September 2007 - Haskell-Cafe

OzHaskell?!?
by Manuel M T Chakravarty 17 Oct '07

17 Oct '07

There is AngloHaskell and now AmeroHaskell. Doesn't that call for OzHaskell? http://haskell.org/haskellwiki/OzHaskell Manuel

2 2

unsafePerformIO: are we safe?
by Jorge Marques Pelizzoni 09 Oct '07

09 Oct '07

Hi, all! This is a newbie question: I sort of understand what unsafePerformIO does but I don't quite get its consequences. In short: how safe can one be in face of it? I mean, conceptually, it allows any Haskell function to have side effects just as in any imperative language, doesn't it? Doesn't it blow up referential transparency for good? Is there anything intrinsic to it that still keeps Haskell "sound" no matter what unsafePerformIO users do (unlikely) or else what are the guidelines we should follow when using it? Thanks in advance. Cheers, Jorge.

11 15

Extract source code from literate Haskell (LHS) files
by Peter Verswyvelen 06 Oct '07

06 Oct '07

This is of course very easy to do manually, but does a command line tool exist for extracting source code from literate Haskell files? Thanks, Peter

7 6

PROPOSAL: New efficient Unicode string library.
by Johan Tibell 03 Oct '07

03 Oct '07

Dear haskell-cafe, I would like to propose a new, ByteString like, Unicode string library which can be used where both efficiency (currently offered by ByteString) and i18n support (currently offered by vanilla Strings) are needed. I wrote a skeleton draft today but I'm a bit tired so I didn't get all the details. Nevertheless I think it fleshed out enough for some initial feedback. If I can get the important parts nailed down before Hackathon I could hack on it there. Apologies for not getting everything we discussed on #haskell down in the first draft. It'll get in there eventually. Bring out your Unicode kung-fu! http://haskell.org/haskellwiki/UnicodeByteString Cheers, Johan Tibell

27 60

Compose
by PR Stanley 01 Oct '07

01 Oct '07

Hi sumSquareEven = (sum.map (^2)).filter even I wrote this thinking i was being very clever. The question is asking to spot the error in f = compose [sum, map (^2), filter even] I've only seen [] used in list comprehension and initialisation. I conducted a mini search on compose earlier but to be honest it just made matters even less clear. So the question is, does compose as a function or keyword exist in Haskell and if so how can the above code frag be corrected? O and, is compose in any way related to curry and uncurry? Thanks v mucho, Paul

3 2

Yampa question
by Ryan Ingram 30 Sep '07

30 Sep '07

I'm deriving an FRP implementation based on the ideas presented in the Yampa Arcade paper (http://www.haskell.org/yale/papers/haskell-workshop03/) to improve my understanding of arrows, but I have a couple of questions. I'm using the following declaration for SF: > type DTime = Double > newtype SF a b = SF { runSF :: DTime -> a -> (b, SF a b) } which I make an arrow in the obvious way: > instance Arrow SF where > arr f = SF sf where > sf dt b = (f b, SF sf) > bc >>> cd = SF sf where > sf dt b = (d, bc' >>> cd') where > (c, bc') = runSF bc dt b > (d, cd') = runSF cd dt c > first bc = SF sf where > sf dt ~(b,d) = ((c,d), sfFirst bc') where > (c, bc') = runSF bc dt b One question I had was about the implementation of "first". Is it important that the pair match be lazy? Or is it safe to make it strict? What are the advantages and disadvantages of each choice? My other question had to do with ArrowChoice. > instance ArrowChoice SF where > left bc = SF sf where > sf dt (Right d) = (Right d, left bc) > sf dt (Left b) = (Left c, left bc') where > (c, bc') = runSF bc dt b What happens to the dt in the "Right d" case? It seems to be invisible to the inner signal function. Is this the right thing to do? I have an alternate implementation which does the following: > sf dt (Right d) = (Right d, left $ addtime dt bc) > addtime :: DTime -> SF a b -> SF a b > addtime add sf = SF sf' where > sf' dt a = runSF sf (dt + add) a but there's a pretty clear space leak here with repeated "Right" calls, and I'm not sure if the inner signal should see the lost time or not. Perhaps signal functions shouldn't be an instance of ArrowChoice at all? Any insights would be appreciated. -- ryan

2 1

Re: [Haskell-cafe] Rewriting filter with foldr
by PR Stanley 30 Sep '07

30 Sep '07

>Well, note that foldr takes a function of x, which produces a >function of xs. This function of xs either conses x onto it, or >leaves it unchanged. We can write this down explicitly by removing >the xs parameter and just writing what function should be produced: > >filter f = foldr (\x -> if (f x) then (x:) else id) [] That's one neat solution but it also raises a few questions for me: foldr :: (a -> b -> b) -> b -> [a] -> b yet you've managed to squeeze in a function that takes only one argument. How is this possible without GHCI blowing its top? 2. Could you please explain the role of the identity function (id) in your code? Where's its argument, or is it sort of implicitly noted? For example, is it the case that the second argument is held in reserve and passed to the first function which would take it, hence (x:) and id? Many thanks, Paul

3 2

Re: [Haskell-cafe] Rewriting filter with foldr
by PR Stanley 30 Sep '07

30 Sep '07

The question is asking for a new definition of filter using foldr. Sorry, I should have mentioned that before. Cheers, Paul At 14:26 30/09/2007, you wrote: >Perhaps a list comprehension better shows the intention of the >filter function: > >filter p xs = [x | x <- xs, p x] > >You can literally read that as "take all x from xs that satisfy p".

1 0

Rewriting filter with foldr
by PR Stanley 30 Sep '07

30 Sep '07

Hi filter :: (a -> Bool) -> [a] -> [a] filter f = foldr (\x -> \xs -> if (f x) then (x:xs) else xs) [] Somehow I feel this could be done more elegantly. What does the list think? Thanks, Paul

3 2

Re: [Haskell-cafe] Rewriting ord (revisited)
by PR Stanley 30 Sep '07

30 Sep '07

The second version looks very neat, certainly, though I am not entirely convinced that it's any more efficient. Still, I may be missing something. The compiler must be one hell of a machine. I wonder if the source code is available to the public. Cheers, Paul At 12:26 30/09/2007, you wrote: >-----BEGIN PGP SIGNED MESSAGE----- >Hash: RIPEMD160 > >I would think that the compiler is smart enough to fuse the two loops. >That doesn't explain the performance difference though. > >Adrian > > >PR Stanley schrieb: > > Hi > > This was my original version plus some modifications as advised by the > > list: > > f c = sum [1 | x <- ['\0'..], x < c] > > > > The following version was sent by a list member: > > f c = length $ takeWhile (<c) ['\0'..] > > > > Now, the sender asserted that the first version was much too slow. I'm > > wondering how the second version is any more efficient than the first. > > Looking at them through my C programmers' eye, as it were, both would > > require at least two loops before returning the ANSI value. > > Any ideas? > > Thanks, Paul > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >-----BEGIN PGP SIGNATURE----- >Version: GnuPG v1.4.7 (MingW32) >Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org > >iD8DBQFG/4f+11V8mqIQMRsRAxegAJ4t5grdRDrWVWby6tjDZDdxgJ72FQCfcD7W >Mntl7FV2GpCGtpY2yP61kbk= >=l+3X >-----END PGP SIGNATURE-----

2 1