November 2008 - Haskell-Cafe

Re: [Haskell-cafe] code generation
by Martin Hofmann 04 Nov '08

04 Nov '08

Sorry for referring to a post, a bit ago. > http://www-users.cs.york.ac.uk/~ndm/derive/ (Deriving Generic Functions > by Example). > Thanks for the pointer, it was already on my to-read-pile :-) > I think using Template Haskell for your work would fit very nicely, so > is a good choice to learn :-) > I already got used to TH a bit, but I am not sure if it is appropriate for my purpose, or at least not completely. I want to load Haskell code into my program at runtime in an abstract representation (as TH), modify it and then type-check it or coerce it into a value (or execute it). For me it looks like I need a combination of Hint and TH. However, Hint can only interpret strings and to get a string from a QExp I have to enter the IO Monad using runQ. So, wouldn't it deteriorate my performance to do it all in the IO? Is there another way? Thanks, Martin

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Network.FastCGI does not emit stderr outputs to lighttpd's error.log?
by Agent Zhang 04 Nov '08

04 Nov '08

Hi, I'm trying Network.FastCGI + lighttpd 1.4.9 to develop a RESTful service platform. I've found that stderr (especially those from Debug.Trace) get lost in the error.log file. But plain CGI mode with Network.CGI works perfectly (all stderr outputs appear in error.log). Besides, my Perl fastcgi script also logs stderr with the same lighttpd. So it does seem that my lighttpd's mod_fastcgi module is OK. Is this a known issue in the Haskell fastcgi library? Is there a workaround or a fix? Thanks in advance :) Cheers, -agentzh

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a minimal Windows program in Haskell?
by Nun Aurbiz 04 Nov '08

04 Nov '08

How do I make a minimal Windows application in Haskell? I know there are Win API bindings in the libraries, and of course the ffi lets you call into and out of C, but I have no sense of how to bring it all together. I can't find any samples either. Is the Haskell code the "main" application and you just ffi into the Win API? Or does it ffi into a win_main.c that I write? Or is the win_main.c the "main" application and it calls into Haskell? Here is what I am talking about by a "minimal Windows application" (written in psuedo C code). #define WIN32_LEAN_AND_MEAN #include <windows.h> LRESULT CALLBACK windowsProcedure(..) { switch(message) { case WM_CLOSE: PostQuitMessage(0); return 0; } return DefWindowProc(..); } int APIENTRY WinMain(..){ WNDCLASS window_class; RegisterClass(&window_class) window = CreateWindow(..); ShowWindow(..); while(true) { /* do whatever */ while(PeekMessage(..)) if(msg.message == WM_QUIT) return msg.wParam; else DispatchMessage(&msg); } return 0; }

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ANNOUNCE: Graphalyze-0.5 and SourceGraph-0.3
by Ivan Lazar Miljenovic 04 Nov '08

04 Nov '08

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I've just submitted my thesis this morning (w00t!!!) and as such I'm announcing the latest versions of Graphalyze [1,2] and SourceGraph [3,4], which fix a couple of bugs in the previous versions (these bugs were fixed whilst waiting for my supervisor to finish reading through my draft, and as such I had to re-write bits of my thesis that talked about the limitations of the software :s ). [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Graphalyze [2] http://code.haskell.org/Graphalyze [3] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SourceGraph [4] http://code.haskell.org/SourceGraph Briefly, SourceGraph is a utility that uses the Graphalyze library to analyse Cabalized Haskell software. Its input/output is rather restrictive at the moment: it takes a single argument, which is the path to a .cabal file, and then produces an HTML report in: <project directory>/SourceGraph/<project name>.html This report contains visualisation of the code for each module, for the imports (similar to what graphmod [5] does) as well as the entire code base. Furthermore, the entire code base is also visualised with functions grouped by the modules they're defined in, as well some simple analyses (such as comparing the export list to what functions are actually roots, similar to the output for top-level functions returned by GHC when using -fwarn-unused-binds). Main changes in Graphalyze since version 0.4: * When writing my thesis, I found some obvious silly mistakes (such as copying the rootsOf function definition to leavesOf, but not changing the function definition). * Images in the document representation are now inline elements. * When using Pandoc [6] for document generation, the size of generated graphs is now customisable on a per-format basis; furthermore, for web pages graph visualisations link to larger versions. Also, graph visualisations can now be created in a sub-directory. Main changes in SourceGraph since version 0.2: * Now uses Cabal 1.6 * Improve visual layout of generated HTML report (still no CSS though). * "Smarter" analysis: don't show trivial analysis results, and when analysing the entire code base don't return cliques, etc. that contain functions all in the same module (as they would have been reported in the per-module section). Note, however, that the parsing limitations for version 0.2 still apply: cpp is still as-yet unsupported (as I'm not sure what I'll do about choosing cpp flags); Template Haskell, HaRP and HXML are still ignored and functions in class declarations/records are still ignored (the former because it's ambiguous which actual function you want, the latter because they're boring :p ). Also, SourceGraph is _still_ not a refactoring tool ;-) If anyone's interested, I'll soon be uploading the final version of my thesis to my blog. Also, if anyone is interested in hacking on any of this, feel free! I'll probably leave it for a while (I'm going to be doing Java coding over summer, blech :s ), but I intend to come back to it later on. - -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.9 (GNU/Linux) iEYEARECAAYFAkkOcaAACgkQfEfFJ9JhvyjhIwCeM9VXsJeSVK2CdWURJDer6zoA A5YAoIfayHtjpw0qt/gyPZhhhypOwzSh =x+/X -----END PGP SIGNATURE-----

