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WolframAlpha
by Vasili I. Galchin 07 May '09

07 May '09

Hello, http://blog.wolframalpha.com/2009/05/01/the-secret-behind-the-computational… <<< this is obviously a Wolfram Inc. blog so maybe not totally objective ... but here is a snippet that speaks in Haskell's favor: As a result, the five million lines of *Mathematica* code that make up Wolfram|Alpha are equivalent to many tens of millions of lines of code in a lower-level language like C, Java, or Python. * I am some what familiar with Mathematica and it's multi-paradigm nature (like F#, OCaml, etc.). In any case, I would like the Haskell community to view WolframAlpha as a challenge. For what is it worth I presently cabalizing Swish .... Based on my reading of WolframAlpha it is a semantic web ... i.e. formal knowledge representation! all Google results => http://www.google.com/search?q=WolframAlpha&ie=utf-8&oe=utf-8&aq=t&rls=com.… Kind regards, Vasili *

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cabal parse problems
by Vasili I. Galchin 06 May '09

06 May '09

Hello, I am trying to cabalize a package (swish .. semantic web) but am running into parse error: vigalchin@ubuntu:~/FTP/Haskell/Swish-0.2.1$ runhaskell Setup.hs configure Setup.hs: swish.cabal:24: Parse of field 'exposed-modules' failed. vigalchin@ubuntu:~/FTP/Haskell/Swish-0.2.1$ Below: is a fragment from my .cabal file. Line #24 starts with "Exposed-modules:" ..... ..... ..... Data-Files: README Exposed-modules: Swish.HaskellRDF.BuiltInDatatypes, Swish.HaskellRDF.BuiltInMap, Swish.HaskellRDF.BuiltInMapTest, Swish.HaskellRDF.BuiltInRules, Swish.HaskellRDF.ClassRestrictionRule, Swish.HaskellRDF.ClassRestrictionRuleTest, Swish.HaskellRDF.Datatype, Swish.HaskellRDF.GraphClass, Swish.HaskellRDF.GraphMatch, Any ideas why I am getting parse error? Pretty vague error message! Kind regards, Vasili

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Vector-like data structure
by Krzysztof Skrzętnicki 06 May '09

06 May '09

Hi I'm looking for a data structure with following characteristics: 1. O(1) lookup 2. O(1) modification 3. amortized O(1) append 4. O(1) size query This roughly characterizes C++ vector<> class. I'm ready to implement it myself, but first I would like to ask if anyone knows package with similar data structure. If there are many, which one would you choose and why? Best regards Christopher Skrzętnicki

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instance Monad (Except err)
by Martijn van Steenbergen 06 May '09

06 May '09

Hello, Mr. McBride and mr. Paterson define in their Applicative paper: > data Except e a = OK a | Failed e > instance Monoid e => Applicative (Except e) where ... Sometimes I'd still like to use >>= on Excepts but this "feels" wrong somehow, because it doesn't use monoids nicely like the Applicative instance does. Are there any good reasons such a Monad instance shouldn't be defined? Does it violate any laws, for example? Thanks, Martijn.

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Current status of Annotations
by Tom Lokhorst 06 May '09

06 May '09

Hello, I've been reading about Annotations [1] as implemented during last years Summer of Code, and about some earlier ideas [3] [2]. It seems though that these annotations have never been merged into HEAD [4]. Could anyone elaborate on the current status of this proposal? Has there been any further work or discussion about this subject that I haven't been able to dig up? - Tom Lokhorst [1] http://hackage.haskell.org/trac/ghc/wiki/Annotations [2] http://www.haskell.org/pipermail/haskell-prime/2006-November/001893.html [3] http://hackage.haskell.org/trac/haskell-prime/ticket/88 [4] http://www.haskell.org/pipermail/haskell-cafe/2008-September/047293.html

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Unfold fusion
by Adrian Neumann 06 May '09

06 May '09

Hello, I'm trying to prove the unfold fusion law, as given in the chapter "Origami Programming" in "The Fun of Programming". unfold is defined like this: unfold p f g b = if p b then [] else (f b):unfold p f g (g b) And the law states: unfold p f g . h = unfold p' f' g' with p' = p.h f' = f.h h.g' = g.h Foremost I don't really see why one would want to fuse h into the unfold. h is executed once, at the beginning and is never needed again. Can someone give me an example? So, this is what I got so far: unfold p f g.h = (\b -> if p b then [] else (f b): unfold p f g (g b).h = if p (h b) then [] else (f (h b)) : unfold p f g (g (h b)) = if p' b then [] else f' b: unfold p f g (h (g' b)) not very much. I kinda see why it works after I "unfold" some more, but I can't really prove it. I suspect I need some technique I haven't learned yet. I've heard about fixpoint induction, that looks promising, but Google knows very little about it. I hope somebody can give me some hints. Regards, Adrian

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setResourceLimit
by brian＠lorf.org 06 May '09

06 May '09

I have a long-lived multithreaded server process that needs to execute programs and interact with them via Handles. The programs could misbehave, like loop or hang, so I need to limit the real and CPU time they can take. I guess System.Posix.Resource.setResourceLimit sets limits on the current process, so I can't use it in the server process. Is it a good approach to write some utility program for the server to call that does setResourceLimit and executes the programs that could misbehave?

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Reply to
by Ashok Gautham 06 May '09

06 May '09

I feel that the haskell mailing lists must have the reply-to field set, so that a person can reply to the list by just clicking reply. I am unsure if I am the only one facing this problem. --- Ashok `ScriptDevil` Gautham

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Haskell Arrows Applications
by Hannousse 06 May '09

06 May '09

Hello, I'm interested to the concept of arrows in Haskell, however, I couldn't find a real application or example using this new technology in a real world application. All what I found are just academic examples and other people developing new specific libraries using arrows.Could someone, please, give me a reference to one of the real applications that uses arrows. Thank you for all Hannousse. -- View this message in context: http://www.nabble.com/Haskell-Arrows-Applications-tp23388417p23388417.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ST.Lazy vs ST.Strict
by Tobias Olausson 06 May '09

06 May '09

Hello! I have a program that is using ST.Strict, which works fine. However, the program needs to be extended, and to do that, lazy evaluation is needed. As a result of that, I have switched to ST.Lazy to be able to do stuff like foo y = do x <- something xs <- foo (y+1) return (x:xs) However, when running the program compiled with ST.Lazy, the following is outputted: [tobsi@wobsi]$ ./runnerLazy looper.hex runnerLazy: <<loop>> The very same program compiled with ST.Strict outputs: [tobsi@wobsi]$ ./runner looper.hex 83298556 The code that is actually computing stuff is this: loopSys :: Int -> CPU s Int loopSys cc = do instr <- fetch if instr == 0xEA --NOP then return cc else do c <- execute instr loopSys $! (cc+c) The CPU type looks as follows: type CPU s a = ReaderT (SysEnv s) (ST s) a The program is run like runReaderT (loopSys 0) which in turn is being runST'd and then printed Does anyone know why the program just outputs <<loop>> when compiled under ghc 6.10.2, and runs perfectly fine under ghc 6.8.2? The program is compiled with --make and -O2 Tobias Olausson tobsan(a)gmail.com

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