May 2007 - Haskell-Cafe

Mathematica
by Andrew Coppin 12 Jun '07

12 Jun '07

Hi folks. How difficult would it be to implement Mathematica in Haskell? I mean, I really like Mathematica, but it costs over £2,000. I can't really afford to pay that much for something that's only really a "toy". (It's not like a *need* it for anything, I just like playing with it.)

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More on the random idea
by Andrew Coppin 11 Jun '07

11 Jun '07

Since the online lambdabot still doesn't seem to want to talk to me, I've been thinking about how I might rectify the situation... Apparently GHC has a flag that makes it execute a Haskell expression directly. For example, C:\> ghc -e "map (2*) [1,2,3]" [2,4,6] Now, if I could just figure out how to make a web server call GHC... I'm laughing! Oh, but there is the *minor* detail that I am literally allowing unauthenticated users to perform arbitrary code execution. For example, C:\> ghc -e "writeFile \"Test.txt\" \"Hi mum!\"" (Generates a file on my harddrive "Test.txt" containing the text "Hi mum!".) AFAIK, Lambdabot dissalows any expression that performs IO. In Haskell, this is beautifully easy: reject any expression having an IO type. And it seems that GHC accepts not just an *expression*, but a *command*. In particular, this works: C:\> ghc -e ":t writeFile \"Test.txt\" \"Hi mum!\"" writeFile "Test.txt" "Hi mum!" :: IO () However, when you consider that the result type could be "IO ()" or "IO String" or "IO [Either (Maybe Int, (String, Bool)) [Either (Int -> String) (Complex Integer)]]", and the expression itself may well contain the "::" sequence... you see we have a nontrivial parsing task here! (Get the parsing wrong and somebody might be able to do Evil Things to the box.) The other possibility is to somehow run GHC under a user context that doesn't *have* write access to anything on the filesystem. That way there is no margin for error. This leaves only the problem of how to make a web server call GHC. I can think of a number of possibilities. - Write my own HTTP server from scratch. (Not keen...) - Configure Apache to do it. (Is that physically possible?) - Use Apache and some bizzare Perl scripting to do the actual call. (Assuming *that* is possible.) - Use Apache and some Perl scripts to write the data to a text file, and write a small Haskell program to poll the filesystem waiting for request files to appear, run then though GHC, and put the result back into a file. Write another Perl script to slurp up the result and send it back to the caller. - Doesn't Java have a free HTTP server implementation? Maybe I could use that... - ...others? (I'm not sure why this should be, but it seems that Don has made several replies to my emails that didn't show up in my inbox, and only show up on the list archives. Oh well, anyway...) I lurk on the POV-Ray NNTP server, and we recently had quite a discussion about Haskell. I'd *love* to be able to say to people "hey, you don't even need to bother working out how to install GHC, just CLICK THIS LINK and you can play with Haskell instantly!" But at the moment I'm not entirely sure how to set this up. Ideas?

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OSCON 2007, who's going?
by Christopher Milton 11 Jun '07

11 Jun '07

Are a lot of Haskellers going to be at OSCON, or just Simon Peyton Jones and myself? http://conferences.oreillynet.com/os2007/ I've never been to Portland, Oregon, before. Chris Milton AIM: cmiltonperl

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Memoization
by Mark Engelberg 08 Jun '07

08 Jun '07

I'd like to write a memoization utility. Ideally, it would look something like this: memoize :: (a->b) -> (a->b) memoize f gives you back a function that maintains a cache of previously computed values, so that subsequent calls with the same input will be faster. I've searched the web for memoization examples in Haskell, and all the examples use the trick of storing cached values in a lazy list. This only works for certain types of functions, and I'm looking for a more general solution. In other languages, one would maintain the cache in some sort of mutable map. Even better, in many languages you can "rebind" the name of the function to the memoized version, so recursive functions can be memoized without altering the body of the function. I don't see any elegant way to do this in Haskell, and I'm doubting its possible. Can someone prove me wrong? --Mark

