- Haskell-Cafe - Haskell.org

In an imperative language, commands & fulling memory cells is easy, but gives the false impression that S/W engineering is easy.
by KC 17 Mar '12

17 Mar '12

-- -- Regards, KC

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In Haskell, because of the fine layers of abstraction that are possible, it pays to hone one's critical thinking skills.
by KC 17 Mar '12

17 Mar '12

-- -- Regards, KC

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Theoretical question: are side effects necessary?
by Christopher Svanefalk 17 Mar '12

17 Mar '12

Dear all, there is a question I have been thinking about a bit. In short, we could simply formulate it like this: Are there any problems which *cannot *be solved a side effect-free language (such as Haskell)? In other words, are there problems that would explicitly demand semantics that can only be provided by a language allowing direct modification of external state variables, such as Java and C++? If not, are there problems which are simply *infeasible *to solve with a side effect-free language? I realize this question is very broad, and I am not expecting an exact answer by any means. I am asking since I am curious about the relation between functional and imperative/procedural languages in general. I primarily come from a Java background, but I can program Haskell and Erlang, and have recently started exploring Scala, so this would be interesting to know. -- Best, Christopher Svanefalk Student, Department of Computer Science and Engineering University of Gothenburg / Chalmers University of Technology

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Re: [Haskell-cafe] Theoretical question: are side effects necessary?
by KC 17 Mar '12

17 Mar '12

On Fri, Mar 16, 2012 at 7:44 PM, Jerzy Karczmarczuk <jerzy.karczmarczuk(a)unicaen.fr> wrote: >... but the question of purity of a program - in my opinion - concerns the program, and nothing else. You might be thinking of software engineering purity. >The networking is not expected to break the referential transparency, or does it? > > > Jerzy Karczmarczuk > What does referential transparency mean to you? -- -- Regards, KC

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Prettier pretty-printing of data types?
by Johan Tibell 17 Mar '12

17 Mar '12

Hi all, The derived Show instance is useful, but I sometimes wish for something that's easier to read for big data types. Does anyone have an implementation of show that draws things in a hierarchical manner? Example: Given data Tree a = Leaf | Bin Int a (Tree a) (Tree a) and value = Bin 1 (Bin 2 Leaf Leaf) (Bin 3 Leaf Leaf) draw as Bin 1 Bin 2 Leaf Leaf Bin 3 Leaf Leaf Cheers, Johan

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A GSoC Proposal
by Kit Barnes 17 Mar '12

17 Mar '12

I have a proposal for a GSoC project that I wish to undertake. Is this a good place for me to begin discussing it? -- KitB

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Re: [Haskell-cafe] Regular Expression with PCRE
by Carter Tazio Schonwald 17 Mar '12

17 Mar '12

There's a lot of reasons why I don't recommend that approach, but I think it's best explained by the following now classic stack overflow post and answer http://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xht… Basically this applies in your case because recognizing if a sequence of characters is in a comment block or not for HTML is likely not expressible using regexes. There may be a way for a very controlled restricted subset of HTML, but it might require some complex regexes. That said, if you're ok with some false positives and dealing with that, a simple regex based solution is the way to go! Cheers, -- Carter Tazio Schonwald On Friday, March 16, 2012 at 7:08 PM, Joseph Bozeman wrote: > My goal is to remove the HTML comments. It probably would be at least as efficient to use an HTML parser, but I usually strip files by hand, and I always use regex then. I didn't want to bother importing yet another package, because if I could just get this line to work, I could get all my stripping done with three functions, and then I have four that I use to apply a template to the text once it's bare. > > On Fri, Mar 16, 2012 at 5:41 PM, Carter Tazio Schonwald <carter.schonwald(a)gmail.com (mailto:carter.schonwald@gmail.com)> wrote: > > have you considered using one of the many amazing HTML parsers on hackage? > > > > If the goal is to just get the HTML comments, that might be a much more effective use of your time > > > > -- > > Carter Tazio Schonwald > > > > > > On Friday, March 16, 2012 at 4:55 PM, Joseph Bozeman wrote: > > > > > > > > > Hey everyone, I'm hoping someone can point me in the right direction. > > > > > > The regex-pcre package exports (=~) and (=~~) as two useful infix functions. They're great! The only problem is, they are a positive match for a regex. I have a file that contains HTML comments (it was generated in Word) and I really just want the barest text. I already have a function that strips out all the tags, and I have a function that finds all the links and sticks those in another file for later perusal. > > > > > > What I'd like is advice on how to implement the (!~) and (!~~) operators. They should have the same types as (=~) and (=~~). I'm stuck, though. Here's the source for both of those functions: they depend on Text.Rege.PCRE package. > > > > > > (=~) :: (RegexMaker (http://hackage.haskell.org/packages/archive/regex-base/0.93.2/doc/html/Text…) Regex (http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…) CompOption (http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…) ExecOption (http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…) source, RegexContext (http://hackage.haskell.org/packages/archive/regex-base/0.93.2/doc/html/Text…) Regex (http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…) source1 target) => source1 -> source -> target > > > (=~) x r = let q :: Regex q = makeRegex r in match q x > > > > > > (=~~) :: (RegexMaker (http://hackage.haskell.org/packages/archive/regex-base/0.93.2/doc/html/Text…) Regex (http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…) CompOption (http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…) ExecOption (http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…) source, RegexContext (http://hackage.haskell.org/packages/archive/regex-base/0.93.2/doc/html/Text…) Regex (http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…) source1 target, Monad (http://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html/Control-M…) m) => source1 -> source -> m target > > > (=~~) x r = do (q :: Regex) <- makeRegexM r matchM q x > > > What I figured I could do was find a function that was the inverse of "match" and "matchM", but I can't find any in the docs. I really hope I don't have to implement that, too. I'm still new at this, and that seems like it would be over my head. > > > > > > > > > > > > _______________________________________________ > > > Haskell-Cafe mailing list > > > Haskell-Cafe(a)haskell.org (mailto:Haskell-Cafe@haskell.org) > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > > > > > > > > >

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Best way to use the google visualization (javascript) API rather than static image charts (hs-gchart)?
by Ryan Newton 17 Mar '12

17 Mar '12

Hello cafe, I've got a benchmarking script which currently generates gnuplot scripts (of simple lines and points) and I'd like to port it to use two** new backends: (1) Chart for PDF generation and gtk viewing (2) Something-or-other for generating pretty interactive charts in the browser, suitable for pasting into blog posts. I was hopeful regarding "hs-gcharts", but it looks like that only works for the charts that generate static images given a URL, as described here: http://code.google.com/apis/chart/image/docs/making_charts.html Rather than interactive ones, like this JSAPI based pie chart: http://code.google.com/apis/chart/interactive/docs/quick_start.html So where to start? Has anybody tried this before? Perhaps the best thing to do is just use HJScript to produce the desired JS code: https://github.com/chrisdone/amelie/blob/master/src/Amelie/View/Script.hs output here: http://hpaste.org/js/amelie.js Thanks, -Ryan ** One would be great but I'm pretty sure no such thing currently exists.

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Regular Expression with PCRE
by Joseph Bozeman 16 Mar '12

16 Mar '12

Hey everyone, I'm hoping someone can point me in the right direction. The regex-pcre package exports (=~) and (=~~) as two useful infix functions. They're great! The only problem is, they are a *positive* match for a regex. I have a file that contains HTML comments (it was generated in Word) and I really just want the barest text. I already have a function that strips out all the tags, and I have a function that finds all the links and sticks those in another file for later perusal. What I'd like is advice on how to implement the (!~) and (!~~) operators. They should have the same types as (=~) and (=~~). I'm stuck, though. Here's the source for both of those functions: they depend on Text.Rege.PCRE package. (=~) :: (RegexMaker<http://hackage.haskell.org/packages/archive/regex-base/0.93.2/doc/html/Text…> Regex<http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…> CompOption<http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…> ExecOption<http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…> source, RegexContext<http://hackage.haskell.org/packages/archive/regex-base/0.93.2/doc/html/Text…> Regex<http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…> source1 target) => source1 -> source -> target (=~) x r = let q :: Regex q = makeRegex r in match q x (=~~) :: (RegexMaker<http://hackage.haskell.org/packages/archive/regex-base/0.93.2/doc/html/Text…> Regex<http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…> CompOption<http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…> ExecOption<http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…> source, RegexContext<http://hackage.haskell.org/packages/archive/regex-base/0.93.2/doc/html/Text…> Regex<http://hackage.haskell.org/packages/archive/regex-pcre/0.94.2/doc/html/Text…> source1 target, Monad<http://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html/Control-M…> m) => source1 -> source -> m target (=~~) x r = do (q :: Regex) <- makeRegexM r matchM q x What I figured I could do was find a function that was the inverse of "match" and "matchM", but I can't find any in the docs. I really hope I don't have to implement *that*, too. I'm still new at this, and that seems like it would be over my head.

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The Theoretical Question would be; are side effects necessary for computational heavy lifting.
by KC 16 Mar '12

16 Mar '12

Casey -- -- Regards, KC

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