December 2008 - Haskell-Cafe

ANNOUNCE: pointless-haskell 0.0.1
by Hugo Pacheco 06 Dec '08

06 Dec '08

Dear Haskelleers, Alcino Cunha and me are pleased to announce the release of the Pointless Haskell library in Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pointless-haskell Pointless Haskell is library for point-free programming with recursion patterns defined as hylomorphisms, inspired in ideas from the PolyP library. The re-implementation of the library using type functions (in opposition to classes with functional dependencies) enables a type-level view of data types as the fixed points of functors and provides a better experience to the users in terms of code sanity. The library also features the visualization of the intermediate data structure of hylomorphisms with GHood. I am giving birth to a website with some examples: http://haskell.di.uminho.pt/wiki/Pointless+Haskell Any feedback is welcome. Cheers, hugo -- www.di.uminho.pt/~hpacheco

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Functional version of this OO snippet
by Andrew Wagner 05 Dec '08

05 Dec '08

Hi all, public interface IEngine { void foo(); void bar(string bah); ... } public class Program { public void Run(IEngine engine){ while(true){ string command = GetLine(); if (command.startsWith("foo")){ engine.foo(); } else if (command.startsWith("bar")){ engine.bar(command); ... else break; } In other words, I want to provide the same UI across multiple implementations of the engine that actually processes the commands. Thanks in advance.

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Fun with type functions
by Simon Peyton-Jones 05 Dec '08

05 Dec '08

Friends GHC has embodied data type families since 6.8, and now type synonym families (aka type functions) in 6.10. However, apart from our initial papers there isn't much published material about how to *use* type families. But that hasn't stopped you: quite a few people are using them already, and of course there is a rich seam of work on using functional dependencies to express type-level computation. Ken Shan and Oleg Kiselyov and I are collaborating to write a paper for an upcoming workshop, under the general rubric of "Fun with type functions" (in homage to Thomas Hallgren's paper "Fun with functional dependencies" and Ralf Hinze's paper "Fun with phantom types"). So this message is to ask you: can you tell us about the most persuasive, fun application you've encountered, for type families or functional dependencies? Simple is good. It doesn't have to be elaborate: just something that does something useful you could not have done otherwise. Pointers to email threads are fine. Don't assume we already know about them (even if we participated in the thread :-) Part of what we're interested in is that *you* found the example compelling. Many thanks Simon, Ken, Oleg PS: I'm broadcasting this message to GHC-users and Haskell-cafe, but to avoid deluging ghc-users, please reply just to us and Haskell cafe. (Interested ghc-users can follow the threads there from the archives if they want.)

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efficient combination of foldl' and foldr -> foldl'r
by Henning Thielemann 05 Dec '08

05 Dec '08

I want to do a foldl' and a foldr in parallel on a list. I assumed it would be no good idea to run foldl' and foldr separately, because then the input list must be stored completely between the calls of foldl' and foldr. I wanted to be clever and implemented a routine which does foldl' and foldr in one go. But surprisingly, at least in GHCi, my clever routine is less efficient than the naive one. Is foldl'rNaive better than I expect, or is foldl'r worse than I hope? module FoldLR where import Data.List (foldl', ) import Control.Arrow (first, second, (***), ) foldl'r, foldl'rNaive :: (b -> a -> b) -> b -> (c -> d -> d) -> d -> [(a,c)] -> (b,d) foldl'r f b0 g d0 = first ($b0) . foldr (\(a,c) ~(k,d) -> (\b -> k $! f b a, g c d)) (id,d0) foldl'rNaive f b g d xs = (foldl' f b *** foldr g d) $ unzip xs test, testNaive :: (Integer, Char) test = second last $ foldl'r (+) 0 (:) "" $ replicate 1000000 (1,'a') {- *FoldLR> test (1000000,'a') (2.65 secs, 237509960 bytes) -} testNaive = second last $ foldl'rNaive (+) 0 (:) "" $ replicate 1000000 (1,'a') {- *FoldLR> testNaive (1000000,'a') (0.50 secs, 141034352 bytes) -}

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Gluing pipes
by Matt Hellige 05 Dec '08