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Typeable and Dynamic
by minh thu 04 Nov '08

04 Nov '08

Hi, Given a TypeRep and a Dynamic value with type corresponding to the TypeRep, I'd like to be able to use fromDyn *without* specifying a type (given as an additional 'default' argument) (since it is somewhat known from the TypeRep). That is, I'd like an undefinedOf that can be used in extract :: Typeable a => TypeRep -> Dynamic -> a extract typerep dyn = fromDyn (undefinedOf typerep) dyn For instance, if TypeRep is Int, extract Int dyn = fromDyn (undefined::Int) dyn Is it possible to write "extract" ? Thanks, Thu

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Typeable and Dynamic
by oleg＠okmij.org 04 Nov '08

04 Nov '08

minh thu asked a tricky question, about writing > extract :: Typeable a => TypeRep -> Dynamic -> a The question here is what determines the type 'a'. One answer is that 'a' is determined from the context, e.g., (extract tr dyn) + 1.0 fixes 'a' to be an Int. In that case, extract is equivalent to `read' (indeed, String is a primitive sort of Dynamic). We do not need any argument TypeRep though as we obtain 'a' from the context. fromDynamic is sufficient there. One can also construe the question to be asking about writing an `inverse' of typeOf. Whereas, typeOf :: Typeable a => a -> TypeRep can give the TypeRep for any (Typeable) type, undefinedOf :: Typeable a => TypeRep -> a could give (an undefined) value for a given TypeRep. Now, the _type_ 'a' is determined by the _value_ of TypeRep: seemingly we need dependent types. To write function of exactly that signature, we need Template Haskell. Oftentimes, we can get away with a weaker function: > data Dyn = forall a. Typeable a => Dyn a > reflect :: TypeRep -> Dyn to be used to implement dynApply, dynFst, dynSnd, dynHead and other operations on Dynamics. Perhaps the following message may help then http://www.haskell.org/pipermail/haskell-cafe/2007-December/036820.html

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Problem while compiling ghc 6.8
by Magicloud 04 Nov '08

04 Nov '08

Hi, I got my ghc 6.8 by `darcs get --partial http://darcs.haskell.org/ghc-6.8/ghc`, and after `darcs-all get`, `sh boot`, `configure`, without any problem, I started to make it. Well, when compiling libraries/stm, it reports "Control/Concurrent/STM/TVar.hs:22:8: Not in scope: `readTVarIO'. I tried to reget the source without '--partial', still the same.

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huge single file syncronization
by Alberto G. Corona 03 Nov '08

03 Nov '08

I need to backup my ubuntu-VMWare image frequently (5 GBits) . I need to know if exist such a utility (in haskell or not) for single file syncronization. I don´t want to reinvent te weel, but I think that it is a few lines of Haskell using the Diff package or something similar.

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Array bug?
by Andrew Coppin 03 Nov '08

