July 2012 - Haskell-Cafe

Is haskell.org down?
by C K Kashyap 05 Jul '12

05 Jul '12

Regards, Kashyap

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Haskell Weekly News: Issue 234
by Daniel Santa Cruz 05 Jul '12

05 Jul '12

Welcome to issue 234 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 24 to 30, 2012. Quotes of the Week * monochrom: shae ericsson is shae erisson's evil twin who prefers erlang to haskell :) * shapr: If I were to do this in Python, it would be done by now. and it would be full of bugs. Top Reddit Stories * Performance: yes, it’s worth looking at the small stuff Domain: serpentine.com, Score: 61, Comments: 11 On Reddit: [1] http://goo.gl/x2pGf Original: [2] http://goo.gl/SHBNB * Awesome Haskell plugins for Vim! Domain: self.haskell, Score: 61, Comments: 32 On Reddit: [3] http://goo.gl/Bm6wI Original: [4] http://goo.gl/Bm6wI * Galois is Hiring! Domain: haskell.org, Score: 56, Comments: 7 On Reddit: [5] http://goo.gl/c7zj8 Original: [6] http://goo.gl/y7nBd * Haskell Symposium 2012 list of accepted papers Domain: haskell.org, Score: 43, Comments: 12 On Reddit: [7] http://goo.gl/Fq0rD Original: [8] http://goo.gl/CGZV3 * Live coding the game of Sokoban with GUI. Please give me your input for future sessions. Domain: youtu.be, Score: 42, Comments: 38 On Reddit: [9] http://goo.gl/HslD9 Original: [10] http://goo.gl/0ZeMA * Mirrored Lenses Domain: comonad.com, Score: 36, Comments: 30 On Reddit: [11] http://goo.gl/UyGco Original: [12] http://goo.gl/9ltWW * The Game of Distributed Systems Programming. Which Level Are You? Incubaid Research Domain: blog.incubaid.com, Score: 30, Comments: 7 On Reddit: [13] http://goo.gl/EYWj7 Original: [14] http://goo.gl/P0vhZ * SugarHaskell: language extensions in sugar modules[pdf] Domain: informatik.uni-marburg.de, Score: 27, Comments: 7 On Reddit: [15] http://goo.gl/Z4fL5 Original: [16] http://goo.gl/I78Md * parallel cabal-install patches Domain: haskell.org, Score: 26, Comments: 4 On Reddit: [17] http://goo.gl/Yr1JR Original: [18] http://goo.gl/P9WPK * Is anybody using Sublime Text for their Haskell editing? Domain: self.haskell, Score: 19, Comments: 16 On Reddit: [19] http://goo.gl/pmor9 Original: [20] http://goo.gl/pmor9 * Runge–Kutta in Haskell (RK4) Domain: stochastix.wordpress.com, Score: 19, Comments: 7 On Reddit: [21] http://goo.gl/9Bd2b Original: [22] http://goo.gl/H27Fv * How to build an example proxy server using conduit 0.5 Domain: yesodweb.com, Score: 19, Comments: 14 On Reddit: [23] http://goo.gl/BHLRd Original: [24] http://goo.gl/PtOxh * Found myself wanting lambda-case again Domain: self.haskell, Score: 16, Comments: 13 On Reddit: [25] http://goo.gl/LIOKa Original: [26] http://goo.gl/LIOKa * What is the advantage of monad transformers over monad coproducts? Domain: self.haskell, Score: 16, Comments: 28 On Reddit: [27] http://goo.gl/lxX2D Original: [28] http://goo.gl/lxX2D * Elm 0.3.5: JavaScript Integration, Signal Filters, space savings, ... Domain: testblogpleaseignore.com, Score: 15, Comments: 2 On Reddit: [29] http://goo.gl/jTwAz Original: [30] http://goo.gl/UzoZL * Scribd's AI Challenge Now Supports Haskell Domain: groups.google.com, Score: 14, Comments: 6 On Reddit: [31] http://goo.gl/xmuwG Original: [32] http://goo.gl/oQsTG * EclipseFP: integrating stylish-haskell Domain: jpmoresmau.blogspot.fr, Score: 11, Comments: On Reddit: [33] http://goo.gl/reK9T Original: [34] http://goo.gl/2416v Top StackOverflow Questions * Bug in Data.Map implementation? votes: 15, answers: 1 Read on SO: [35] http://goo.gl/xcgC1 * Monads with Join() instead of Bind() votes: 15, answers: 6 Read on SO: [36] http://goo.gl/phgVU * Haskell numbers and type system? votes: 11, answers: 2 Read on SO: [37] http://goo.gl/nv7Be * How to put constraints on the associated data? votes: 10, answers: 2 Read on SO: [38] http://goo.gl/6zOr9 * How to run Snap haskell webapp in production? votes: 9, answers: 3 Read on SO: [39] http://goo.gl/kx2DR * How can I search a hackage package for a function? votes: 8, answers: 2 Read on SO: [40] http://goo.gl/9fm4O * In