- Haskell-Cafe - Haskell.org

We *have* been accepted into the Google Summer of Code
by Edward Kmett 18 Mar '12

18 Mar '12

Just to clarify, since I've been getting a ton of emails on the topic, we *have* been accepted to the Google Summer of Code this year. The list on their site is still updating, and we should appear there shortly. -Edward

3 3

cabal-install problem with alex dependency in bytestring-lexing
by Erik de Castro Lopo 17 Mar '12

17 Mar '12

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? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

6 8

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

3 2

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

4 4

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

Re: [Haskell-cafe] Theoretical question: are side effects necessary?
by Christopher Svanefalk 16 Mar '12

16 Mar '12

Many thanks for the replies and references all of you! I will continue to read up on this from here, and you have all boosted my interest in investigating how complex systems could be developed using functional languages. On Fri, Mar 16, 2012 at 1:46 PM, Dimitri Scheftelowitsch < d.scheftelowitsch(a)googlemail.com> wrote: > Hi, > > it depends on what your definition of „problem“ is. If a problem is just > something that is defined by „given input A, generate output B such that > some constraints are met“, then you can go with anything Turing-complete. > > If you think of problems in the sense of software engineering and disallow > side effects, then you cannot do anything — any sensible software requires > I/O. > > If you want to compare the programming models of lambda calculus vs. > random access machines, then you have nothing worse than a polynomial > slowdown in terms of runtime. Strong Church-Turing thesis and so on. > > HTH > Dimitri > > On 16.03.2012, at 13:23, Christopher Svanefalk wrote: > > > 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 > > -- Best, Christopher Svanefalk

1 0

is the evaluation order deterministic when using applicative with IO
by Rouan van Dalen 16 Mar '12

16 Mar '12

Hi everyone. I was wondering if I can make assumptions about the evaluation order of the following code: isTrue :: Int -> IO Bool isTrue val = pure (||) <*> boolTest1 val <*> boolTest2 val {- boolTest1 is an inexpensive, quick check -} boolTest1 :: Int -> IO Bool boolTest1 val = undefined {- boolTest2 is a very expensive check -} boolTest2 :: Int -> IO Bool boolTest2 val = undefined When using Applicative in the isTrue function, I would like to make use of the short-circuit behaviour of || and rely on the fact that the boolTest1 will be executed first. The reason I am asking is because the boolTest functions are in the IO monad, instead of just returning pure Bool values. Regards Rouan.

5 4

Re: [Haskell-cafe] HXT: how to get sibling element
by Никитин Лев 16 Mar '12

16 Mar '12

Oh, yes! In this situation with so poor structured source I can try to use tagsoup. (or I'll take a look at xml-conduit). Nevertheless for better undestanding HXT it will be interesting to solve this problem in HXT. Or is it impossible? 15.03.2012, 20:08, "Asten, W.G.G. van (Wilfried, Student B-TI)" <w.g.g.vanasten(a)student.utwente.nl>: > You might want to check out the xml-conduit package. It has preceding > and following sibling Axis. I am not sure how the package works > exactly, but it seems to be a good starting point. > > 2012/3/15 Никитин Лев <leon.v.nikitin(a)pravmail.ru>: > >> I absolutly agree with you but unfortunetly, it is not my xml file. >> It is extraction from html page of public web server. I cannot to change >> format of this html page. >> Sorry. I had to explain it in first letter. >> >> But than what about to get sibling text (geting sibling is an separate >> interesting tasks with no matter for my contrete case). >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe

2 2

Too much inlining on text package
by Felipe Almeida Lessa 16 Mar '12

16 Mar '12

Hello! tl;dr: text package's pack function is creating huge chunks of code everywhere. Michael Snoyman and I have been trying to nail the performance problems of persistent's Template Haskell code -- GHC was taking a lot of memory and CPU time to compile these. What we found out is that the code size was getting increased 20-fold by the simplifier on phase 0 on GHC 7.0 (c.f. http://groups.google.com/group/yesodweb/msg/9f625fcf85575263) So, what was increasing in size? Consider this extremely simple module (attached as Bug.hs): module Bug (text) where import qualified Data.Text as T text :: T.Text text = T.pack "text" Until simplifier phase 0, the code size floats but tops at 12. Here's the core: Bug.text :: Data.Text.Internal.Text [LclIdX, Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=0, Value=False, ConLike=False, Cheap=False, Expandable=False, Guidance=IF_ARGS [] 11 0}] Bug.text = Data.Text.Fusion.unstream (Data.Text.Fusion.Common.streamList @ GHC.Types.Char (GHC.Base.map @ GHC.Types.Char @ GHC.Types.Char Data.Text.Internal.safe (GHC.Base.unpackCString# "text"))) Which is straightforward. However, on simplifier phase 0 the code size jumps to 391 (!!), a 32-fold increase. I've attached the core (after.hs) since it's too large to copy here on the body. So it seems that the (unstream . streamList) pair above is getting inlined to a HUGE chunk of code (at least Data.Text.Array.new is getting inlined). Worse yet, this happens for every single pack that you use, even those packs hidden by OverloadedStrings! Does anyone have any ideas why GHC is inlining so much code everywhere? While everything I said here was tested on GHC 7.0, we have evidence that GHC 7.4 suffers from the same problem. We don't know about GHC 6.12, though. This seems to be a problem for everyone who uses text, which we hope is everyone using Haskell ;-). Cheers, -- Felipe.

