April 2009 - Haskell-Cafe

ANN: curl-1.3.5
by Sigbjorn Finne 20 Apr '09

20 Apr '09

Hi, a refresh release of the Haskell (lib)curl package has been uploaded to Hackage, http://hackage.haskell.org/cgi-bin/hackage-scripts/package/curl It enables 6.10.2 use, taking into account the updated story on how to register Haskell-based finalizers. Thanks to Carl Howells for reporting this problem. Bug reports / feature requests etc. most welcome, btw. enjoy --sigbjorn

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Petr Ročkai's darcs project accepted for Haskell GSoC
by Eric Kow 20 Apr '09

20 Apr '09

Hi everybody, I'm pleased to announce that Petr Ročkai's project on optimising darcs has been accepted for Google Summer of Code. More details can be found here: http://socghop.appspot.com/org/home/google/gsoc2009/haskell Petr's project fits into an ongoing effort by the darcs community to improve our performance so that one day we will able to recommend darcs for medium sized and larger projects. Until early 2008, we have been focused on darcs 2: which provides the darcs 2 theory (to solve most of the exponential merge problems), and also the "hashed" format repositories (including darcs 2). The project for this summer will allow us to complete the work on hashed repositories. In their current state, hashed format repositories make darcs a lot more robust and also allow us to fetch repositories faster (with darcs get --lazy and with a global patch cache). But in their current form, hashed repositories can can also slow down day-to-day operations like darcs whatsnew and darcs record. Petr has identified the cause of these delays has provided a work plan which we think will get hashed storage right. If his project is successful, future versions of darcs should make operations like darcs record and whatsnew much more comfortable in practice. Congratulations and good luck to Petr! And many thanks to the darcs and Haskell communities for their support. -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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RE: [Announce] primes
by Niemeijer, R.A. 20 Apr '09

20 Apr '09

Sebastian Fischer wrote: > I am pleased to announce the package 'primes' that implements lazy > wheel sieves for efficient, purely functional generation of prime > numbers in Haskell. > > The implementation is reasonably efficient. The query > > > primes !! 1000000 > 15485867 > > answers after a few seconds. > > Feel free to contribute more functionality to this package. The > sources are on Github: > > http://github.com/sebfisch/primes > > If you fork my project, I'll be happy to merge your changes. I have just finished benchmarking all the implementations provided in http://www.cs.hmc.edu/~oneill/code/haskell-primes.zip (the zip file linked to from the Haskell wiki article on primes). NaurPrimes.hs is by far the fastest version, and at least 2 or 3 times faster than your current implementation. I'm pretty sure it also uses less memory. I want to find efficient algorithms for the other proposed functions before forking, but I figured I'd let you know in the meantime.

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[ANN] Hack: a sexy Haskell Webserver Interface ^^
by Jinjing Wang 20 Apr '09

20 Apr '09

Simplest app should look like this module Main where import Hack import Hack.Handler.Kibro hello :: Application hello = \env -> return $ Response { status = 200 , headers = [ ("Content-Type", "text/plain") ] , body = "Hello World" } main = run hello Hack is a brainless port of the brilliant Ruby Rack framework. Rack is very modular and that's very important. Rack utilities and middlewares should be able to be ported with minimal effort, I hope. link / source: http://github.com/nfjinjing/hack/tree/master -- jinjing

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Re: [Haskell-cafe] CPS and the product function
by michael rice 20 Apr '09

