- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] [Haskell] ANNOUNCE: notcpp-0.0.1
by Henning Thielemann 13 Apr '12

13 Apr '12

On Fri, 13 Apr 2012, Ben Millwood wrote: > But I never liked using CPP: it completely defeats haskell-src-exts > and hence things like SourceGraph, and anyway it's not designed for > Haskell and doesn't at all understand its structure, or fit with its > syntax. With a little thought, I wondered if creative use of template > haskell might not achieve the same goal. It turned out it did, and > emboldened with this knowledge I set out to write a new package making > this technique available to others. I never liked CPP in Haskell, too. The problem is: I also do not like Template Haskell. :-) Whereever possible I use a Cabal flag and a conditional Hs-Source-Dirs. http://www.haskell.org/haskellwiki/Cabal/Developer-FAQ#Adapt_to_different_s…

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is this an arrow?
by Johannes Waldmann 13 Apr '12

13 Apr '12

Dear Cafe, I have the following types: type Computer a b = ( a -> IO ( Maybe b ) ) type Transformer a b c d = Computer a ( b, c -> d ) For example, a SAT solver: minisat :: Computer CNF Assignment, and when I use it to solve an application problem via transformation to SAT, I need t :: Transformer Problem SAT Assignment Solution Now - what is the proper abstraction? Is this an arrow somehow? And is there already a type and a library that would contain useful combinators like some of these: http://autolat.imn.htwk-leipzig.de/gitweb/?p=box;a=blob;f=src/exotic/Strate… Thanks - J.W.

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Can cabal detect property of the platform ?
by Vincent Hanquez 13 Apr '12

13 Apr '12

Hi Cafe, Is there a way to have (some) cabal flags to autoconfigure themselves ? For example, a SSE flag, i want the configure to default to on when the building machine has SSE instruction available, and default to off otherwise. (of course, it wouldn't affect the user ability to override the default with -f) It doesn't appears possible just using the cabal file. Is there a way with the Setup.hs maybe ? or some other cunning plan ? -- Vincent

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open source project for student
by Dan Cristian Octavian 13 Apr '12

13 Apr '12

Hello, I am a second year computer science student who is very interested in working on a haskell open source project. I have no particular focus on a certain type of application. I am open to ideas and to exploring new fields. What kind of project should I look for considering that I am a beginner? (Any particular project proposals would be greatly appreciated). Is the entry bar too high for most projects out there for somebody lacking experience such as me so that I should try getting some experience on my own first? Would it be a better idea to try to hack on my own project rather than helping on an existing one? Thank you very much for your help.

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prettyprint with IO
by Warren Harris 12 Apr '12

12 Apr '12

I wrote a parsec parser that does symbols lookups during the parsing process (ParsecT String Store IO a). Now I'd like to write a pretty printer that does the reverse. Unfortunately there doesn't appear to be a transformer version of Text.PrettyPrint.HughesPJ. Can anyone suggest a way to do this? Thanks, Warren

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I Need a Better Functional Language!
by Grigory Sarnitskiy 12 Apr '12

12 Apr '12

Hello! I've just realized that Haskell is no good for working with functions! First, what are 'functions' we are interested at? It can't be the usual set-theoretic definition, since it is not constructive. The constructive definition should imply functions that can be constructed, computed. Thus these are computable functions that should be of our concern. But computable functions in essence are just a synonym for programs. One could expect from a language that bears 'functional' as its characteristic to be able to do everything imaginable with functions. However, the only thing Haskell can do with functions is to apply them to arguments and to feed them as arguments, run in parallel (run and concatenate programs). Obviously, that's not all of the imaginable possibilities. One also can rewrite programs. And write programs that rewrite programs. And write programs that rewrite programs that rewrite the first programs and so on. But there is no such possibility in Haskell, except for introducing a DSL. So now I wonder, what are the languages that are functional in the sense above? With a reasonable syntax and semantics, thus no assembler. I guess Lisp might be of this kind, but I'm not sure. In addition, I'm not a fan of parentheses. What else? Pure? Mathematica? Maxima? Note, that the reflectivity is important.

