- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Is ListT a valid MonadPlus?
by oleg＠okmij.org 09 Feb '12

09 Feb '12

First of all, ListT is not a monad transformer, since it breaks the law of associativity of bind: *Control.Monad.List> let one = (lift $ putStrLn "1") :: ListT IO () *Control.Monad.List> let two = (lift $ putStrLn "2") :: ListT IO () *Control.Monad.List> let choice = return 1 `mplus` return 2 :: ListT IO Int *Extensible Control.Monad.List> runListT $ choice >> (one >> two) 1 2 1 2 [(),()] *Extensible Control.Monad.List> runListT $ (choice >> one) >> two 1 1 2 2 [(),()] > It appears to me that the MonadPlus instance for ListT breaks the > following MonadPlus law > m >> mzero = mzero The precise set of MonadPlus laws isn't actually agreed upon. The above law is the least agreed upon. Incidentally, this law is certainly violated for _any_ back-tracking monad transformer. Consider (lift <any action in base monad>) >> mzero for example (lift (putStrLn "printed")) >> mzero Is it reasonable to expect that printing should be undone? That the paper should be fed back into the printer and the toner lifted? There are back-tracking monad transformers; for example, LogicT.

1 0

Is ListT a valid MonadPlus?
by Richard Wallace 09 Feb '12

09 Feb '12

It appears to me that the MonadPlus instance for ListT breaks the following MonadPlus law m >> mzero = mzero I've tried with every implementation of ListT I could find - the one in mtl, transformers, List, even "ListT done right" and it's alternative. They all seem to violate the above law. Following is a ghci session that shows this with the mtl impl. Prelude> import Control.Monad.List Prelude Control.Monad.List> let nt = ListT Nothing :: ListT Maybe Int Prelude Control.Monad.List> let e = mzero :: ListT Maybe Int Prelude Control.Monad.List> runListT nt Nothing Prelude Control.Monad.List> runListT e Just [] Prelude Control.Monad.List> runListT $ nt >> e Nothing According to the MonadPlus law above, that last line should evaluate to `Just []`. So is ListT a valid MonadPlus? Or am I missing something? If not, can a valid MonadPlus instance for ListT be written? I've tried a few things and come up short. Thanks, Rich

