May 2009 - Haskell-Cafe

Bidirectional programming in Haskell
by Shin-Cheng Mu 27 May '09

27 May '09

Hi, On Tue May 26 01:21:28 EDT 2009, Artyom Shalkhakov wrote: > Are there any libraries for "bidirectional" [1] programming > in Haskell? Any related work? Some of the early work in the PSD project (closely related to the "lenses") were developed in Haskell, http://www.iis.sinica.edu.tw/~scm/2007/inv/ It is not in active maintenance now, but if you are interested in doing something with it, I'd be happy to help. :) Ref: [1] S-C. Mu, Z. Hu and M. Takeichi, An injective language for reversible computation. In Mathematics of Program Construction 2004, LNCS 3125, pp. 289-313, July 2004. [2] Z. Hu, S-C. Mu and M. Takeichi, A programmable editor for developing structured documents based on bidirectional transformations. In Partial Evaluation and Semantics-Based Program Manipulation, pp. 178-189. August 2004

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The essence of my monad confusion
by Paul Keir 27 May '09

27 May '09

On the wiki page for Applicative Functors (http://www.haskell.org/haskellwiki/Applicative_functor) a familiar characteristic of monads is quoted; that they "allow you to run actions depending on the outcomes of earlier actions". I feel comfortable with Functors and Applicative Functors, but I don't yet get that "extra power" that monads provide. An example immediately follows that quotation on the wiki: do text <- getLine if null text then putStrLn "You refuse to enter something?" else putStrLn ("You entered " ++ text) For simplicity's sake, I modified it to avoid using the IO monad; the "text" binding is now provided by the first parameter, and (=<<) is used due to its similarity to fmap: bar :: Monad m => m String -> m String bar as = (=<<) (\a -> if null a then return "nothing" else return "something") as This works fine, so bar ["Blah"] gives ["something"], and bar (Just "") gives ["nothing"]. But, I can get the same effect using a Functor (replacing (=<<) with fmap): bar2 :: Functor f => f String -> f String bar2 as = fmap (\a -> if null a then "nothing" else "something") as Can anyone help me out of the swamp? Cheers, Paul

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Purely logical programming language
by Matthias Görgens 27 May '09

27 May '09

There are a number of ways to marry purely functional programming languages with IO. To name just two possibilities: Clean uses linear types, threading exactly one "World" through functions, Haskell uses Monads. The model in Prolog, however, looks more like the model used in most strict functional languages. It uses impure predicates to affect the outside world. Do you know of any attempt to do for logic programming what Monads did for functional programming?

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May all logos be freely used?
by Hans van Thiel 27 May '09

27 May '09

Hello, What's the status of the logos on http://www.haskell.org/logos/results.html I'd like to use one or more in an about message in a Haskell program. Is this allowed, and if so, are there any conditions? I've just looked at them again, and it's a really fine collection, I think. Best Regards, Hans van Thiel

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Bidirectional programming in Haskell
by Artyom Shalkhakov 27 May '09

27 May '09

Hello, Are there any libraries for "bidirectional" [1] programming in Haskell? Any related work? All I've found is a paper "There and back again: arrows for invertible programming", which I couldn't find the full text of. Cheers, Artyom Shalkhakov. [1] something along the lines of Boomerang <www.seas.upenn.edu/~harmony/>

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How does GHC's implementation of par work?
by Ryan Ingram 26 May '09

26 May '09

This is related to the long thread on a bug in cross-module strictness analysis for the "lazy" function in GHC. Simon posted a response to that bug ( http://hackage.haskell.org/trac/ghc/ticket/3259 ), which shows the Haskell source for GHC's "par" function: > {-# INLINE par #-} > par :: a -> b -> b > par x y = case (par# x) of { _ -> lazy y } While I understand that in GHC Core, case always reduces its argument to WHNF, this isn't true of Haskell in general. What causes par# to get executed? Does GHC overload case for unboxed return values to force the evaluation of the scrutinee even if its return value isn't used? -- ryan On Tue, May 26, 2009 at 10:29 AM, Duncan Coutts <duncan.coutts(a)worc.ox.ac.uk> wrote: > On Mon, 2009-05-25 at 09:40 -0400, Mario Blazevic wrote: >> >> $diff main.simpl imported.simpl >> >> ... >> >> 223c232 >> >> < a_s1rs [ALWAYS Just L] :: GHC.Integer.Internals.Integer >> >> --- >> >>> a_s1sV [ALWAYS Just S] :: GHC.Integer.Internals.Integer >> >> ... >> > >> > Good find! >> > >> >> Does this S vs. L difference have anything to do with strictness and laziness? >> > >> > Yes. >> > >> > So, I think we should open a ghc but report with all the details, >> > particularly the example's source, the ghc version and that highlight of >> > that strictness difference. > >> I have logged the ticket at >> >> http://hackage.haskell.org/trac/ghc/ticket/3259 > > Thanks for being so persistent Mario! > > Simon has diagnosed the bug. It's really rather subtle. > > Duncan > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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[wxHaskell] ghc-pkg: /usr/local/lib/imports doesn't exist ...
by jose.benjaminps 26 May '09

26 May '09

Hello, I'm installing wxhaskell from darcs, almost all found good, but after make "# runghc Setup.lhs install" the last lines this is: cat config/wxcore.pkg | sed -e "s|\${wxhlibdir}|/usr/local/lib|" | ghc-pkg update - Reading package info from stdin ... done. ghc-pkg: /usr/local/lib/imports doesn't exist or isn't a directory (use --force to override) make: *** [wxcore-register] Error 1 I don't know solve this, I'm using linux ubuntu-jaunty with ghc-6.10.1 and wxGTK-2.8.10 thanks, --benjo

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Haskell programmers in São Carlos - SP - Brazil?
by Maurício 26 May '09

26 May '09

Anybody else around here? Best, Maurício

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Re: Re: Re: Re: [Haskell-cafe] Re: A problem with par and modules boundaries...
by Duncan Coutts 26 May '09

26 May '09

On Sun, 2009-05-24 at 12:48 -0400, Mario Blažević wrote: > > How about diffing the whole core output (and using -ddump-simpl). If > > there's a performance difference then there must be a difference in the > > core code too. > I can't exactly use diff because the generated identifier names are not the same, > but after poring over with Emacs ediff I have found only one difference that's > not attributable to identifiers: > > $diff main.simpl imported.simpl > ... > 223c232 > < a_s1rs [ALWAYS Just L] :: GHC.Integer.Internals.Integer > --- > > a_s1sV [ALWAYS Just S] :: GHC.Integer.Internals.Integer > ... Good find! > Does this S vs. L difference have anything to do with strictness and laziness? Yes. So, I think we should open a ghc but report with all the details, particularly the example's source, the ghc version and that highlight of that strictness difference. Duncan

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The Computer Language Benchmarks Game: pidigits
by Isaac Gouy 26 May '09

26 May '09

On Mon May 25 16:18:29 EDT 2009, Arnaud Payement wrote: > ... I thought it is better to show Haskell as one would naturally write it. One would naturally first write it in C ? :-)

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