2 1

Haskell Weekly News: Issue 218
by Daniel Santa Cruz 16 Mar '12

16 Mar '12

Welcome to issue 218 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue cover the weeks of February 26 to March 10, 2012. You can find the HTML version at: http://contemplatecode.blogspot.com/2012/03/haskell-weekly-news-issue-218.h… Quotes of the Week * monochrom: shapr is not "normal". he got a decent job, then went to school. * quicksilver: typeclasses are absolutely not sugar for a record of functions quicksilver: they're unsugar. quicksilver: they're a benighted bitter poison. * edwardk: i'm not willing to give up orders of magnitude worth of performance to get a bad hack edwardk: if you want that, go code in scheme * (on pointless black magic) mgsloan: welcome to excessively pointless plumbing operators :) byorgey: mgsloan: that's... terrifying DanBurton: you should put it on hackage * cheater_: maybe you should try /part since we're talking about perfect-world situations * edwardk: i used to be a huge proponent of unicode syntax everywhere, then i used agda ;) * cmccann: free advice is, of course, any advice which satisfies the properties expected given the definition of "advice", but nothing further ;] * shachaf: "magic" is just another word for "primitive" * dylukes: I'm getting way more acquainted with the GHC build system than I'd like to be. * JoeyA: Inform7 is a programming language. A programming language tells a computer what to do. The computer is in a room. You are facing West. * mauke: sorry, the current time is not a constant lukish: It's a pity * dfletcher: > ((++" Batman!") . take 48 . cycle . show) (0/0) Top Reddit Stories * New unordered-containers release improves performance by as much as 2x, using HAMTs Domain: blog.johantibell.com, Score: 64, Comments: 46 On Reddit: [1] http://goo.gl/D6yTb Original: [2] http://goo.gl/5YUph * A humble suggestion for visual pure functional programming Domain: i.imgur.com, Score: 59, Comments: 59 On Reddit: [3] http://goo.gl/eUMrk Original: [4] http://goo.gl/DSFnj * Spike: experimental web browser in Haskell Domain: self.haskell, Score: 53, Comments: 22 On Reddit: [5] http://goo.gl/ZTR1j Original: [6] http://goo.gl/ZTR1j * GHC 7.4.1 speeds up arbtt by a factor of 22 Domain: joachim-breitner.de, Score: 51, Comments: 18 On Reddit: [7] http://goo.gl/XVBDt Original: [8] http://goo.gl/IL6EL * Eff 3.0 is out, a functional language with first-class computational effects and handlers (an alternative to monads) Domain: math.andrej.com, Score: 49, Comments: 12 On Reddit: [9] http://goo.gl/1Idw0 Original: [10] http://goo.gl/iwHlG * What's the most clever piece of Haskell code you know? Domain: self.haskell, Score: 47, Comments: 64 On Reddit: [11] http://goo.gl/T1u9t Original: [12] http://goo.gl/T1u9t * Diagrams 0.5 released: powerful, flexible embedded domain-specific language for creating vector graphics Domain: byorgey.wordpress.com, Score: 40, Comments: 7 On Reddit: [13] http://goo.gl/e75Yx Original: [14] http://goo.gl/565c1 * Faster Javascript Through Category Theory Domain: johnbender.us, Score: 39, Comments: 5 On Reddit: [15] http://goo.gl/UgvTr Original: [16] http://goo.gl/TLK4k * I'm writing a Haskell book. Almost finished the 3rd chapter. Would like feedback. Domain: bit.ly, Score: 36, Comments: 48 On Reddit: [17] http://goo.gl/AzwhR Original: [18] http://goo.gl/AzwhR * Level 0 -- a Snake clone using SDL, with a nice interactive map editor Domain: quasimal.com, Score: 28, Comments: 5 On Reddit: [19] http://goo.gl/x9gZk Original: [20] http://goo.gl/meSPo * Parallel Haskell Digest 8: MVar, summer school, Simon PJ @ YOW, and more Domain: well-typed.com, Score: 27, Comments: 1 On Reddit: [21] http://goo.gl/MDzrI Original: [22] http://goo.gl/OC6O6 * Avoid cabal hell: find nirvana Domain: yesodweb.com, Score: 27, Comments: 30 On Reddit: [23] http://goo.gl/ryd5e Original: [24] http://goo.gl/Yyzqd * Improvements to HashMap and HashSet creation Domain: blog.johantibell.com, Score: 26, Comments: 1 On Reddit: [25] http://goo.gl/yh2zs Original: [26] http://goo.gl/u9uhg * The Day Python Embarassed Imperative Programming » What, then, Shall We Say? Domain: the-27th-comrade.appspot.com, Score: 25, Comments: 20 On Reddit: [27] http://goo.gl/5KIOx Original: [28] http://goo.gl/Wi47v * Improbable uses of unsafeCoerce Domain: gist.github.com, Score: 25, Comments: 25 On Reddit: [29] http://goo.gl/cN0LE Original: [30] http://goo.gl/cxfSc Top StackOverflow Questions * Haskell: Lists, Arrays, Vectors, Sequences votes: 32, answers: 1 Read on SO: [31] http://goo.gl/YHVpC * Why type classes in Haskell were difficult to implement? votes: 20, answers: 1 Read on SO: [32] http://goo.gl/LoLHe * runST and function composition votes: 18, answers: 3 Read on SO: [33] http://goo.gl/6NSvR * How do you compute the difference between successive elements of a list of unknown size, functionally? votes: 16, answers: 7 Read on SO: [34] http://goo.gl/rnc8l * Do we care about the 'past' in FRP? votes: 16, answers: 1 Read on SO: [35] http://goo.gl/oR2rd * “evaluate” function votes: 16, answers: 1 Read on SO: [36] http://goo.gl/ShLMT * What's preferred among liftM, lilftA, etc votes: 15, answers: 1 Read on SO: [37] http://goo.gl/y8apA * Name of type pattern: R a b = Q (a -> (R a b,b)) votes: 14, answers: 3 Read on SO: [38] http://goo.gl/kRMgn * Haskell offline documentation? votes: 12, answers: 2 Read on SO: [39] http://goo.gl/Hih3z * Program design in Haskell: how to do simulation without mutability votes: 12, answers: 3 Read on SO: [40] http://goo.gl/ODJjT Until next time, Daniel Santa Cruz References 1. http://blog.johantibell.com/2012/03/announcing-unordered-containers-02.html 2. http://www.reddit.com/r/haskell/comments/ql9m5/new_unorderedcontainers_rele… 3. http://i.imgur.com/TOZZZ.png 4. http://www.reddit.com/r/haskell/comments/q7m8i/a_humble_suggestion_for_visu… 5. http://www.reddit.com/r/haskell/comments/qhce6/spike_experimental_web_brows… 6. http://www.reddit.com/r/haskell/comments/qhce6/spike_experimental_web_brows… 7. http://www.joachim-breitner.de/blog/archives/546-GHC-7.4.1-speeds-up-arbtt-… 8. http://www.reddit.com/r/haskell/comments/q86iw/ghc_741_speeds_up_arbtt_by_a… 9. http://math.andrej.com/eff/ 10. http://www.reddit.com/r/haskell/comments/qosr7/eff_30_is_out_a_functional_l… 11. http://www.reddit.com/r/haskell/comments/qn7nk/whats_the_most_clever_piece_… 12. http://www.reddit.com/r/haskell/comments/qn7nk/whats_the_most_clever_piece_… 13. http://byorgey.wordpress.com/2012/03/09/announcing-diagrams-0-5/ 14. http://www.reddit.com/r/haskell/comments/qpkxb/diagrams_05_released_powerfu… 15. http://johnbender.us/2012/02/29/faster-javascript-through-category-theory/ 16. http://www.reddit.com/r/haskell/comments/qbuk8/faster_javascript_through_ca… 17. http://bit.ly/xDsDSj 18. http://www.reddit.com/r/haskell/comments/qquc5/im_writing_a_haskell_book_al… 19. http://quasimal.com/projects/level_0.html 20. http://www.reddit.com/r/haskell/comments/qcrhj/level_0_a_snake_clone_using_… 21. http://www.well-typed.com/blog/64 22. http://www.reddit.com/r/haskell/comments/qefzt/parallel_haskell_digest_8_mv… 23. http://www.yesodweb.com/blog/2012/03/cabal-nirvana 24. http://www.reddit.com/r/haskell/comments/qkqdj/avoid_cabal_hell_find_nirvan… 25. http://blog.johantibell.com/2012/03/improvements-to-hashmap-and-hashset.html 26. http://www.reddit.com/r/haskell/comments/qp7w1/improvements_to_hashmap_and_… 27. http://the-27th-comrade.appspot.com/blog/ahJzfnRoZS0yN3RoLWNvbXJhZGVyDAsSBU… 28. http://www.reddit.com/r/haskell/comments/q7ykt/the_day_python_embarassed_im… 29. https://gist.github.com/1934179 30. http://www.reddit.com/r/haskell/comments/qadwm/improbable_uses_of_unsafecoe… 31. http://stackoverflow.com/questions/9611904/haskell-lists-arrays-vectors-seq… 32. http://stackoverflow.com/questions/9602176/why-type-classes-in-haskell-were… 33. http://stackoverflow.com/questions/9468963/runst-and-function-composition 34. http://stackoverflow.com/questions/9512513/how-do-you-compute-the-differenc… 35. http://stackoverflow.com/questions/9526836/do-we-care-about-the-past-in-frp 36. http://stackoverflow.com/questions/9553943/evaluate-function 37. http://stackoverflow.com/questions/9509350/whats-preferred-among-liftm-lilf… 38. http://stackoverflow.com/questions/9487760/name-of-type-pattern-r-a-b-q-a-r… 39. http://stackoverflow.com/questions/9454793/haskell-offline-documentation 40. http://stackoverflow.com/questions/9548913/program-design-in-haskell-how-to…