Haskell, + is a function, (+ 2) is a function, (+ 2 3) is 5. What exactly is going on there? votes: 8, answers: 4 Read on SO: [41] http://goo.gl/PP2X2 * How can I emulate pointers in Haskell? votes: 8, answers: 1 Read on SO: [42] http://goo.gl/Yq01G * Haskell Precedence: Lambda and operator votes: 8, answers: 2 Read on SO: [43] http://goo.gl/X8yvV * Binding name in type signature using DataKind votes: 8, answers: 2 Read on SO: [44] http://goo.gl/rYcS8 * Missing something with Reader monad - passing the damn thing around everywhere votes: 7, answers: 5 Read on SO: [45] http://goo.gl/zQuBx Until next time, Daniel Santa Cruz References 1. http://www.serpentine.com/blog/2012/06/25/yes-its-worth-looking-at-the-smal… 2. http://www.reddit.com/r/haskell/comments/vkp8t/performance_yes_its_worth_lo… 3. http://www.reddit.com/r/haskell/comments/vomw0/awesome_haskell_plugins_for_… 4. http://www.reddit.com/r/haskell/comments/vomw0/awesome_haskell_plugins_for_… 5. http://www.haskell.org/pipermail/haskell-cafe/2012-June/101978.html 6. http://www.reddit.com/r/haskell/comments/vkwa6/galois_is_hiring/ 7. http://www.haskell.org/haskell-symposium/2012/#schedule 8. http://www.reddit.com/r/haskell/comments/vqjj5/haskell_symposium_2012_list_… 9. http://youtu.be/mtvoOIsN-GU 10. http://www.reddit.com/r/haskell/comments/vkkqc/live_coding_the_game_of_soko… 11. http://comonad.com/reader/2012/mirrored-lenses/ 12. http://www.reddit.com/r/haskell/comments/vk7fy/mirrored_lenses/ 13. http://blog.incubaid.com/2012/03/28/the-game-of-distributed-systems-program… 14. http://www.reddit.com/r/haskell/comments/vnvnw/the_game_of_distributed_syst… 15. http://www.informatik.uni-marburg.de/~seba/projects/sugarj/sugarhaskell.pdf 16. http://www.reddit.com/r/haskell/comments/vtie8/sugarhaskell_language_extens… 17. http://www.haskell.org/pipermail/cvs-ghc/2012-June/074131.html 18. http://www.reddit.com/r/haskell/comments/vmni7/parallel_cabalinstall_patche… 19. http://www.reddit.com/r/haskell/comments/vqhke/is_anybody_using_sublime_tex… 20. http://www.reddit.com/r/haskell/comments/vqhke/is_anybody_using_sublime_tex… 21. http://stochastix.wordpress.com/2012/06/29/runge-kutta-in-haskell/ 22. http://www.reddit.com/r/haskell/comments/vtp5l/rungekutta_in_haskell_rk4/ 23. http://www.yesodweb.com/blog/2012/06/conduit-0-5 24. http://www.reddit.com/r/haskell/comments/vu5hf/how_to_build_an_example_prox… 25. http://www.reddit.com/r/haskell/comments/vu0bb/found_myself_wanting_lambdac… 26. http://www.reddit.com/r/haskell/comments/vu0bb/found_myself_wanting_lambdac… 27. http://www.reddit.com/r/haskell/comments/vuty0/what_is_the_advantage_of_mon… 28. http://www.reddit.com/r/haskell/comments/vuty0/what_is_the_advantage_of_mon… 29. http://www.testblogpleaseignore.com/2012/06/29/announcing-elm-0-3-5-javascr… 30. http://www.reddit.com/r/haskell/comments/vu7z7/elm_035_javascript_integrati… 31. https://groups.google.com/forum/?fromgroups#!topic/scribd-fruit-hunt/Uz5hmP… 32. http://www.reddit.com/r/haskell/comments/vpoza/scribds_ai_challenge_now_sup… 33. http://jpmoresmau.blogspot.fr/2012/06/eclipsefp-integrating-stylish-haskell… 34. http://www.reddit.com/r/haskell/comments/vsgog/eclipsefp_integrating_stylis… 35. http://stackoverflow.com/questions/11180546/bug-in-data-map-implementation 36. http://stackoverflow.com/questions/11234632/monads-with-join-instead-of-bind 37. http://stackoverflow.com/questions/11231859/haskell-numbers-and-type-system 38. http://stackoverflow.com/questions/11258536/how-to-put-constraints-on-the-a… 39. http://stackoverflow.com/questions/11214167/how-to-run-snap-haskell-webapp-… 40. http://stackoverflow.com/questions/11199853/how-can-i-search-a-hackage-pack… 41. http://stackoverflow.com/questions/11217874/in-haskell-is-a-function-2-is-a… 42. http://stackoverflow.com/questions/11222991/how-can-i-emulate-pointers-in-h… 43. http://stackoverflow.com/questions/11237076/haskell-precedence-lambda-and-o… 44. http://stackoverflow.com/questions/11243227/binding-name-in-type-signature-… 45. http://stackoverflow.com/questions/11226338/missing-something-with-reader-m…