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Is type class 'Addressable' already exists.
by 陈文龙 12 Apr '12

12 Apr '12

To get element in List,Map... in python's way. Python: > strMap["apple"] Haskell: > strMap # "apple" https://gist.github.com/2364395 {-# LANGUAGE TypeFamilies #-} module Addressable where import qualified Data.Map as M import Prelude class Addressable a where type Key a type Value a (#!) :: a -> Key a -> Value a (#) :: a -> Key a -> Maybe (Value a) instance Addressable [a] where type Key [a] = Int type Value [a] = a (#!) = (!!) xs # i | i < 0 = Nothing [] # _ = Nothing (x:_) # 0 = Just x (_:xs) # n = xs # (n-1) instance (Ord k) => Addressable (M.Map k v) where type Key (M.Map k v) = k type Value (M.Map k v) = v a #! i = a M.! i a # i = M.lookup i a main :: IO () main = do let strMap = M.fromList [("one","1"),("two","2"),("three","3")] let strList = ["1","2","3"] print $ strMap # "two" -- Just "2" print $ strMap #! "two" -- "2" print $ strList # 0 -- Just "1" print $ strList #! 0 -- "1" print $ strMap # "no-exist" -- Nothing print $ strList # 100 -- Nothing print $ strMap #! "no-exist" -- error print $ strList #! 100 -- error

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Haskell Weekly News: Issue 222
by Daniel Santa Cruz 11 Apr '12

11 Apr '12

Welcome to issue 222 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of April 1 to 7, 2012. Quotes of the Week * tgeeky_: "What I cannot create, I do not understand." -- Richard Feynman, on his blackboard when he died in 1988. * elliott: ... [a] is more of a control structure than a data structure. * Cale: Basically, we've known how to implement first class functions efficiently for 20 or 30 years now, and we've known about their importance to abstraction in programming since before the advent of electronic computers. There's no excuse to still be writing new programs in languages without them. * edwardk: When someone throws an exception at you, duck. * luite: I think it uses levenshtein distance (he invented this distance because nobody could spell his name correctly) * edwardk: 'hey ghc devs can you fix this thing that might or might not be a bug so we can break a huge pile of invariants in your compiler to put go faster stripes on something nobody uses? k thx' Top Reddit Stories * Tfoo, my simple Five in a Row game. Online, with server-sent events, deployed to Heroku, open source. Domain: tfoo.herokuapp.com, Score: 49, Comments: 18 On Reddit: [1] http://goo.gl/Klevh Original: [2] http://goo.gl/PtjKk * Interacting with inner-functions in GHCi [StackOverflow] Domain: stackoverflow.com, Score: 48, Comments: 12 On Reddit: [3] http://goo.gl/23l0G Original: [4] http://goo.gl/2H5kI * ANN: acme-http. 221,693.0 req/s on the PONG benchmark. Domain: groups.google.com, Score: 42, Comments: 4 On Reddit: [5] http://goo.gl/XROlu Original: [6] http://goo.gl/PuOt3 * Recent haskell-mode changes screencast Domain: youtube.com, Score: 39, Comments: 15 On Reddit: [7] http://goo.gl/JyX0s Original: [8] http://goo.gl/VWLIF * An Haskell tutorial. What do you think? Domain: yannesposito.com, Score: 39, Comments: 47 On Reddit: [9] http://goo.gl/eLiQT Original: [10] http://goo.gl/yBDnQ * Galois' Open-Source code available on github Domain: corp.galois.com, Score: 38, Comments: 3 On Reddit: [11] http://goo.gl/xoz4K Original: [12] http://goo.gl/OhlaA * Wadler’s Law Revisited Domain: comonad.com, Score: 27, Comments: 9 On Reddit: [13] http://goo.gl/8p9As Original: [14] http://goo.gl/7iB53 * With Linux supporting x32 ABI, should GHC follow suit? Domain: self.haskell, Score: 23, Comments: 6 On Reddit: [15] http://goo.gl/Nkuob Original: [16] http://goo.gl/Nkuob * cabal-meta: transcending to dependency heaven Domain: yesodweb.com, Score: 22, Comments: 27 On Reddit: [17] http://goo.gl/nsVhn Original: [18] http://goo.gl/U2cDK * [Haskell-cafe] A Modest Records Proposal Domain: haskell.org, Score: 21, Comments: 7 On Reddit: [19] http://goo.gl/MvbRq Original: [20] http://goo.gl/RgaRU Top StackOverflow Questions * What is the best way to test and interact with inner functions defined inside a toplevel function? votes: 44, answers: 1 Read on SO: [21] http://goo.gl/2H5kI * How can a Windows service application be written in Haskell? votes: 15, answers: 1 Read on SO: [22] http://goo.gl/kz8TA * What does ':..' mean in Haskell? votes: 12, answers: 2 Read on SO: [23] http://goo.gl/Whi8a * Is the whole Map copied when a new binding is inserted? votes: 