1 0

puzzling polymorphism behavior (7.0.3 windows)
by gladstein＠gladstein.com 16 Mar '12

16 Mar '12

Why does the following program compile and produce the results it does? It seems like 3 and "x" got interpreted as the same type, (). Thanks in advance for your help. import Data.IORef import System.IO.Unsafe cell = unsafePerformIO $ newIORef [] push i = modifyIORef cell (++ [i]) main = do push 3 push "x" readIORef cell >>= return *Main> :browse cell :: GHC.IORef.IORef [a] push :: a -> IO () main :: IO [a] *Main> main [(),()]

4 3

Trying to use more than one array in runSTUArray
by Juan Miguel Vilar 15 Mar '12

15 Mar '12

Hello, café: I am trying to use more than one array with runSTUArray but I don't seem to be able to understand how it works. My first try is this: test1 n = runSTUArray $ do a <- newArray (1, n) (2::Int) b <- newArray (1, n) (3::Int) forM_ [1..n] $ \i -> do v <- readArray a i writeArray b i (v+1) return b but it does not work. However, when I write test2 n = runSTUArray $ do let createArray v n = newArray (1, n) (v::Int) a <- createArray 2 n b <- createArray 0 n forM_ [1..n] $ \i -> do v <- readArray a i writeArray b i (v+1) return b everything is fine although I expected the two versions to be equivalent. To further complicate matters, the following createArray v n = newArray (1, n) (v::Int) test3 n = runSTUArray $ do a <- createArray 2 n b <- createArray 3 n forM_ [1..n] $ \i -> do v <- readArray a i writeArray b i (v+1) return b does not work either. Where can I find an explanation for this behaviour? Furthermore, what I am after is to use two arrays with different types (Int and Bool), is it possible? Thanks in advance, Juan Miguel -- Juan Miguel Vilar Torres Profesor titular de universidad Vicedirector de la ESTCE para ITIG e ITIS Departamento de Lenguajes y Sistemas Informáticos Escuela Superior de Tecnología y Ciencias Experimentales Universitat Jaume I Av. de Vicent Sos Baynat s/n 12071 Castelló de la Plana (Spain) Tel: +34 964 72 8365 Fax: +34 964 72 8435 jvilar(a)lsi.uji.es

3 5

HXT: how to get sibling element
by Никитин Лев 15 Mar '12

15 Mar '12

1 0

Functional programming podcast
by Christopher Done 15 Mar '12

15 Mar '12

Show of hands, who would be interested in working on a podcast weekly or biweekly and what would you like to provide? Light banter is an acceptable answer. Some points that might be covered on such a podcast might be: * Latest FP conferences/hackathons/etc * Competitions * Interesting papers (new, or old and dug up) * Interesting blog posts or stackoverflow questions * New and interesting libraries/tech released * Developments in the communities, funding, business developments * Interviews with prominent FP chappies This bleeds over a little with Haskell Weekly News, but it's more like FP weekly "interesting stuff", and therefore there would be a lot more news to pack in. It would be something to listen to while you're walking to work with your ipod. On 21 February 2012 15:55, Mats Rauhala <mats.rauhala(a)gmail.com> wrote: > On 15:15 Tue 21 Feb , Christopher Done wrote: >> I recently thought it would be Pretty Cool to make an FP podcast, sort >> of a spoken Haskell Weekly News but covering all FP, blogs, packages, >> conferences, papers, standards, mailing lists, even stackoverflow, >> whatever's interesting in FP. We could use Gtalk or Mumble (both quite >> high quality audio) to conduct it and cut it up in audacity. > > I for one would be an interested listener

6 8

HXT: how to get sibling element
by Никитин Лев 15 Mar '12

15 Mar '12

Hello, haskellers. Suppose we have this xml doc (maybe, little stupid): <div> <span>Some story</span> <span>Description</span>: This story about... <span>Author</span>: Tom Smith </div> In the end I whant to get list: [("Title", "Some story"), ("Description","This story about..."), ("Author", "Tom Smith")], or, maybe this: Book "Some story" [("description","This story about..."), ("Author", "Tom Smith")] (Book = Book String [(String, String)]. First span is a special case then others and I undestand how to process it: =============== import Text.XML.HXT.Core import Text.XML.HXT.Curl import Text.XML.HXT.HTTP pageURL = "http://localhost/test.xml" main = do r <- runX (configSysVars [withCanonicalize no, withValidate no, withTrace 0, withParseHTML no] >>> readDocument [withErrors no, withWarnings no, withHTTP []] pageURL >>> getChildren >>> isElem >>> hasName "div" >>> listA (getChildren >>> hasName "span") >>> getTitle <+> getSections) putStrLn "Статьи:" putStr "<" mapM_ putStr $ map (\i -> (fst i) ++ ": " ++ (snd i) ++ "| ") r putStrLn ">" getTitle = arr head >>> getChildren >>> getText >>> arr trim >>> arr ("Title",) getSections = arr tail >>> unlistA >>> ((getChildren >>> getText >>> arr trim) &&& (getChildren >>> getText >>> arr trim)) ltrim [] = [] ltrim (' ':x) = ltrim x ltrim ('\n':x) = ltrim x ltrim ('\r':x) = ltrim x ltrim ('\t':x) = ltrim x ltrim x = x rtrim = reverse . ltrim . reverse trim = ltrim . rtrim =================== And I' get list: [("Title", "Some story"), ("Description","Description"), ("Author", "Author")] (Maybe, there is a better way to get this list?) But I cannot find a way to get text that followes some span. I suppose that I have to use function from Data.Tree.NavigatableTree.XPathAxis, but I don't "puzzle out" how to do it. Please, help me.