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Is there a GHC flag that will allow mutable top level state while you are debugging and then ...
by KC 05 Jul '12

05 Jul '12

you can turn the flag off when you are ready to do the computational heavy lifting so that you don't have to modify your code base? That is, GHC can then apply its algebraic transformation optimizations to the "code algebra" of the pure functions. -- -- Regards, KC

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Inferring Safety
by Ivan Lazar Miljenovic 05 Jul '12

05 Jul '12

When it says up the top of Haddock docs that a module is "Safe-Infered" (which should probably Safe-Infer*r*ed), what does that actually mean? My understanding of Safe Haskell is that there's no unsafe* functions, etc. being used unless the module author swears (by using -XSafe) that the module is indeed safe... Yet as I pointed out on Google+ [1], the Data.GraphViz [2] module of my graphviz package is labelled as being safe-infered, despite having this function in there: dotizeGraph :: (Ord cl, Graph gr) => GraphvizParams Node nl el cl l -> gr nl el -> gr (AttributeNode nl) (AttributeEdge el) dotizeGraph params gr = unsafePerformIO $ graphToGraph params' gr where params' = params { fmtCluster = const [] , fmtNode = const [] , fmtEdge = const [] } This function uses `dot -Tcanon` to add in positional (as well as possibly other) information to FGL graphs. Now, as far as I know this function _is_ safe (assuming that you have Graphviz installed), but I certainly haven't proven it enough that for every possible input graph the same positional attributes will be attached back (i.e. that the graph will always be laid out the same), and thus I haven't used -XSafe. So what's going on here? [1]: https://plus.google.com/101302416956767249890/posts/3NUWxd9P6xV [2]: http://hackage.haskell.org/packages/archive/graphviz/2999.13.0.3/doc/html/D… -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Martin Odersky on "What's wrong with Monads"
by Jonathan Geddes 05 Jul '12

05 Jul '12

Cafe, I was watching a panel on languages[0] recently and Martin Odersky (the creator of Scala) said something about Monads: "What's wrong with Monads is that if you go into a Monad you have to change your whole syntax from scratch. Every single line of your program changes if you get it in or out of a Monad. They're not polymorphic so it's really the old days of Pascal. A monomorphic type system that says 'well that's all I do' ... there's no way to abstract over things. " [0, 53:45] Thoughts? --J Arthur [0] - http://css.dzone.com/articles/you-can-write-large-programs

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Using "syntactic" to implement the lambda calculus
by Alex Rozenshteyn 05 Jul '12

05 Jul '12

(This is intended as a simplification of the problem I actually need to solve.) I'm trying to implement the lambda calculus (with CBV evaluation) using the "syntactic" package, in such a way that a simple extension is also simple to implement. I am stuck on the fact that it seems that the Value type needs to be parametrized over the Expr type and I can't seem to figure out how to do it. I've read this post<http://www.haskell.org/pipermail/haskell-cafe/2011-May/091770.html>from the archives, but I still seem to be missing something. Does anyone have any suggestions? > -- Lambda calculus > type Ident = String > data Value = VLam Ident Expr > data Expr = Var Ident | Lam Ident Expr | App Expr Expr > eval :: Expr -> Value > eval e = > case e of > Var _ -> error "not closed" > Lam i e' -> VLam i e' > App e1 e2 -> > case eval e1 of > Lam i e' -> subst e' (eval e2) i > _ -> error "illegal application" > -- Lambda calculus with integers and addition > data Value = VLam Ident Expr | VInt Integer > data Expr = Var Ident | Lam Ident Expr | App Expr Expr | Plus Expr Expr > eval e = > case e of > ... > Plus e1 e2 -> > case (eval e1, eval e2) of > (VInt x1, VInt x2) -> VInt $ x1 + x2 > _ -> error "illegal addition" -- Alex R

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translation between two flavors of lexically-scoped type variables
by Kangyuan Niu 05 Jul '12