10, answers: 3 Read on SO: [24] http://goo.gl/OabLu * Haskell functions left-to-right votes: 10, answers: 1 Read on SO: [25] http://goo.gl/rhCm3 * How can I avoid writing boilerplate code for functions performing pattern matching? votes: 9, answers: 3 Read on SO: [26] http://goo.gl/6cvXJ * Controlling how test data is generated in QuickCheck votes: 8, answers: 1 Read on SO: [27] http://goo.gl/BNSug * Can I get warnings about overly-restrictive type signatures? votes: 8, answers: 1 Read on SO: [28] http://goo.gl/X7PkN * Slowdown when using parallel strategies in Haskell votes: 8, answers: 1 Read on SO: [29] http://goo.gl/ghV3C * Dealing with boilerplate in Haskell votes: 7, answers: 2 Read on SO: [30] http://goo.gl/Um4Fm Until next time, Daniel Santa Cruz References 1. http://tfoo.herokuapp.com/ 2. http://www.reddit.com/r/haskell/comments/rx20z/tfoo_my_simple_five_in_a_row… 3. http://stackoverflow.com/questions/9987739/what-is-the-best-way-to-test-and… 4. http://www.reddit.com/r/haskell/comments/rr7se/interacting_with_innerfuncti… 5. https://groups.google.com/group/happs/browse_thread/thread/9b078d23b59a7374 6. http://www.reddit.com/r/haskell/comments/ro77b/ann_acmehttp_2216930_reqs_on… 7. http://www.youtube.com/watch?v=E6xIjl06Lr4 8. http://www.reddit.com/r/haskell/comments/rtctu/recent_haskellmode_changes_s… 9. http://yannesposito.com/Scratch/en/blog/Haskell-the-Hard-Way/ 10. http://www.reddit.com/r/haskell/comments/rw61r/an_haskell_tutorial_what_do_… 11. http://corp.galois.com/blog/2012/4/4/galois-open-source-projects-on-github.… 12. http://www.reddit.com/r/haskell/comments/rt8m3/galois_opensource_code_avail… 13. http://comonad.com/reader/2012/wadlers-law-revisited/ 14. http://www.reddit.com/r/haskell/comments/rpyhr/wadlers_law_revisited/ 15. http://www.reddit.com/r/haskell/comments/rpbcx/with_linux_supporting_x32_ab… 16. http://www.reddit.com/r/haskell/comments/rpbcx/with_linux_supporting_x32_ab… 17. http://www.yesodweb.com/blog/2012/04/cabal-meta 18. http://www.reddit.com/r/haskell/comments/rurn4/cabalmeta_transcending_to_de… 19. http://www.haskell.org/pipermail/haskell-cafe/2012-April/100527.html 20. http://www.reddit.com/r/haskell/comments/rnwgn/haskellcafe_a_modest_records… 21. http://stackoverflow.com/questions/9987739/what-is-the-best-way-to-test-and… 22. http://stackoverflow.com/questions/10037654/how-can-a-windows-service-appli… 23. http://stackoverflow.com/questions/9996740/what-does-mean-in-haskell 24. http://stackoverflow.com/questions/10002956/is-the-whole-map-copied-when-a-… 25. http://stackoverflow.com/questions/10003796/haskell-functions-left-to-right 26. http://stackoverflow.com/questions/10024455/how-can-i-avoid-writing-boilerp… 27. http://stackoverflow.com/questions/9977734/controlling-how-test-data-is-gen… 28. http://stackoverflow.com/questions/9986508/can-i-get-warnings-about-overly-… 29. http://stackoverflow.com/questions/10009361/slowdown-when-using-parallel-st… 30. http://stackoverflow.com/questions/9969638/dealing-with-boilerplate-in-hask…

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Nice article on FP and concurrency from a born-again C++ programmer
by Obscaenvs 11 Apr '12

11 Apr '12

http://fpcomplete.com/the-downfall-of-imperative-programming/ /F

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Memory leak from close fields in conduit
by Michael Snoyman 11 Apr '12

11 Apr '12

Hi all, Hideyuki Tanaka alerted me[1] to a memory leak in conduit. Long story short: it appears that Pipe composition leads to collection of a large number of `return ()` actions for unnecessary memory cleanup. We came up with a possible solution: a Finalize type[2]. In both of our testing, this eliminates the memory leak in question. So now I have a few questions: 1. Does anyone have a different idea on how to approach this problem? 2. Are there any refinements to be applied to the solution? 3. Technically, this is an API breakage, but (a) conduit 0.4 was released very recently, (b) this is a fairly important change, and (c) the real-world breakage from this is very minimal. Will anyone call foul if I release such a change as 0.4.1? Just to stress 3c: I compiled all of Yesod against this modified version of conduit, and only http-conduit had to be tweaked at all. Michael [1] https://github.com/snoyberg/conduit/issues/40 [2] https://github.com/snoyberg/conduit/commit/ebc7a68e0f1f0c1b80c0105916b00060…

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