March 2012 - Haskell-Cafe

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

13 14

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

3 3

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

11 11

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

2 1

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 > > > > > > > > > > > > > >

2 1

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.

1 0

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.

2 1

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

16 Mar '12

Casey -- -- Regards, KC

1 0

Are side effects necessary is a non question; you compute so as to change the state of the world.
by KC 16 Mar '12

16 Mar '12

Casey -- -- Regards, KC

1 0

Re: [Haskell-cafe] cabal-install problem with alex dependency in bytestring-lexing
by wren ng thornton 16 Mar '12

16 Mar '12

On 3/16/12 6:00 AM, Erik de Castro Lopo wrote: > Hi all, > > With a base system with just ghc and cabal-install, if I try to install > bytestring-lexing I get: > > $ cabal install bytestring-lexing > Resolving dependencies... > Configuring bytestring-lexing-0.4.0... > cabal: The program alex version>=2.3 is required but it could not be found. > cabal: Error: some packages failed to install: > bytestring-lexing-0.4.0 failed during the configure step. The exception was: > ExitFailure 1 > > The cabal file for bytestring-lexing contains > > Build-Tools: alex>= 2.3 > > Is there any way to make the cabal install of bytestring-lexing force > the install of alex first? FWIW, the Alex dependency is part of the code I inherited when taking over bytestring-lexing. Apparently older versions of the library included pre-generated versions of the files that Alex builds, prior to when Cabal understood about build tools at all. I'm willing to go back that direction if that's what folks want--- especially if there's a way to tell Cabal to switch between the pregenerated files vs using Alex. I've noticed that the code generated by Alex causes a lot of warnings, some of them are of the "it will break in the future" variety. I've considered rewriting the parsers for Double manually, but I'm not sure exactly what the performance bottlenecks are in that context nor whether anyone is actually using the current functions in performance-critical regions. To Erik specifically: my emails to you off-list have been bouncing/timing out. Is there a chance I've ended up on a blacklist or there's some configuration issue at mega-nerd.com? -- Live well, ~wren

1 0