2 1

Re: [Haskell-cafe] Unable to call function from DLL using FFI
by rajendra prasad 15 Mar '12

15 Mar '12

Hi, Thanks for very quick reply Claude. Your suggestion resolved the issue. Using extern "C" worked for me. Thank you very much. Thanks to all. Regards, Rajendra > > ---------- Forwarded message ---------- > From: Claude Heiland-Allen <claude(a)goto10.org> > Date: Wed, Mar 14, 2012 at 7:43 PM > Subject: Re: [Haskell-cafe] Unable to call function from DLL using FFI > To: haskell-cafe(a)haskell.org > > > On 14/03/12 14:01, rajendra prasad wrote: > >> My c++ code(HelloWorld.cpp) looks like this: >> > > Try adding extern "C" { ... } to use the C ABI instead of a C++ ABI (which > usually features symbol name mangling to add type information, among other > things). (This may not solve the entire problem, but is an important > puzzle-piece). > > > __declspec(dllexport) int doubleMe(int a) >> { >> return 2*a; >> } >> > > > Claude > > ______________________________**_________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/**mailman/listinfo/haskell-cafe<http://www.haskell.org/mailman/listinfo/haskell-cafe> > >

1 0

Mapping string to a function
by Haisheng Wu 15 Mar '12

15 Mar '12

Hi there, Do you have any comments / suggestions for the following scenario? I have two list and a function over list testdata :: [Int] testdata2 :: [Int] f testdata = map g testdata What I like to do is choosing what test data via command line arguments. i.e. test.hs testdata2 will run against testdata2 I could make it using pattern match between argument and data definition but it is annoy. code here: https://github.com/freizl/dive-into-haskell/blob/master/sandbox/one-in-arit… I'm wondering it can be done simply in haskell. Thanks a lot. -Haisheng

3 3

A new home for haskell-mode
by Johan Tibell 15 Mar '12

15 Mar '12

Hi all, After a discussion with Philip Weaver and a few other interested parties in the community we've moved Philip's repo from https://github.com/pheaver/haskell-mode to the new official location at: https://github.com/haskell/haskell-mode You can file bugs/feature requests at: https://github.com/haskell/haskell-mode/issues Hosting the repo under the Haskell GitHub organization gives us a few benefits: * It's easier to find. (We'll update all the references out there.) * We can add co-maintainers, to offload Philip and make sure pull requests get merged in a timely manner. * The repo doesn't have to move if Philip no longer has time to maintain haskell-mode some time in the future. Cheers, Johan

1 0

using mutable data structures in pure functions
by E R 15 Mar '12

15 Mar '12

Consider the following idea for implementing pure functions: A pure function can allocate and modify memory as long as a) it never returns a reference to the memory or b) it never again modifies the memory once it returns (i.e. it returns an immutable object). This is based on the idea of "value objects" in object-oriented languages - the only mutation of a value object occurs in the constructor. Once the constructor is finished initializing the object it becomes immutable. My first question is whether or not this is accurate or does it need some qualifications / fixing up. Secondly, where does this idea fit into the Haskell philosophy? For example, consider the definition of Data.List.nub: nub l = nub' l [] where nub' [] _ = [] nub' (x:xs) ls | x `elem` ls = nub' xs ls | otherwise = x : nub' xs (x:ls) Clearly the memory allocated to ls never escapes nub', so it seems that ls could be replaced with a mutable data structure (with an eye towards improving performance in special cases). For another example, consider Data.Map.fromList. I kind of expected fromList to build up the map using a mutable data structure and then "seal it up" before returning it, but it seems to call the same insert that one would call to add to the map after it has been constructed. So, again, what is the Haskell philosophy towards using mutable data structures in pure functions? Is it: 1. leave it to the compiler to find these kinds of opportunities 2. just use the immutable data structures - after all they are just as good (at least asymptotically) 3. you don't want to use mutable data structures because of _____ 4. it does happen, and some examples are ______ Thanks, ER

5 5

[ANNOUNCE] JuicyPixels 1.1 - image loading library (maintenance release)
by Vincent Berthoux 14 Mar '12

14 Mar '12

Hello list, This is a maintenance release without new functionality, the central change is the switch from Haskell Array to Data.Vector (to ease it's use for OpenGL texture loading). Juicy.Pixels 1.1 change log : - Fixing automatic bitmap decoding, the decodeImage function wasn't trying to decode bitmap - Converting Haskell array types to Data.Vector (thanks to Jason Dagit) - Some tiny documentation enhancement Hackage : http://hackage.haskell.org/package/JuicyPixels GitHub : https://github.com/Twinside/Juicy.Pixels Thanks Vincent Berthoux

1 1

An idea to document inter department dependencies in Haskell
by C K Kashyap 14 Mar '12

14 Mar '12

My dear Haskell folks, I work in a software company where I develop components that go into a really complex system that's built of several components developed by different teams that are geographically distributed. The components themselves run in a distributed manner across client and several servers. All our design documents are in wiki's (fashionably so). As a result of the above situation and the fact that our code base is not in Haskell, we are almost always dealing with "Oh I did not know this would effect that" and "Oh I have no clue what all this change will impact". I've been wondering if it would be a good idea to try and create a spec for the system in Haskell, such that would could get a better handle on the dependencies. Perhaps an EDSL, that would allow Product Managers to introduce new requirements - compilation failures would indicate the areas that would need to be touched. How is it different from having a spec in a diagram - well, in that case, we are requiring humans to look at the diagram to detect dependencies instead of the compiler telling me about the dependencies. I am about to start off with some implementation to capture my idea - I can articulate it better then. However, I just wanted to throw it out there to check if anyone's had some thought in this direction or if there is some prior art here. Regards, Kashyap

2 2

Is there a better way to subtyping?
by Jeff Shaw 14 Mar '12

14 Mar '12

More specifically, if I have a record type from which I construct multiple sub-record types, and I want to store these in a collection which I want to map over while preserving the ability to get at the sub-fields, is there a better way to do it than to have an enumeration for the sub-types and then use Dynamic? I also have a nastier version that doesn't require the enumeration, which throws an exception when fromDynamic can't return a value with one of the expected types. {-# LANGUAGE Rank2Types, DeriveDataTypeable #-} module Super where import Data.Dynamic import Data.Typeable import Data.Maybe data Super a = Super { commonFields :: (), subFields :: a } deriving Typeable data SubTypes = SubA | SubB | SubC data A = A { aFields :: () } deriving Typeable data B = B { bFields :: () } deriving Typeable data C = C { cFields :: () } deriving Typeable doSomethingWithSubType :: (Super A -> ()) -> (Super B -> ()) -> (Super C -> ()) -> (SubTypes, Dynamic) -> Maybe () doSomethingWithSubType a _ _ (SubA, dynamic) = fromDynamic dynamic >>= return . a doSomethingWithSubType _ b _ (SubB, dynamic) = fromDynamic dynamic >>= return . b doSomethingWithSubType _ _ c (SubC, dynamic) = fromDynamic dynamic >>= return . c doSomethingWithSubType2 :: (Super A -> ()) -> (Super B -> ()) -> (Super C -> ()) -> Dynamic -> () doSomethingWithSubType2 a b c dynamic = let dynamicAsA = fromDynamic dynamic :: Maybe (Super A) dynamicAsB = fromDynamic dynamic :: Maybe (Super B) dynamicAsC = fromDynamic dynamic :: Maybe (Super C) in head $ catMaybes [ dynamicAsA >>= return . a , dynamicAsB >>= return . b , dynamicAsC >>= return . c]

