May 2008 - Haskell-Cafe

Data.Tree.Zipper in the standard libraries
by Krasimir Angelov 24 May '08

24 May '08

Hello Guys, We have Data.Tree in the standard libraries for a long time but for some reason we still don't have standard implementation for Zipper. I wrote recently one implementation for Yi but there are many other versions hanging around. At least I know for some. I propose to add one in the standard libraries i.e. the "containers" package. The version that I use currently is here: http://code.haskell.org/yi/Data/Tree/Zipper.hs If you would like to do code review I will be happy to hear comments. After the API is settled down I will write test cases also. One thing that is worying me is that the current version uses State monad which is in the "mtl" package while the natural place for Data.Tree.Zipper is in "containers". This will create an extra dependency. Is this acceptable? Regards, Krasimir

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gtk2hs linking problems
by Nabil Alsharif 24 May '08

24 May '08

I downloaded the gtk2hs 0.9.12.1 tarball and did the standard configure, make, make install but when I go to compile a test program I get a bunch of errors that look like: compilation IS NOT required [..snip..] (.data+0x38): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziTypes_zdf548_closure' gui.o: In function `r2JC_srt': (.data+0x60): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziButtonsziButton_onClicked_closure' gui.o: In function `r2JC_srt': (.data+0x68): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziTypes_zdf662_closure' gui.o: In function `r2JE_srt': (.data+0x90): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziButtonsziButton_buttonLabel_closure' gui.o: In function `r2JE_srt': (.data+0x98): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziTypes_zdf662_closure' gui.o: In function `r2JG_srt': (.data+0xc0): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziAbstractziContainer_containerChild_closure' gui.o: In function `r2JG_srt': (.data+0xc8): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziTypes_zdf662_closure' gui.o: In function `r2JG_srt': (.data+0xd0): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziTypes_zdf548_closure' gui.o: In function `r2JG_closure': (.data+0x108): undefined reference to `glibzm0zi9zi12zi1_SystemziGlibziAttributes_set_closure' gui.o: In function `r2JG_closure': (.data+0x110): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziGeneralziGenera This was done on a Ubuntu system. It looks like it can't find a reference to a library but it doesn't say which one. If any one has ran into this problem before or knows what wrong help would be appreciated.

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Re: [Haskell-cafe] Announce: ghc-core, command line pager for reading GHC Core
by Henning Thielemann 23 May '08

23 May '08

On Fri, 23 May 2008, Ketil Malde wrote: > Henning Thielemann <lemming(a)henning-thielemann.de> writes: > >>> An even more advanced tool could show differences between two Core >>> listings [..] >> In this special example, actually simple 'diff' spotted the critical >> difference > > I would suggest Emacs's ediff for this, it lets you refine differing > chunks and highlights the actual differences. If you are an Emacs > user, if not, there are probably other nice diff tools out there.. :-) tkdiff, kompare

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Re: [Haskell-cafe] Newbie: State monad example questions
by Olivier Boudry 23 May '08

23 May '08

On Fri, May 23, 2008 at 2:20 PM, Thomas Hartman <tphyahoo(a)gmail.com> wrote: > > The big benefit I got from using the State Monad was that I was able to > reorder the functions > by just copy/pasting the function name from one > place to another. > > I don't understand... why do you need state to do this? Couldn't you > have a function pipeline using dots for composition like > > .... ( > ... > parseAttn . > parsePoBox . > ... > ) address ... > > and have the functions be equally switchable? (well, the first and > last can't quite be copy pasted, but close enough.) > > Introducing state seems like a lot of trouble to me if the only one is > easier reorderability of lines. > Agreed, in fact I started with a function pipeline and then switched to using the State Monad. As the program was written months ago I don't remember exactly why. Maybe I don't like to read backwards. ;-) Funtions running in the state monad can call other functions with the same `State s a` signature (and so on as deep as you want). You never have to care about passing parameters and restarting a new pipeline. But of course you can easily do without the State Monad. I don't think the State Monad allows to do thing that you can't do with basic Haskell. It just makes code more readable (in my opinion at least). Olivier.

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RE: [Haskell-cafe] force inlining in GHC (fwd)
by Henning Thielemann 23 May '08

23 May '08

maybe also of interest in the ghc-core thread ---------- Forwarded message ---------- Date: Fri, 23 May 2008 21:21:03 +0200 (CEST) From: Henning Thielemann <lemming(a)henning-thielemann.de> To: Simon Peyton-Jones <simonpj(a)microsoft.com> Cc: glasgow-haskell-users(a)haskell.org Subject: RE: [Haskell-cafe] force inlining in GHC On Wed, 30 Apr 2008, Henning Thielemann wrote: > On Tue, 29 Apr 2008, Simon Peyton-Jones wrote: > > > As luck would have it, I'm working on INLINE pragmas for Roman right at > this moment. > > > > Could you spare a moment to give me a concrete test case, to make sure I > > hit your case too? If you can give me a program that doesn't optimise as > > you expect, I'm much more likely to get it right. I examined some more examples and found out that the behaviours of GHC-6.4.1 and GHC-6.8.2 are quite consistent, that is, accelerations I achieved for GHC-6.8.2 also worked for GHC-6.4.1. Main problem in both GHC-6.4.1 and GHC-6.8.2 remains that sometimes GHC decides to SPECIALISE a function that is tagged with INLINE. I think that SPECIALISE should either copy the INLINE tag to the specialised function (confluent rewriting, right?) or should not SPECIALISE an INLINE function at all. I can now present a simple example, however it needs a (self-contained) package: http://darcs.haskell.org/storablevector/ module Main where import qualified Data.StorableVector.Lazy as SV zipData :: SV.Vector Double zipData = SV.take 500000 $ SV.zipWith (+) (SV.iterate SV.defaultChunkSize ((1-1e-6)*) 0.5) (SV.iterate SV.defaultChunkSize (1e-6 +) 0) main :: IO () main = do SV.writeFile "test-data" zipData ghc-core -o dist/build/fusiontest/fusiontest -O -Wall -fexcess-precision -package storablevector speedtest/SpecialiseTest.hs I get quite at the beginning of the Core: a1 :: Data.StorableVector.Lazy.ChunkSize -> (Double -> Data.Maybe.Maybe (Double, Double)) -> Double -> [Data.StorableVector.Base.Vector Double] This looks much like specialised SV.unfoldr, the expansion of SV.iterate. It seems to be that GHC detects that SV.unfoldr is called twice with the same type and thus specialises it to Double - while forgetting that SV.unfoldr should be inlined anyway.