05 Dec '08

>From time to time, I've wanted to have a more pleasant way of writing point-free compositions of curried functions. One might want to transform both the first and second arguments, for instance, or only the third argument, of a curried function. I've never known a (non-cryptic) way to do this. For example, given: f :: a -> b -> c g :: a1 -> a h :: b2 -> b I'd like to be able to write something like: \ x y -> f (g x) (h y) in a way that is both point-free and applicative. In other words, I'd like to apply a function to "pipes" or transformers rather than to values. Recent posts by Conal Elliott [1,2] got me thinking about this again, and I've found a simple solution. I use two of Conal's combinators: argument = flip (.) result = (.) And now we define: infixr 2 ~> f ~> g = argument f . result g infixl 1 $. ($.) = flip ($) Which lets us write: -- transform both arguments f $. g ~> h ~> id -- transform just the second argument f $. id ~> h ~> id The name ($.) is chosen to indicate that this is (roughly) a composition disguised as an application. The transformer spec to the right of ($.) looks like the type of the function to the left, and consists of a transformer for each argument and one for the result type. Of course, (~>) is right associative, and id can match the entire tail "in bulk", so we can also write something like: f $. g ~> id And of course, each transformer can be a pipeline, so assuming proper types, we can do things like: f $. id ~> (length.snd.unWrap) ~> wrap More details here: http://matt.immute.net/content/pointless-fun Questions: 1. Is there already another well-known way to do this? It seems a common enough problem... 2. Do these particular combinators already exist somewhere with other names? 3. Are there better names for these functions? Thanks! Matt [1] http://conal.net/blog/posts/semantic-editor-combinators/ [2] http://conal.net/blog/posts/prettier-functions-for-wrapping-and-wrapping/

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sending email via gmail Re: [Haskell-cafe] Sending email from a Haskell program
by Thomas Hartman 05 Dec '08

05 Dec '08

I ran into this problem when I wanted to send email from a HAppS web app, routing it through gmail. There actually used to be an SMTP client in HAppS but it was dropped a while ago. The most common workaround appears to be, just use sendmail. But I couldn't figure out a way to get sendmail to interface with gmail. (I think there is a way, there are blogs on it, I just wasn't patient enough to follow the directions.) What I finally did is, I have successfully followed the instructions in http://www.ericstockwell.com/?p=3 to send myself email from the command line. So I will probably use HSH to run shell commands in my haskell web apps, to send email. This process only took a few minutes, and seemed easier than the postfix/sendmail solutions I came across. Of course, this only works in unixy environments. thomas. 2007/9/27 brad clawsie <clawsie(a)fastmail.fm>: >> jmuk's HaskellNet project from last year? >> >> http://darcs.haskell.org/SoC/haskellnet/HaskellNet/IMAP.hs > > sweet! was there any documentation created for this? examples? > anything? have people tried to make this work with ssl/tls libs? > > by the way there looks like some other gems in the haskellnet > dir. what exactly was haskellnet - a project to code to the major > network protocols? are these libs stable? how does the HTTP lib stack > up against Network.HTTP? any info on this project would be > appreciated. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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deleting spam on the wiki
by Duncan Coutts 05 Dec '08

05 Dec '08

Who is able to delete wiki spam? http://haskell.org/haskellwiki/?title=Special:Contributions&target=Tomso123 All the pages created by this user appear to be spam (check the google translation) so the account should probably be deleted too. As I understand it, any registered user can revert changes to a page: http://www.haskell.org/haskellwiki/Help:Editing However if a registered user creates new spam pages then other ordinary registered users cannot revert that change. Is there some way of reporting spam pages to the appropriate people that I missed? Duncan

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instance Applicative f => Applicative (StateT s f)
by Martijn van Steenbergen 05 Dec '08

05 Dec '08

Hello all, How do I implement the following? instance Applicative f => Applicative (StateT s f) The implementation of pure is trivial, but I can't figure out an implementation for <*>. Is it possible at all, or do I need to require f to be a monad? Thanks in advance, Martijn.

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Quickcheck in industry - own experience.
by Gianfranco Alongi 05 Dec '08

05 Dec '08

http://writert.blogspot.com/2008/12/quickchecking-code-i-c.html /Gf

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propogation of Error
by brad clawsie 05 Dec '08

05 Dec '08

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 hi. i have a partial library for parsing ogg files here: http://hpaste.org/12705 i have a question about an aspect of the code. in the function checkHeader there are a sequence of functions to check various elements in the header of a ogg file. if i test this function against a file that *isn't* an ogg file, i.e. badFile = "/home/user/.bashrc" :: String main = parseOgg badFile >>= (\x -> print x) i would expect to get back the Error from the *first* function in the sequence of functions in checkHeader (oggHeaderError from the oggHeader function). but instead i always see the Error from the *last* function in the sequence, OggPacketFlagError from the OggPacketFlag function. why is this? is there any way i can get the desired behavior...i.e. see the Error from the first function in the sequence that fails? thanks brad -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEARECAAYFAkk4tQYACgkQxRg3RkRK91PD9gCePloWFIvE9WjcfApxR2RmnHQ0 pUgAn0WAzRQR/y2yE8yeYP1s7eKHyKDh =EcOM -----END PGP SIGNATURE-----

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