5 4

Adding type annotations to an AST?
by Stephen Tetley 14 Mar '12

14 Mar '12

Hello All How do I add type annotations to interior locations in an abstract syntax tree? I have a small ML language where programs are a single expression, but the expression type has let-rec so it supports recursive function definitions. Source programs have no type annotations. I want to annotate the lec-rec with type annotations, but my type inference is a simple implementation of Algorithm J so it seems "monolithic" to me - i.e. the type it infers is the type of the whole program, I would also like the types of any internal let-rec definitions so I can label my AST. What is the idiom for accomplishing this? I'm using Eijiro Sumii's MinCaml as reference - this embeds mutable references for types in the AST. So this appears a non-starter. My other thought is to uniquely label all type locations during parsing then collect them in a map (location -> type) whilst computing the outermost type - I presume I can identify inner types as they are resolved during Algorithm J's run... Any solutions? Many thanks Stephen

2 3

Unable to call function from DLL using FFI
by rajendra prasad 14 Mar '12

14 Mar '12

Hi, I am new to Haskell and I need to call a c function by loading dynamic link library in Haskell. I started with very simple code to create a dll in visual studio 2008 and then trying to load it in haskell. I am listing here the steps I follwed to achieve this: *Step 1: * My c++ code(HelloWorld.cpp) looks like this: __declspec(dllexport) int doubleMe(int a) { return 2*a; } I created the dll using following command: cl /D_USRDLL /D_WINDLL HelloWorld.cpp /link /dll /out:LoadDLL.dll *Step 2:* My Haskell code(Main.hs) looks like this: {-# LANGUAGE ForeignFunctionInterface #-} module Main where import Foreign import Foreign.C foreign import ccall "LoadDLL doubleMe" twice :: CInt -> IO (CInt) main = do let x = 4 y <- twice x print y I am compling haskell code using following command: ghci Main.hs -L. -lLoadDLL I am finally getting the following error: During interactive linking, GHCi couldn't find the following symbol: doubleMe This may be due to you not asking GHCi to load extra object files, archives or DLLs needed by your current session. Restart GHCi, specifying the missing library using the -L/path/to/object/dir and -lmissinglibname flags, or simply by naming the relevant files on the GHCi command line. Alternatively, this link failure might indicate a bug in GHCi. If you suspect the latter, please send a bug report to: glasgow-haskell-bugs(a)haskell.org Please help me in identifying the mistake I am doing. If possible please let me know the exact steps to be followed to solve this issue. Thank you so much in advance, Regards, Rajendra

2 1

Why so many strings in Network.URI, System.Posix and similar libraries?
by Jason Dusek 14 Mar '12

14 Mar '12

The content of URIs is defined in terms of octets in the RFC, and all Posix interfaces are byte streams and C strings, not character strings. Yet in Haskell, we find these objects exposed with String interfaces: > :info Network.URI.URI data URI = URI {uriScheme :: String, uriAuthority :: Maybe URIAuth, uriPath :: String, uriQuery :: String, uriFragment :: String} -- Defined in Network.URI > :info System.Posix.Env.getEnvironment System.Posix.Env.getEnvironment :: IO [(String, String)] -- Defined in System.Posix.Env But there is no law that environment variables must be made of characters: :; export x=$'\xFF' ; echo -n $x | xxd -p ff :; locale LANG="en_US.UTF-8" That the relationship between bytes and characters can be confusing, both in working with UNIX and in dealing with web protocols, is undeniable -- but it seems unwise to limit the options available to Haskell programmers in dealing with these systems. -- Jason Dusek pgp // solidsnack // C1EBC57DC55144F35460C8DF1FD4C6C1FED18A2B

5 10

Re: [Haskell-cafe] using mutable data structures in pure functions
by Ryan Ingram 14 Mar '12

14 Mar '12

On Sun, Mar 11, 2012 at 8:38 PM, E R <pc88mxer(a)gmail.com> wrote: > A pure function can allocate and modify memory as long as a) it never > returns a reference to the memory or b) it never again modifies the > memory once it returns (i.e. it returns an immutable object). > That's a reasonable first approximation to the problem, yes. It gets a bit more complicated due to laziness (what if the mutation gets delayed until some later part of the output is examined?) So, again, what is the Haskell philosophy towards using mutable data > structures in pure functions? Is it: > > 1. leave it to the compiler to find these kinds of opportunities > 2. just use the immutable data structures - after all they are just as > good (at least asymptotically) > 3. you don't want to use mutable data structures because of _____ > 4. it does happen, and some examples are ______ > There are two ways in which Haskell encourages the use of mutable data structures in a pure way. The first is in the inherent mutation caused by laziness. For example: type Positive = Integer -- trie of binary representation of positive number -- [1] -> tOne -- x ++ [0] -> lookup x . tEven -- x ++ [1] -> lookup x . tOdd data Trie a = Trie { tOne :: a, tEven :: NatTrie a, tOdd :: NatTrie a } lookupTrie :: Trie a -> Positive -> a lookupTrie t 1 = tOne t lookupTrie t n | even n = lookupTrie (tEven t) (n `div` 2) | otherwise = lookupTrie (tOdd t) (n `div` 2) -- div drops remainder makeTrie :: (Positive -> a) -> Trie a makeTrie f = Trie (f 1) e o where e = makeTrie $ \n -> f (2*n) o = makeTrie $ \n -> f (2*n + 1) memoize :: (Positive -> a) -> (Positive -> a) memoize = lookupTrie . makeTrie collatz_rec :: (Positive -> Integer) -> Positive -> Integer collatz_rec f 1 = 0 collatz_rec f n | even n = 1 + f (n `div` 2) | otherwise = 1 + f (3*n + 1) collatz = memoize (collatz_rec collatz) In this case, makeTrie creates a thunk, and it's only evaluated where requested by lookupTrie. You can call collatz at many different values and later calls will be much faster, as the mutation caused by lazy evaluation 'remembers' the values. The second is by explicitly documenting that you are using a temporarily mutable structure, which is the ST monad: instance Monad (ST s) newSTRef :: a -> ST s (STRef s a) readSTRef :: STRef s a -> ST s a writeSTRef :: STRef s a -> a -> ST s () -- and similar interface for mutable STArrays runST :: (forall s. ST s a) -> a -- note higher rank type A computation in the ST monad is an impure computation that can modify memory, but only memory allocated within that same computation. The higher rank type in runST makes it safe to do so--references from one ST computation cannot escape to any other ST computation. So even though internally some pure value might rely on an impure computation, it's safe to do so in a pure context. Here's a sample implementation of ST: -- DO NOT EXPORT THESE CONSTRUCTORS newtype ST s a = ST (IO a) newtype STRef s a = STRef { getRef :: IORef a } runST :: (forall s. ST s a) -> a runST (ST act) = unsafePerformIO act -- Actually safe! newSTRef :: a -> ST s (STRef s a) newSTRef a = ST $ liftM STRef (newIORef a) readSTRef :: STRef s a -> ST s a readSTRef (STRef r) = ST $ readIORef r writeSTRef :: STRef s a -> a -> ST s () writeSTRef (STRef r) a = ST $ writeIORef r a There's some usage examples at http://www.haskell.org/haskellwiki/Monad/ST -- ryan