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Re: throwDynIO ?
by Aleksey Nogin 23 May '08

23 May '08

Back in October'06 Carlos Pita sent a message to this list: > Currently I'm trying to grasp haskell exception handling and although > I find it mostly clear I'm a bit confused regarding the lack of a > throwDynIO action "paralelling" throwIO, as there are dynamic variants > for the other throw functions: > > throwDyn - throw > throwDynTo - throwTo > > I understand that the same effect could be conjured up by > > throwIO (DynException (...))) > > magic. But the lack of symmetry still disturbes me. Am I missing something? As far as I can tell, his question was left unanswered. As another Haskell newcomer (crossing over from the OCaml world), I found the absence of throwDynIO weird as well - as far as I understand things, throwing an error in the IO monad seems like a safer thing to do (as I'd know precisely when to expect it to be raised) than throwing it from a pure code and then relying on the "imprecise exceptions" mechanism to get it right. Looking at the ghc-6.8.2/libraries/base/Control/Exception.hs in the GHC source code, I see the following definitions: throw = unsafePerformIO . throwIO throwDyn exception = throw (DynException (toDyn exception)) so I guess I can get define throwDynIO myself by "taking back" the unsafePerformIO: throwDynIO exception = throwIO (DynException (toDyn exception)) but I am still wondering whether the library is simply incomplete here, or whether there is some reason for not defining/using throwDynIO. Thanks in advance for any clarifications! Aleksey

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Trying to write a very simple HTTP Client
by Eric 23 May '08

23 May '08

Hi all, I've written the following program to connect to a submit an HTTP GET request to a server and print the response: module Main where import Network import System.IO main = withSocketsDo go go = do putStrLn "Connecting..." out <- connectTo "haskell.org" (PortNumber 80) hPutStrLn out "GET /\r" hPutStrLn out "Host: haskell.org\r" hPutStrLn out "\r" cs <- hGetLine out hClose out print cs When I run the program, however, I get the following error: HTTPClient: <socket: 1872>: hGetLine: end of file Can anyone see what's wrong? E.

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Newbie: State monad example questions
by Dmitri O.Kondratiev 23 May '08

23 May '08

I am trying to understand State monad example15 at: http://www.haskell.org/all_about_monads/html/statemonad.html Example 15 uses getAny that I don't understand at all how it works: getAny :: (Random a) => State StdGen a getAny = do g <- get (x,g') <- return $ random g put g' return x Questions: 1) random has type: random :: (Random a, RandomGen g) => g -> (a, g) and for State monad: return a = State (\s -> (a, s)) then: return (random g) = State (\s -> ((a,g), s)) Is it correct? 2) What x and g' will match to in: do ... (x,g') <- return $ random g which, as I understand equals to: do ... (x,g') <- State (\s -> ((a,g), s)) What x and g' will match to in the last expression? 3) In general, in do expression (pseudo): do { x <- State (\s -> (a, s)); ...} What x will refer to? Will x stand for a whole lambda function: \s -> (a, s) ? 4) How 'g <- get' works in this function (getAny) ? 5) Why we need 'put g'? Thanks!

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What to do when GHC works, RUNGHC fails
by Peter Verswyvelen 23 May '08

23 May '08

I'm experimenting creating a Cabal script on Windows that installs a module that refers to some DLLs (freetype, png, ...) via C wrappers When configuring, building, and installing using cabal, a test program that uses this installed module runs fine when compiled with GHC --make, but fails miserably when run with RUNGHC (or GHCi) The first thing I noticed is that RUNGHC looks for a DLL called "png.dll" (which it doesn't find and then bails out with an error), while the executable build with GHC uses the correct "libpng3.dll". When I rename the libpng3.dll into png.dll, RUNGHC goes a bit further but then fails with an "unknown symbol". Obviously I did something wrong in my Cabal script, but I have no idea what it is. There seems to be a big difference when it comes to linking between of GHCi and GHC . Any guidelines on what to do in cases like the one above? I really like to use the interpreter because it provides much shorter turnaround times, and things like this stop me from using it. Thanks, Peter

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London Haskell Get Together
by Dominic Steinitz 23 May '08

23 May '08

For information, not an "official" user group get together but I believe the evening was enjoyed by all participants. Dominic. http://enhyper.blogspot.com/2008/05/haskell-get-together-22nd-may-2008.html

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