20 Apr '09

That last definition multiplied zeros coming out of the recursion, but I believe this one is good: myprod l = prod l id id where prod [] k b = k 1 prod (x:xs) k b = if x == 0 then b 0 else prod xs (\ z -> k (x * z)) b My goal was to use CPS to do it. Did I succeed? Also, is there another way to do the same thing without passing b through all those recursions? I'm not good enough with Haskell's syntax to see how to do it. Michael --- On Mon, 4/20/09, michael rice <nowgate(a)yahoo.com> wrote: From: michael rice <nowgate(a)yahoo.com> Subject: Re: [Haskell-cafe] CPS and the product function To: haskell-cafe(a)haskell.org, "Tillmann Rendel" <rendel(a)cs.au.dk> Date: Monday, April 20, 2009, 11:28 AM This also seems to work: myprod l = prod l id where prod [] k = k 1 prod (x:xs) k = if x == 0 then 0 else prod xs (\ z -> k (x * z)) *Main Data.List> :load prod [1 of 1] Compiling Main ( prod.hs, interpreted ) Ok, modules loaded: Main. *Main Data.List> myprod [1,2,3,4,5,0,6,7,8,9] 0 *Main Data.List> myprod [1,2,3,4,5] 120 *Main Data.List> myprod [0..] 0 *Main Data.List> Michael --- On Mon, 4/20/09, Tillmann Rendel <rendel(a)cs.au.dk> wrote: From: Tillmann Rendel <rendel(a)cs.au.dk> Subject: Re: [Haskell-cafe] CPS and the product function To: haskell-cafe(a)haskell.org Date: Monday, April 20, 2009, 11:07 AM michael rice wrote: > I've been looking at CPS in Haskell and wondering how many multiplications the product function performs if it encounters a zero somewhere in the input list. Zero? > > Does anyone know the definition of the product function? You can use Hoogle [1] to search for product [2]. The documentation page [3] has a link to the source code [4]. Depending on some flag, it is either product = foldl (*) 1 or an explicit loop with an accumulator. That means that even for a non-strict (*), the whole input list would be processed before a result could be returned. > Does anyone know how to define it to avoid that? You have to define a multiplication function which is non-strict in the second argument if the first is 0. mult 0 b = 0 mult a b = a * b Now we can foldr this multiplication function over a list, and evaluation will stop at the first 0. foldr mult 1 ([1..100] ++ [0 ..]) However, this solution seems not to run in constant space. We can write it with a strict accumulator to avoid this problem: product = product' 1 where product' acc [] = acc product' acc (0 : xs) = 0 product' acc (x : xs) = (product' $! acc * x) xs Tillmann [1] http://www.haskell.org/hoogle/ [2] http://www.haskell.org/hoogle/?hoogle=product [3] http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:produ… [4] http://haskell.org/ghc/docs/latest/html/libraries/base/src/Data-List.html#p… _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell] ANN: persistent-map-0.0.0
by Alberto G. Corona 20 Apr '09

20 Apr '09

Interesting. It seems similar to TCache. However the design is different. persistent-map store different finite maps whenever any of them are modified transactionally, while TCache handles a single hashTable of object of type "a" and from time to time store all the transactionally modified objects and rearange the hashTable size. 2009/4/19 Peter Robinson <thaldyron(a)gmail.com>: > The persistent-map package [1] provides a thread-safe (STM) frontend > for finite map types together with a backend interface for persistent > storage. The TMap data type is thread-safe, since all access functions > run inside an STM monad . Any type that is an instance of > Data.Edison.Assoc.FiniteMapX (see EdisonAPI) can be used as a map > type. > > If a TMap is modified within an STM transaction, a corresponding > backend IO-request is added using the onCommit hook (cf. stm-io-hooks > package). To ensure consistency, the (Adv)STM monad runs these > requests iff the transaction commits. Additional backends (e.g. HDBC) > can be added by instantiating class Backend. > > Cheers, > Peter > > [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/persistent-map > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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Re: [Haskell-cafe] General function to count list elements?
by michael rice 20 Apr '09

20 Apr '09

Though I haven't tried it out, it's trying to use my function to count functions. The first argument is the identity function. The second argument is a list of a different form of the identity function. Though the two identity functions, given the same input, would produce the same output, I doubt they would be equal. So my guess at an answer would be zero. Michael --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: From: Eugene Kirpichov <ekirpichov(a)gmail.com> Subject: Re: [Haskell-cafe] General function to count list elements? To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Saturday, April 18, 2009, 11:03 AM What should count (\x -> x) (replicate 10 (\y -> if 1==1 then y else undefined)) return? 2009/4/18 michael rice <nowgate(a)yahoo.com>: > Is there a general function to count list elements. I'm trying this > > count :: a -> [a] -> Int > count x ys = length (filter (== x) ys) > > with this error upon loading > > ============= > > [michael@localhost ~]$ ghci count > GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help > Loading package ghc-prim ... linking ... done. > Loading package integer ... linking ... done. > Loading package base ... linking ... done. > [1 of 1] Compiling Main ( count.hs, interpreted ) > > count.hs:2:29: > Could not deduce (Eq a) from the context () > arising from a use of `==' at count.hs:2:29-32 > Possible fix: > add (Eq a) to the context of the type signature for `count' > In the first argument of `filter', namely `(== x)' > In the first argument of `length', namely `(filter (== x) ys)' > In the expression: length (filter (== x) ys) > Failed, modules loaded: none. > Prelude> > > ============= > > Not sure what it's trying to tell me other than I need an (Eq a) somewhere. > > Michael > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Eugene Kirpichov Web IR developer, market.yandex.ru