1 0

Polymorphic addition function with variable number of arguments?
by Some One 13 Mar '12

13 Mar '12

Dear Haskellers, I'm trying to define a polymorphic "add" function which takes a variable number of arguments (instance of Num) and returns their sum. I don't want to specify the types of the arguments while calling the function, I just want to , at most, specify the return type (*it should infer that the return type is the type of the arguments*). class Add a b where add :: a -> b instance Num a => Add a a where add = id instance (Num a, Add a b) => Add (a -> b) where add x y = add (x + y) Can someone tell me why this is not working and propose a fix for this, if possible? Thanks in advance

2 3

Inecxact map?
by Morten Olsen Lysgaard 13 Mar '12

13 Mar '12

I'm running a project where i want to generate a map of a rather large data collection. The map is if the form: Data.Map.Map String a I'd like to be able to do inexact lookups on the map. Firstly, ignore the difference between upper lower case, that's easy. But secondly, have a function: search :: (Num b) => Data.Map.Map String a -> String -> b -> [a] This function is like the normal lookup, but it takes an extra argument b. B is the allowed character error rate. That is the Levenstein distance divided on the length. It should return all the strings that has an error rate lower that b and in a ordered manner, best first. Is it possible to design such a datastructure efficiently? Does there already exist something like this? If not, where would i begin to create this? Any advices or knowledge appreciated. -- Morten

3 2

cabal test-suite library recompilation
by Ozgur Akgun 13 Mar '12

13 Mar '12

Hi all, I've recently started to move my tests to use the new cabal test-suite framework. Old setup: The cabal file compiles one library + a few executables. The library contains almost all of the code and exposes the necessary modules for use by the executables. The executables mainly consist of one main file. This way, the library is only compiled once and used multiple times by the executables. Great. One of the executables compile a test runner, and is handled similarly. New setup, using test-suite: I've deleted the executable section for the test runner and added a test-suite section instead. To my surprise, the library is compiled twice now. Hence, it takes twice as long. And it is a clear regression compared to my old setup. While waiting for a build to finish [ :) ], I just wanted to write an email and check if this is intentional or only an oversight. Or maybe a technical limitation for the time being? Thanks, Ozgur

2 2

ANNOUNCE: join (UHac <$> DHD)
by Sean Leather 13 Mar '12

13 Mar '12

You can still join us at the DHD >>= UHac, but the time is running out. http://www.haskell.org/haskellwiki/DHD_UHac Deadline: April 1 (no fooling!) We've also have a few updates: * We put the DHD program up, at least as much of it as we have. http://www.haskell.org/haskellwiki/DHD_UHac/DHD_Program * As you can see from the program, there is still room for more speakers. In particular, we'd like to hear from people coming outside Utrecht and the Netherlands. We'd also like to see more lightning talks. Send Sean an email with your idea: leather(a)cs.uu.nl * UHac on Friday has moved to a "new" location. Actually, it's the same location as on Saturday and Sunday, so that should make things easier. See you in Utrecht! The Organizers: Sean Leather Jurriën Stutterheim Jurriaan Hage P.S. Make sure your Monad is also a Functor. P.P.S. Apologies for multiple copies.

1 0

Re: [Haskell-cafe] Empty Input list
by Kevin Clees 13 Mar '12

13 Mar '12

Hey Chris, thank you for your help! Your last comment with the (!!)-thing was a very good idea! Now my function looks like this: tmp:: [(Int, Int)] -> Int -> (Int, Int) tmp [] y = (0,0) tmp xs y = xs !! (y-1) If the function returns (0,0) it will blocked by another function. If I want to use the "maybe" return, I get some new trouble that I don't like - so I chose that way :) Thank you again Kevin

5 4

Leksah Install Issue (cabal experts please help)
by Hamish Mackenzie 13 Mar '12

13 Mar '12

pozic raised an install issue on IRC. Basically he was getting... ~$ cabal install leksah-server Resolving dependencies... cabal: Couldn't read cabal file "leksah-server/0.12.0.3/leksah-server.cabal" To reproduce this issue. Use GHC 6.12.3, 7.0.3 or 7.2.2 to "cabal install cabal-install" then run "cabal install leksah-server". To work around it install GHC 7.0.4 or 7.4.1 and use it to run "cabal install cabal-install" (then you can go back to the old GHC if you want). Cabal experts, is there a better work around than this? The current leksah-server.cabal requires Cabal 1.10.2 or greater because it has a "test-suite" section for "cabal test". This is fine if you have installed cabal-install with GHC 7.0.4 or 7.4.1, but if you install cabal-install with older versions of GHC (including 7.2.2) then it can't handle the file format. I could upload a new leksah-server to hackage (with the unit tests removed), but that would not stop cabal-install from failing to read leksah-server 0.12.0.3 and bombing out (there is already a 0.12.0.4 and it is not getting to that one). Even if you "cabal unpack" and then modify the .cabal file and run "cabal install" it still fails with the same error. If you can think of a better solution than asking users to install GHC 7.0.4 or 7.4.1 to build cabal-install, then please let me know.

2 7

Empty Input list
by Kevin Clees 12 Mar '12

12 Mar '12

Dear Haskell friends, what can I do, if a function gets an empty input list? I want, that it only returns nothing. This is my source code: tmp:: [(Int, Int)] -> Int -> (Int, Int) tmp (x:xs) y | y == 1 = x | y > 1 = tmp xs (y-1) If this function gets an empty list, he throws "Exception: sortAlgo.hs:(18,1)-(21,44): Non-exhaustive patterns in function Main.tmp" *Main> tmp [(1,2),(3,2)] 1 (1,2) *Main> tmp [(1,2),(3,2)] 2 (3,2) *Main> tmp [] 1 *** Exception: sortAlgo.hs:(20,1)-(22,44): Non-exhaustive patterns in function Main.listElementIntInt Thank you for any help ! Best regards Kevin

2 2

Re: [Haskell-cafe] IO() and other datatypes
by Kevin Clees 12 Mar '12

12 Mar '12

Great, Thank you !

