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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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Code Golf
by Sebastian Fischer 20 Apr '09

20 Apr '09

Fancy some Codegolf? I wrote the following function for list diagonalization: > diag l = foldr (.) id ((sel l . flip sel) ((:[]).(:))) [] > where > sel = foldr (\a b c -> id : mrg (a c) (b c)) (const []) . map (flip id) > > mrg [] ys = ys > mrg xs [] = xs > mrg (x:xs) (y:ys) = (x.y) : mrg xs ys Self explanatory, isn't it? Here is a test case: *Main> take 10 $ diag [[ (m,n) | n <- [1..]] | m <- [1..]] [(1,1),(1,2),(2,1),(1,3),(2,2),(3,1),(1,4),(2,3),(3,2),(4,1)] I was trying to golf it down [^1] but my brain explodes. If you succeed in reducing keystrokes, I'd be happy to know! Cheers, Sebastian [^1]: http://codegolf.com/

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Re: [Haskell-cafe] ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
by Niemeijer, R.A. 20 Apr '09

20 Apr '09

> data EShow = forall a. Show a => EShow a > > data E t = forall a. E (a->t) a > > smallPrint_ t = concatMap (\f-> f t) [show . foo, show . bar, show . baz] Yeah, I am aware of these solutions, but like Dan says: > but first-class existentials are still desirable because introducing a new > type for every existential is annoying. It's comparable to having to write a > new class for every combination of argument and result types to mimic first > class functions in Java (aside from first class functions being more > ubiquitous in their usefulness, although perhaps it only appears that way > because we don't have easy use of existential types). I've personally always looked at the extra data type or repeated functions as ugly hacks around the fact that GHC doesn't have real existential typing. Since Haskell is otherwise virtually free of ugly hacks (at least at the level I work at, which doesn't require things like unsafePerformIO and unboxed arrays) this has always annoyed me a bit. So I figured that since we now have a working implementation it would be worth a shot to ask if this language wart can be removed from GHC.

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

20 Apr '09

I know functions can be compared in Scheme Welcome to DrScheme, version 4.1 [3m]. Language: Swindle; memory limit: 128 megabytes. > (equal? equal? equal?) #t > but apparently not in Haskell [michael@localhost ~]$ ghci 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. Prelude> (==) (==) (==) <interactive>:1:0: No instance for (Eq (a -> a -> Bool)) arising from a use of `==' at <interactive>:1:0-13 Possible fix: add an instance declaration for (Eq (a -> a -> Bool)) In the expression: (==) (==) (==) In the definition of `it': it = (==) (==) (==) Prelude> though I'm new at Haskell and may not be posing the question properly. I would think a language with 1st-class support for functions would certainly include comparing them. To compare two functions in C, I would compare their machine addresses. 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, 11:39 AM Could you then provide an example of two functions that *are* equal, or, even better, a definition of equality for arbitrary functions? Since Haskell may be compiled into C, this must be a definition that is implementable in C. 2009/4/18 michael rice <nowgate(a)yahoo.com>: > 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 > > -- Eugene Kirpichov Web IR developer, market.yandex.ru

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