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Re: [Haskell-cafe] CPS and the product function
by michael rice 20 Apr '09

20 Apr '09

This also seems to work: myprod l = prod l id where prod [] k = k 1 prod (x:xs) k = if x == 0 then 0 else prod xs (\ z -> k (x * z)) *Main Data.List> :load prod [1 of 1] Compiling Main ( prod.hs, interpreted ) Ok, modules loaded: Main. *Main Data.List> myprod [1,2,3,4,5,0,6,7,8,9] 0 *Main Data.List> myprod [1,2,3,4,5] 120 *Main Data.List> myprod [0..] 0 *Main Data.List> Michael --- On Mon, 4/20/09, Tillmann Rendel <rendel(a)cs.au.dk> wrote: From: Tillmann Rendel <rendel(a)cs.au.dk> Subject: Re: [Haskell-cafe] CPS and the product function To: haskell-cafe(a)haskell.org Date: Monday, April 20, 2009, 11:07 AM michael rice wrote: > I've been looking at CPS in Haskell and wondering how many multiplications the product function performs if it encounters a zero somewhere in the input list. Zero? > > Does anyone know the definition of the product function? You can use Hoogle [1] to search for product [2]. The documentation page [3] has a link to the source code [4]. Depending on some flag, it is either product = foldl (*) 1 or an explicit loop with an accumulator. That means that even for a non-strict (*), the whole input list would be processed before a result could be returned. > Does anyone know how to define it to avoid that? You have to define a multiplication function which is non-strict in the second argument if the first is 0. mult 0 b = 0 mult a b = a * b Now we can foldr this multiplication function over a list, and evaluation will stop at the first 0. foldr mult 1 ([1..100] ++ [0 ..]) However, this solution seems not to run in constant space. We can write it with a strict accumulator to avoid this problem: product = product' 1 where product' acc [] = acc product' acc (0 : xs) = 0 product' acc (x : xs) = (product' $! acc * x) xs Tillmann [1] http://www.haskell.org/hoogle/ [2] http://www.haskell.org/hoogle/?hoogle=product [3] http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:produ… [4] http://haskell.org/ghc/docs/latest/html/libraries/base/src/Data-List.html#p… _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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CPS and the product function
by michael rice 20 Apr '09

20 Apr '09

I've been looking at CPS in Haskell and wondering how many multiplications the product function performs if it encounters a zero somewhere in the input list. Zero? Does anyone know the definition of the product function? Michael

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Re: [Haskell-cafe] CPS and the product function
by michael rice 20 Apr '09

20 Apr '09

Hi Eugene, Clever solution. It didn't come right back so I interrupted it. Guess it would do all those multiplies by zero. Does anyone know how to define it to avoid that? Thanks. Michael --- On Mon, 4/20/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: From: Eugene Kirpichov <ekirpichov(a)gmail.com> Subject: Re: [Haskell-cafe] CPS and the product function To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Monday, April 20, 2009, 9:40 AM Check that by experiment in ghci or by looking at the source: Prelude*> product [0..] Or search "product" at http://holumbus.fh-wedel.de/hayoo/hayoo.html and click on one of the Source links. 2009/4/20 michael rice <nowgate(a)yahoo.com>: > I've been looking at CPS in Haskell and wondering how many multiplications > the product function performs if it encounters a zero somewhere in the input > list. Zero? > > Does anyone know the definition of the product function? > > Michael > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Eugene Kirpichov Web IR developer, market.yandex.ru

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