1 0

Helper classes for Generics
by Reiner Pope 12 Mar '12

12 Mar '12

Hi all, I've been playing with GHC's new generics features (see http://www.haskell.org/ghc/docs/latest/html/users_guide/generic-programming…) All the documentation I've seen suggests creating a "helper class" -- for instance, the GSerialize class in the above link -- on which one defines generic instances. It seems to me that this isn't necessary. For example, here's the the example from the GHC docs, but without a helper class: > -- set the phantom type of Rep to (), to avoid ambiguity > from0 :: Generic a => a -> Rep a () > from0 = from > > data Bit = O | I > > class Serialize a where > put :: a -> [Bit] > > default put :: (Generic a, Serialize (Rep a ())) => a -> [Bit] > put = put . from0 > > instance Serialize (U1 x) where > put U1 = [] > > instance (Serialize (a x), Serialize (b x)) => Serialize ((a :*: b) x) where > put (x :*: y) = put x ++ put y > > instance (Serialize (a x), Serialize (b x)) => Serialize ((a :+: b) x) where > put (L1 x) = O : put x > put (R1 x) = I : put x > > instance (Serialize (a x)) => Serialize (M1 i c a x) where > put (M1 x) = put x > > instance (Serialize a) => Serialize (K1 i a x) where > put (K1 x) = put x Is there a reason to prefer using helper classes? Or perhaps we should update the wiki page (http://www.haskell.org/haskellwiki/Generics) to avoid using helper classes? Regards, Reiner

3 5

ANNOUNCE: pipes-core 0.0.1
by Paolo Capriotti 12 Mar '12

12 Mar '12

I'm pleased to announce the release of version 0.0.1 of pipes-core, a library for efficient, safe and compositional IO, similar in scope to iteratees and conduits. http://hackage.haskell.org/package/pipes-core This is a reimplementation of the original 'Pipe' concept by Gabriel Gonzales. The package documentation contains an introduction to Pipes and a detailed tutorial, so here I'll just outline some of the differences from the original version: - A single composition operator. "Strict" composition has been removed, since its role wasn't actually that clear, and its purpose for resource finalization has been replaced by better constructs (see below). - A new primitive 'tryAwait', which makes it possible to recover from termination of an upstream pipe. Using 'tryAwait' you can write pipes that are able to consume all their input and return a value (like iteratees or Conduit's sinks can), and stateful pipes that can make use of their final state before terminating. - Full exception safety and exception-handling primitives. Pipes have been augmented with 'catch' and 'finally' primitives, which allow you to recover from exceptions and ensure finalization of resources within the Pipe monad. There's no more need to use resource-simple or monadic regions together with Pipes. - Multi-channel pipes. This is a generalization of Arrows using sums instead of pairs for contexts. We provide a number of combinators that allow you to combine Pipes in much the same way as Arrows, although there is not unfortunately an alternative for the Arrow syntax. Together with pipes-core, I also released a number of accessory packages with various utilities. Here is a complete list: * pipes-extra: File readers and writers, chunk pipes. http://hackage.haskell.org/package/pipes-extra * pipes-attoparsec: Utilities to convert a parser into a pipe. http://hackage.haskell.org/package/pipes-attoparsec * pipes-conduit: Conduit adapters. http://hackage.haskell.org/package/pipes-conduit * pipes-network: Utilities to deal with sockets. Ported from conduit. http://hackage.haskell.org/package/pipes-network * pipes-zlib: Pipes to deal with zipped data. Ported from conduit. http://hackage.haskell.org/package/pipes-zlib This is an experimental release, but it should be equipped with all the functionality needed to write serious applications. I encourage people to try it out and send feedback if anything looks too simplistic or too limiting. BR, Paolo Capriotti

4 25

Global Arrays
by Clark Gaebel 12 Mar '12

12 Mar '12

In Haskell, what's the canonical way of declaring a top-level array (Data.Vector of a huge list of doubles, in my case)? Performance is key in my case. The straightforward way would just be something like: globalArray :: V.Vector Double globalArray = V.fromList [ huge list of doubles ] {-# NOINLINE globalArray #-} However, I don't want to have to run the fromList at runtime. Not only would this mean a bigger executable (having to store a linked list, instead of an array), it would be quite inefficient since we don't even use the source list! Therefore, I was thinking of storing the array in a C file: static const double globalArray[] = { huge list of doubles }; double* getGlobalArray() { return globalArray; } int getGlobalArraySize() { return sizeof(globalArray)/sizeof(globalArray[0]); } And importing it in haskell witht he FFI, followed with an unsafeCast: foreign import ccall unsafe "getGlobalArray" c_globalArray :: Ptr CDouble foreign import ccall unsafe "getGlobalArraySize" c_globalArraySize :: CInt globalArray :: V.Vector Double globalArray = V.unsafeCast $ unsafeFromForeignPtr0 (unsafePerformIO $ newForeignPtr_ c_globalArray) (fromIntegral c_globalArraySize) {-# NOINLINE globalArray #-} But this version (clearly) is full of "unsafe"ty. Is there a better way that I haven't thought of? Regards, - clark

5 8

Question about concurrency, threads and GC
by Paul Graphov 12 Mar '12

12 Mar '12

Hello Cafe! I am trying to implement networked application in Haskell. It should accept many client connections and support bidirectional conversation, that is not just loop with Request -> Response function but also sending notifications to clients etc. NB: I came from C++ background and used to explicit asynchonous operations and pools of OS threads. (Boost.Asio + Boost.Thread) There seem to be lots of goodies around (green threads, STM). The problem is that I cannot figure out how to design such applications. First of all now I have to start some threads for each client. Client state and behaviour may change depending on data that comes from it or other clients. So it seems that I have to start at least three threads: one that reads data from socket, one that sends queued messages to the socket and one that represents clients main behaviour loop. The last one will supposedly read from some STM sources, one of them being TChan/TMVar filled by thread that reads socket. Here I get stuck: how to manage that threads? I tried to search STM examples at hackage and the most interesting packages were: 1) combinatorrent - quite complex application, but it seem to be highly Erlang-inspired, so I am not sure that it is the simplest way of doing thing. 2) network-connection package - it is a library that wraps socket and uses STM internally but exposes almost the same interface with blocking operations. Thread management is quite explicit there. But this package is marked obsolete. I wondered if one can rely upon garbage collector to reap dead threads when using STM? Or is it too error prone (accidental references to STM object that prevent it from being collected) and unpredictable? I have read this post: http://blog.ezyang.com/2011/07/blockedindefinitelyonmvar/ It shows that relying upon handling BlockedIndefinitelyOn* exceptions is a bad idea but is it OK just to rely upon that blocked forever thread will die and not to kill it explicitly? And also if it is really reliable, what are exact rules when some object goes out of scope and becomes collectable? For example in blog post mentioned above in "main1" example "lock" variable is accessible until the end of do-block, i.e. if I add "takeMVar lock" line to its end, it will compile. But it is not used actually. Is it guaranteed that explicit performGC will reap it? Possibly the best answer would be just pointing me to some recent examples of concurrent network applications. Thanks!