05 Jul '12

The paper "Lexically scoped type variables" by Simon Peyton Jones and Mark Shields describes two ways to give type variables lexical scoping. They state that one of the advantages of the GHC-style type-sharing approach over the SML-style type-lambda approach is that the former allows for existential quantification in addition to universal quantification. As an example, they give this code: data Ap = forall a. Ap [a] ([a] -> Int) The constructor `Ap` has the type: Ap :: forall a. [a] -> ([a] -> Int) -> Ap And one can write a function: revap :: Ap -> Int revap (Ap (xs :: [a]) f) = f ys where ys :: [a] ys = reverse xs with the annotations on `xs` and `ys` being existential instead of universal. But I'm a little confused about *why* type-lambdas don't allow this. Aren't both Haskell and SML translatable into System F, from which type-lambda is directly taken? What does the translation for the above code even look like? Why isn't it possible to write something like: fun 'a revap (Ap (xs : 'a list) f) = f ys where ys :: 'a list ys = reverse xs in SML? -Kangyuan Niu

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existential types and cast
by Corentin Dupont 04 Jul '12

04 Jul '12

Hi all, I read somewhere (here: http://stackoverflow.com/questions/2300275/how-to-unpack-a-haskell-existent…) that it's bad to try to unbox an existential type using a cast. OK, but without I really can't figure out how to do what I want: *data NewPlayer = NewPlayer deriving (Typeable, Eq) data NewRule = NewRule deriving (Typeable, Eq) class (Eq e, Typeable e) => Event e where data EventData e instance Event NewPlayer where data EventData NewPlayer = P Int instance Event NewRule where data EventData NewRule = R Int instance Typeable1 EventData where typeOf1 _ = mkTyConApp (mkTyCon "EventData") [] data EventHandler = forall e . (Event e) => EH e (EventData e -> IO ()) addEvent :: (Event e) => e -> (EventData e -> IO ()) -> [EventHandler] -> [EventHandler] addEvent e h ehs = (EH e h):ehs triggerEvent :: (Event e) => e -> (EventData e) -> [EventHandler] -> IO () triggerEvent e d ehs = do let r = find (\(EH myEvent _) -> cast e == Just myEvent) ehs case r of Nothing -> return () Just (EH _ h) -> case cast h of Just castedH -> castedH d Nothing -> return ()* How to remove the casts from triggerEvent? All that I want is to apply the handler found on the data passed in parameter. I tried to add a function apply in the class, without success: *apply :: (EventData e -> IO ()) -> (EventData e) -> IO () apply = ($)* Thanks! Corentin

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haskell.org website ?
by Qi Qi 04 Jul '12

04 Jul '12

Hi, Haskell.org website seems dropping offline at this moment. Anyone know what's happening to it? Thanks, Qi

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Re: [Haskell-cafe] Martin Odersky on "What's wrong with Monads"
by Facundo Domínguez 04 Jul '12

04 Jul '12

> That precedent supports the view that e.g. a command-line flag > shouldn't affect behavior without the type reflecting it, e.g. by > doing IO, but the de facto use of the unsafe IO trick means not > everyone agrees. For those interested, here's [1] a case where treating command line arguments as top level constants went wrong. Look specifically at the section named "Crime Doesn't Pay". Best, Facundo [1] http://www.aosabook.org/en/ghc.html > Date: Tue, 3 Jul 2012 17:49:48 -0400 > From: Alvaro Gutierrez <radical(a)google.com> > Subject: Re: [Haskell-cafe] Martin Odersky on "What's wrong with > Monads" > To: Dominique Devriese <dominique.devriese(a)cs.kuleuven.be> > Cc: haskell-cafe(a)haskell.org > Message-ID: > <CAC6k+tpuiDrtaC_cCFA9ewawDpG1adaht96rgMc_S5wtVJwLVw(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > On Thu, Jun 28, 2012 at 2:53 PM, Dominique Devriese > <dominique.devriese(a)cs.kuleuven.be> wrote: >> 2012/6/27 Tillmann Rendel <rendel(a)informatik.uni-marburg.de>: >>> How would you implement this requirement in Haskell without changing the >>> line "amount (Leaf x) = x"? >> >> I may be missing the point here, but having worked on large code bases >> with a wide variety contributors before, I find it very advantageous >> that programmers are prevented from writing an amount function whose >> behaviour depends on command line arguments without at least an >> indication in the type. The fact that the function can not perform >> stuff like that is precisely the guarantee that the Haskell type gives >> me... > > I don't think there's an answer that's uniformly right; it depends on > whether you think of the input to the program, e.g. the environment, > command-line arguments, etc. as 'constant' and in some sense, pure. > The latter are constant in the sense that they never change, but they > are not fixed at compile-time. Other languages effectively treat them > as pure (by passing them directly to main), whereas Haskell chooses > not to, which is probably the reason why getArgs has IO in its type > (something that seems unintuitive at first.) > > That precedent supports the view that e.g. a command-line flag > shouldn't affect behavior without the type reflecting it, e.g. by > doing IO, but the de facto use of the unsafe IO trick means not > everyone agrees. >

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