03 Nov '08

Consider the following GHCi session: GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :m Data.Array.IO Prelude Data.Array.IO> t <- newArray ((0,0),(5,4)) 0 :: IO (IOUArray (Int,Int) Int) Loading package array-0.1.0.0 ... linking ... done. Prelude Data.Array.IO> getBounds t ((0,0),(5,4)) Prelude Data.Array.IO> writeArray t (10,10) 5 *** Exception: Error in array index Prelude Data.Array.IO> writeArray t (7,3) 5 *** Exception: Error in array index Prelude Data.Array.IO> writeArray t (3,7) 5 Prelude Data.Array.IO> So the array has 0 <= x <= 5 and 0 <= y <= 4, and writing to (10,10) or (7,3) throws an exception. However, writing to (3,7) triggers no exception - despite being clearly out of range. Judging by the behaviour of the large, complex program I was debugging when I stumbled upon this, the coordinates "wrap round" to the next column. (!!) Obviously, writing to non-existent coordinates and not getting an exception is a Very Bad Thing. I was counting on writeArray (and readArray) to detect out-of-range coordinates so I could fix my code. But as you can see, it doesn't actually work as advertised. You have *no idea* how long I spent trying to track this bug down! >_< Is this a known bug? Is it likely to be fixed any time soon? (I'm guessing the bug is as simple is converting indicies to integers and then checking the integers are in-range, rather than the underlying index type.)

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Re: [Haskell] ANNOUNCE: RefSerialize-0.2.1
by Alberto G. Corona 03 Nov '08

03 Nov '08

I uploadad RefSerialize<http://hackage.haskell.org/cgi-bin/hackage-scripts/package/RefSerialize> to Hackage . Read, Show and Data.Binary do not check for repeated references to the same data address. As a result, the data is serialized multiple times when serialized. This is a waste of space in the filesystem and also a waste of serialization time. but the worst consequence is that, when the serialized data is read, it allocates multiple copies in memory for the same object referenced multiple times. Because multiple referenced data is very typical in a pure language such is Haskell, this means that the resulting data loose the beatiful economy of space and processing time that referential transparency permits. This package allows the serialization and deserialization of large data structures without duplication of data, with the result of optimized performance and memory usage. It is also useful for debugging purposes. There are automatic derived instances for instances of Read/Show, lists and strings. the deserializer contains a almos complete set of Parsec.Token parsers for deserialization. Every instance of Show/Read is also a instance of Data.RefSerialize The serialized string has the form "expr( var1, ...varn) where var1=value1,..valn=valueN " so that the string can ve EVALuated. See demo.hs and tutorial. I presumably will add a entry in haskell-web.blogspot.com To develop: -derived instances for Data.Binary -serialization to/from ByteStings I wrote this module because I needed to serialize lists of verisions of the same data with slight modifications between each version. This is a short tutorial (in tutorial.txt) runW applies showp, the serialization parser of the instance Int for the RefSerialize class Data.RefSerialize>let x= 5 :: Int Data.RefSerialize>runW $ showp x "5" every instance of Read and Show is an instance of RefSerialize. rshowp is derived from showp, it labels the serialized data with a variable name Data.RefSerialize>runW $ rshowp x " v8 where {v8= 5; }" Data.RefSerialize>runW $ rshowp [2::Int,3::Int] " v6 where {v6= [ v9, v10]; v9= 2; v10= 3; }" while showp does a normal show serialization Data.RefSerialize>runW $ showp [x,x] "[5, 5]" rshowp variables are serialized memory references: no piece of data that point to the same addrees is serialized but one time Data.RefSerialize>runW $ rshowp [x,x] " v9 where {v6= 5; v9= [ v6, v6]; }" This happens recursively Data.RefSerialize>let xs= [x,x] in str = runW $ rshowp [xs,xs] Data.RefSerialize>str " v8 where {v8= [ v10, v10]; v9= 5; v10= [ v9, v9]; }" the rshowp serialized data is read with rreadp. The showp serialized data is read by readp Data.RefSerialize>let xss= runR rreadp str :: [[Int]] Data.RefSerialize>print xss [[5,5],[5,5]] this is the deserialized data the deserialized data keep the references!! pointers are restored! That is the whole point! Data.RefSerialize>varName xss !! 0 == varName xss !! 1 True rShow= runW rshowp rRead= runR rreadp Data.RefSerialize>rShow x " v11 where {v11= 5; }" In the definition of a referencing parser non referencing parsers can be used and viceversa. Use a referencing parser when the piece of data is being referenced many times inside the serialized data. by default the referencing parser is constructed by: rshowp= insertVar showp rreadp= readVar readp but this can be redefined. See for example the instance of [] in RefSerialize.hs This is an example of a showp parser for a simple data structure. data S= S Int Int deriving ( Show, Eq) instance Serialize S where showp (S x y)= do xs <- rshowp x -- rshowp parsers can be inside showp parser ys <- rshowp y return $ "S "++xs++" "++ys readp = do symbol "S" -- I included a (almost) complete Parsec for deserialization x <- rreadp y <- rreadp return $ S x y there is a mix between referencing and no referencing parser here: Data.RefSerialize>putStrLn $ runW $ showp $ S x x S v23 v23 where {v23= 5; } (I corrected some errors in this file here)

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