,5::Int) .*. hNil) testApplyDistSum1 = hFoldr ApplyDistSum (0::Double) ( (4::Float,5.1::Float) .*. (4::Int,5::Int) .*. hNil)

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newbie q about types
by Logesh Pillay 02 Feb '08

02 Feb '08

I have a list. Each component is a list with 2 whole numbers. I want to multiply the second number by the log of the first eg tail ([519432,525806]) * log (head [519432,525806]). I get the ffg error message: No instance for (Floating [t]) arising from a use of `log' at <interactive>:1:25-50 Possible fix: add an instance declaration for (Floating [t]) In the second argument of `(*)', namely `log (head [519432, 525806])' In the expression: tail ([519432, 525806]) * log (head [519432, 525806]) In the definition of `it': it = tail ([519432, 525806]) * log (head [519432, 525806]) I've tried the only type signature I know "fromIntegral" but it does not help. How to fix? Thanks Logesh

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parsec3 pre-release
by Derek Elkins 01 Feb '08

01 Feb '08

I'm currently getting Paolo Martini's Google Summer of Code project, an updated version of Parsec, into a releasable state, and I will be maintaining it for at least a while. Paolo's major additions are: * The Parser monad has been generalized into a Parser monad transformer * The parsers have been generalized to work over a stream of any type, in particular, with bytestrings. I have made a few minor additions as well: * There is Haddock documentation for almost all functions * The Parser monad now has Applicative/Alternative instances Currently, I am looking for people to give it a go reporting any bugs in the library or documentation, troubles building it, or changes/features they would like. I'm also interested in performance information. Most old Parsec code should be relatively easy but not trivial to port. There is a darcs repository on code.haskell.org. If nothing comes up, I'll put a package on Hackage in about a week or so. To get the code: darcs get http://code.haskell.org/ To build it, the standard cabal commands should work: http://haskell.org/haskellwiki/Cabal/How_to_install_a_Cabal_package Alternatively, you can use the cabal-install application: http://hackage.haskell.org/trac/hackage/wiki/CabalInstall The documentation can be generated also via the normal cabal routine, or via cabal-install.

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reading from the cdrom drive on Red Hat
by Galchin Vasili 01 Feb '08

01 Feb '08

Hello, I wrote a simple program to read the contents of a cdrom: module Main where import Text.Printf import System.IO import System.Posix.Types import System.Posix.IO main = do fd <- openFd "/dev/cdrom" ReadOnly Nothing defaultFileFlags readCdRom fd 4096 closeFd fd readCdRom fd byteCount = do (buf, actualByteCount) = fdRead fd bytecount readCdRom fd byteCount When it executed it read thousands of 4096 blocks as I expected. It then got a "hardware error" which I suspect was the "end of the CD", i.e. the begininng of the "unlasered" part of the CD. Is there a better and more graceful way to detect and handle "end of data"? Thanks, ,Vasili

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Simple network client
by Timo B. Hübel 01 Feb '08

01 Feb '08

Hello, I am using the very simple interactTCP example from [1] to play around with Haskell network programming but I just can't get a simple client for that example to work (it works like a charm with my telnet client, as described in the article). This is what I am trying to do with the client: main = withSocketsDo $ do hdl <- connectTo "localhost" (PortNumber 1234) hSetBuffering hdl NoBuffering hPutStr hdl "test message" res <- hGetContents hdl putStrLn (show res) The server looks like this: interactTCP :: Int -> (String -> IO String) -> IO () interactTCP port f = withSocketsDo $ do servSock <- listenOn $ PortNumber (fromIntegral port) waitLoop f servSock waitLoop f servSock = do bracket (fmap (\(h,_,_)->h) $ accept servSock) hClose (\h -> do hSetBuffering h NoBuffering hGetContents h >>= f >>= hPutStr h) waitLoop f servSock main = interactTCP 1234 (return . map toUpper) But is seems as some deadlocking occurs. Both programs just hang around doing nothing. By inserting some debug output I was able to make sure that the client successfully connects, but the data interchange just does not start. Because the whole thing works using telnet, I suspect that I am doing something fundamentally wrong in the client ... Any hints are greatly appreciated. Thanks, Timo [1] http://stephan.walter.name/blog/computers/programming/haskell/interacttcp.h…

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