5 9

Understanding GC time
by Thiago Negri 12 Mar '12

12 Mar '12

Hi all. I wrote a very simple program to try out parallel Haskel and check how it would look like to make use of more than one core in this language. When I tried the program with RTS option -N1, total time shows it took 2.48 seconds to complete and around 65% of that time was taken by GC. Then I tried the same program with RTS options -N2 and total time decreased to 1.15 seconds as I expected a gain here. But what I didn't expect is the GC time to drop to 0%. I guess I'm having trouble to understand the output of the RTS option -s. Can you enlighten me? The source for the testing program: > module Main where > > import Data.List (foldl1') > import Control.Parallel (par, pseq) > import Control.Arrow ((&&&)) > > f `parApp` (a, b) = a `par` (b `pseq` (f a b)) > seqApp = uncurry > > main = print result > where result = (+) `parApp` minMax list > minMax = minlist &&& maxlist > minlist = foldl1' min > maxlist = foldl1' max > list = [1..19999999] The results on a Windows 7 64bits with an Intel Core 2 Duo, compiled with GHC from Haskell Platform: c:\tmp\hs>par +RTS -s -N1 par +RTS -s -N1 20000000 803,186,152 bytes allocated in the heap 859,916,960 bytes copied during GC 233,465,740 bytes maximum residency (10 sample(s)) 30,065,860 bytes maximum slop 483 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 1523 collections, 0 parallel, 0.80s, 0.75s elapsed Generation 1: 10 collections, 0 parallel, 0.83s, 0.99s elapsed Parallel GC work balance: nan (0 / 0, ideal 1) MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 0.00s ( 0.90s) 0.00s ( 0.06s) Task 1 (worker) : 0.00s ( 0.90s) 0.00s ( 0.00s) Task 2 (bound) : 0.86s ( 0.90s) 1.62s ( 1.69s) SPARKS: 1 (0 converted, 0 pruned) INIT time 0.00s ( 0.00s elapsed) MUT time 0.86s ( 0.90s elapsed) GC time 1.62s ( 1.74s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2.48s ( 2.65s elapsed) %GC time 65.4% (65.9% elapsed) Alloc rate 936,110,032 bytes per MUT second Productivity 34.6% of total user, 32.4% of total elapsed gc_alloc_block_sync: 0 whitehole_spin: 0 gen[0].sync_large_objects: 0 gen[1].sync_large_objects: 0 c:\tmp\hs>par +RTS -s -N2 par +RTS -s -N2 20000000 1,606,279,644 bytes allocated in the heap 74,924 bytes copied during GC 28,340 bytes maximum residency (1 sample(s)) 29,004 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 1566 collections, 1565 parallel, 0.00s, 0.01s elapsed Generation 1: 1 collections, 1 parallel, 0.00s, 0.00s elapsed Parallel GC work balance: 1.78 (15495 / 8703, ideal 2) MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 0.00s ( 0.59s) 0.00s ( 0.00s) Task 1 (worker) : 0.58s ( 0.59s) 0.00s ( 0.01s) Task 2 (bound) : 0.58s ( 0.59s) 0.00s ( 0.00s) Task 3 (worker) : 0.00s ( 0.59s) 0.00s ( 0.00s) SPARKS: 1 (1 converted, 0 pruned) INIT time 0.00s ( 0.00s elapsed) MUT time 1.15s ( 0.59s elapsed) GC time 0.00s ( 0.01s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 1.15s ( 0.61s elapsed) %GC time 0.0% (2.4% elapsed) Alloc rate 1,391,432,695 bytes per MUT second Productivity 100.0% of total user, 190.3% of total elapsed gc_alloc_block_sync: 90 whitehole_spin: 0 gen[0].sync_large_objects: 0 gen[1].sync_large_objects: 0

6 7

Associative prefix operators in Parsec
by Troels Henriksen 12 Mar '12

12 Mar '12

Consider a simple language of logical expressions: > import Control.Applicative > import Text.Parsec hiding ((<|>), many) > import Text.Parsec.String > import Text.Parsec.Expr > > data Expr = Truth > | Falsity > | And Expr Expr > | Or Expr Expr > | Not Expr > deriving (Show, Eq) I define a simple expression parser using Parsec: > expr :: Parser Expr > expr = buildExpressionParser table (lexeme term) > <?> "expression" > > term :: Parser Expr > term = between (lexeme (char '(')) (lexeme (char ')')) expr > <|> bool > <?> "simple expression" > > bool :: Parser Expr > bool = lexeme (string "true" *> pure Truth) > <|> lexeme (string "false" *> pure Falsity) > > lexeme :: Parser a -> Parser a > lexeme p = p <* spaces > > table = [ [prefix "!" Not ] > , [binary "&" And AssocLeft ] > , [binary "|" Or AssocLeft ] > ] > > binary name fun assoc = Infix (do{ lexeme (string name); return fun }) assoc > prefix name fun = Prefix (do{ lexeme (string name); return fun }) Now this doesn't work: > test1 = parseTest expr "!!true" But this does: > test2 = parseTest expr "!(!true)" I have studied the code for buildExpressionParser, and I know why this happens (prefix operators are treated as nonassociative), but it seems like one would often want right-associative prefix operators (so test1 would work). Is there a common workaround or solution for this problem? I assume the nonassociativity in Parsec is by design and not a bug. -- \ Troels /\ Henriksen

2 3

Re: [Haskell-cafe] Why so many strings in Network.URI, System.Posix and similar libraries?
by Jason Dusek 12 Mar '12

12 Mar '12

2012/3/11 Thedward Blevins <thedward(a)barsoom.net>: > On Sun, Mar 11, 2012 at 13:33, Jason Dusek <jason.dusek(a)gmail.com> wrote: > > The syntax of URIs is a mechanism for describing data octets, > > not Unicode code points. It is at variance to describe URIs in > > terms of Unicode code points. > > This claim is at odds with the RFC you quoted: > > 2. Characters > > The URI syntax provides a method of encoding data, presumably for the sake > of identifying a resource, as a sequence of characters. The URI characters > are, in turn, frequently encoded as octets for transport or presentation. > This specification does not mandate any particular character encoding for > mapping between URI characters and the octets used to store or transmit > those characters. > > (Emphasis is mine) > > The RFC is specifically agnostic about serialization. I generally agree that > there are a lot of places where ByteString should be used, but I'm not > convinced this is one of them. Hi Thedward, I am CC'ing the list since you raise a good point that, I think, reflects on the discussion broadly. It is true that intent of the spec is to allow encoding of characters and not of bytes: I misread its intent, attending only to the productions. But due to the way URIs interact with character encoding, a general URI parser is constrained to work with ByteStrings, just the same. The RFC "...does not mandate any particular character encoding for mapping between URI characters and the octets used to store or transmit those characters..." and in Section 1.2.1 it is allowed that the encoding of may depend on the scheme: In local or regional contexts and with improving technology, users might benefit from being able to use a wider range of characters; such use is not defined by this specification. Percent-encoded octets (Section 2.1) may be used within a URI to represent characters outside the range of the US-ASCII coded character set if this representation is allowed by the scheme or by the protocol element in which the URI is referenced. It seems possible for any octet, 0x00..0xFF, to show up in a URI, and it is only after parsing the scheme that we can say whether the octet belongs there are not. Thus a general URI parser can only go as far as splitting into components and percent decoding before handing off to scheme specific validation rules (but that's a big help already!). I've implemented a parser under these principles that handles specifically URLs: http://hackage.haskell.org/package/URLb Although the intent of the spec is to represent characters, I contend it does not succeed in doing so. Is it wise to assume more semantics than are actually there? The Internet and UNIX are full of broken junk; but faithful representation would seem to be better than idealization for those occasions where we must deal with them. I'm not sure the assumption of "textiness" really helps much in practice since the Universal Character Set contains control codes and bidi characters -- data that isn't really text. -- Jason Dusek pgp // solidsnack // C1EBC57DC55144F35460C8DF1FD4C6C1FED18A2B

2 1