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haskell wiki indexing
by Vincent Kraeutler 08 Jun '07

08 Jun '07

as was pointed out on the programming reddit [1], crawling of the haskell wiki is forbidden, since http://www.haskell.org/robots.txt contains User-agent: * Disallow: /haskellwiki/ and indeed, a google search gives the old wiki http://www.google.ch/search?q=haskell+wiki i.e. http://haskell.org/hawiki/FrontPage rather than http://haskell.org/haskellwiki/Haskell in other words, it seems like the most relevant search engines do not index the haskell wiki. i have reported this on #haskell about a week ago[2], and this setting was acknowledged to be a measure deliberately taken to curb excessive load from spiders crawling the wiki. i still believe this a highly harmful stance, and just to make sure that whoever may feel concerned is at least aware of the problem, i am now cross-posting to this list as well. kind regards, v. [1] http://programming.reddit.com/info/1qzjx/comments/c1r3ok [2] http://tuukka.iki.fi/tmp/haskell-2007-05-16.html

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Scope of type variables in associated types
by Matthew Sackman 08 Jun '07

08 Jun '07

The following doesn't seem to work. Is this a limitation of the current implementation or will it never work? Are there any work arounds without introducing extra type params into the data type E? > class G a b | a -> b where > data E a :: * > wrap :: b -> E a > unwrap :: E a -> b > instance G a b where > data E a = EC b -- this line - the b is not in scope. > wrap = EC > unwrap (EC b) = b I get "Not in scope: type variable `b'". I was rather hoping it would be in scope. I've tried: > instance forall a b . G a b where > ... but that doesn't seem to extend the scope as it does with functions. I realise this is bleeding edge stuff. But I can't seem to work out from the various wiki pages on this whether this is going to be eventually supported. This is all with today's GHC HEAD. Cheers, Matthew -- Matthew Sackman http://www.wellquite.org/

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faster factorial function via FFI?
by Dan Drake 07 Jun '07

07 Jun '07

Hello everyone, I have some code in which the bottleneck is the factorial function. I began by using a naive fac n = product [1..n] but it looks like there are faster ways to do it. I could try to look up those faster algorithms and implement them, but I'm guessing that using libgmp's factorial function is the best way to do it. I'm a poor programmer, so I don't trust myself to implement those algorithms properly. Hence I need to use FFI. ...but ghc already uses libgmp for Integers, so shouldn't I be able to somehow call libgmp's factorial function? Using regular FFI stuff, it looks like this might get complicated, since the function doesn't return one of the usual foreign types, so I'd need to mess around to get the result into an Integer, which is what I need. Is there an easy way to do this? It might also be faster to use a lookup table, since most of the time I'll be taking factorials of relatively small numbers. Thanks, Dan -- Ceci n'est pas une .signature.

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What puts False before True?
by PR Stanley 06 Jun '07

06 Jun '07

Hi What is the basic philosophy for Bool being a member of Ord? What justifies False < True? many Thanks in advance, Paul

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RE: [Haskell] boilerplate boilerplate
by Simon Peyton-Jones 04 Jun '07

04 Jun '07

[Redirecting to haskell-cafe; the main list is a good place to open discussions but not so good for continuing them] | > derive (Ord,Eq,Read,Show,Typeable) (BlogEntry Name Title Body Emai) | | according to the most recent status report, the syntax is not quite | settled yet, so what is in ghc head might still change. more info here: | | http://haskell.org/haskellwiki/GHC/StandAloneDeriving Indeed... but nothing much is happening at the moment because there's not been much feedback so it's drifted off the radar (so to speak). Refining the wiki and evolving a consensus would be great. Simon

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small hack on hasktags to make it collect class names
by Marc Weber 04 Jun '07

04 Jun '07

You can download the modified version from http://mawercer.de/marcweber/hasktags.hs Marc Weber

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