1 0

Haskell Weekly News: Issue 214
by Daniel Santa Cruz 09 Feb '12

09 Feb '12

Welcome to issue 214 the HWN, a newsletter covering stories, questions, and remarks seen around the web during the week of January 29 to February 4, 2012. You can find the HTML version at: http://contemplatecode.blogspot.com/2012/02/haskell-weekly-news-issue-214.h… Quotes of the Week * Jafet: [on memoising an O(n) algorithm:] This is like trying to save work in a police investigation by having each detective determine whether another detective has already removed all evidence from his area by walking through it to search for evidence in the area. * CodeWeaver: keep in mind encryption's only as good as how much you trust that the implementors got it right. * cmccann: length [1..10^20] lambdabot: mueval-core: Time limit exceeded cmccann: looks like 10^20 is infinity * elliott: race condition waiting to happen * elliott: o'reilly publishes attoparsec tutorial: exactly the same as their parsec tutorial, but 10^-18th the size * rwbarton: Num works for Float/Double, it's Float/Double that doesn't work for Num * ddarius: Space isn't that big. * Brent Yorgey: > Sometimes, I suspect that Haskell not only makes easier of the hard > things for imperative programming languages but also makes harder of > some easy things. This is true. But usually these are things that you don't *want* to be easy, like shooting yourself in the kidney. (But these features are often described in ways that make them sound great, like "flexible usage of bullet-placement tools".) Inserting arbitrary print statements into the middle of some code is difficult for a reason. Top Reddit Stories * GHC 7.4.1 is out. Here's the release notes Domain: haskell.org, Score: 107, Comments: 59 On Reddit: [1] http://goo.gl/su4jl Original: [2] http://goo.gl/Nstve * Coming soon, to a bookstore near you Domain: yesodweb.com, Score: 82, Comments: 16 On Reddit: [3] http://goo.gl/6gZXg Original: [4] http://goo.gl/zSW1l * ImplicitCAD is a Programmatic CAD Tool/3D Modelling Engine in Haskell. Also my first big Haskell project. Feedback, advice? Domain: github.com, Score: 63, Comments: 23 On Reddit: [5] http://goo.gl/YHMRa Original: [6] http://goo.gl/39rc3 * Using monadic effects to reverse a merge sort Domain: github.com, Score: 42, Comments: 4 On Reddit: [7] http://goo.gl/dVuyp Original: [8] http://goo.gl/jL9qj * Exciting changes coming to conduit 0.2 Domain: yesodweb.com, Score: 38, Comments: 7 On Reddit: [9] http://goo.gl/neyW2 Original: [10] http://goo.gl/1ikN5 * Vectorisation without Replication in Data Parallel Haskell Domain: disciple-devel.blogspot.com, Score: 35, Comments: 4 On Reddit: [11] http://goo.gl/iJ4OD Original: [12] http://goo.gl/uMp4S * If software purity in functional languages is such an awesome idea, why are we still writing Python, Ruby or Java? Domain: self.haskell, Score: 33, Comments: 136 On Reddit: [13] http://goo.gl/F0QCY Original: [14] http://goo.gl/F0QCY * Released Data.Array.Accelerate 0.9.0.0 — the Haskell array library for GPUs Domain: justtesting.org, Score: 32, Comments: 3 On Reddit: [15] http://goo.gl/NqUwp Original: [16] http://goo.gl/JYTQ4 * Fun With -XPolyKinds : Polykinded Folds Domain: typesandotherdistractions.com, Score: 24, Comments: 1 On Reddit: [17] http://goo.gl/daxL8 Original: [18] http://goo.gl/u9gHi * Introduction to using acid-state, IxSet and data-lens as a native Haskell, noSQL, RAM-cloud database Domain: happstack.com, Score: 23, Comments: 21 On Reddit: [19] http://goo.gl/9bliO Original: [20] http://goo.gl/ZGXyW * Can't wait for the Haskell Platform? -- Compiling GHC 7.4.1 from source has never been easier. Domain: blog.jasonknight.us, Score: 21, Comments: 8 On Reddit: [21] http://goo.gl/IKjm6 Original: [22] http://goo.gl/dgMF2 * Generating random functions Domain: mainisusuallyafunction.blogspot.com, Score: 20, Comments: 6 On Reddit: [23] http://goo.gl/DB4f6 Original: [24] http://goo.gl/aOAPo * How to deal with undesired values when designing types Domain: self.haskell, Score: 19, Comments: 28 On Reddit: [25] http://goo.gl/aPCbQ Original: [26] http://goo.gl/aPCbQ * Guarded Pipes or How to write Conduit.List Combinators in Pipes. Domain: pcapriotti.wordpress.com, Score: 17, Comments: 16 On Reddit: [27] http://goo.gl/vo8ip Original: [28] http://goo.gl/mhnfH * UHC JavaScript Backend Project Page (Work in Progress) Domain: uu-computerscience.github.com, Score: 16, Comments: 0 On Reddit: [29] http://goo.gl/42xXs Original: [30] http://goo.gl/Cz8R6 Top StackOverflow Questions * What are the rules about concurrently accessing a persistent database votes: 17, answers: 1 Read on SO: [31] http://goo.gl/m3QhB * Functions don't just have types: They ARE Types. And Kinds. And Sorts. Help put a blown mind back together votes: 15, answers: 2 Read on SO: [32] http://goo.gl/fKjQR * Defining polymorphic constants in Haskell votes: 12, answers: 2 Read on SO: [33] http://goo.gl/3H3ke * Do guarded pipes behave the same as pipes using await? votes: 12, answers: 2 Read on SO: [34] http://goo.gl/Mf3HQ * What is the purpose of the IdentityT transformer? votes: 11, answers: 2 Read on SO: [35] http://goo.gl/L7Syk * call/cc implementation? votes: 10, answers: 5 Read on SO: [36] http://goo.gl/tfLLz * Understanding bind function in Haskell votes: 9, answers: 4 Read on SO: [37] http://goo.gl/amtSd * How to show that a monad is a functor and an applicative functor? votes: 8, answers: 2 Read on SO: [38] http://goo.gl/Cg0EQ * Avoiding lift with Monad Transformers votes: 7, answers: 2 Read on SO: [39] http://goo.gl/fBhCz * Container element types votes: 7, answers: 1 Read on SO: [40] http://goo.gl/UfZnG Until next time, Daniel Santa Cruz References 1. http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/release-7-4-1.html 2. http://www.reddit.com/r/haskell/comments/p886g/ghc_741_is_out_heres_the_rel… 3. http://www.yesodweb.com/blog/2012/01/coming-soon 4. http://www.reddit.com/r/haskell/comments/p4hpt/coming_soon_to_a_bookstore_n… 5. https://github.com/colah/ImplicitCAD 6. http://www.reddit.com/r/haskell/comments/p9jzh/implicitcad_is_a_programmati… 7. https://github.com/DanBurton/Blog/blob/master/Literate%20Haskell/recovery.l… 8. http://www.reddit.com/r/haskell/comments/p2kar/using_monadic_effects_to_rev… 9. http://www.yesodweb.com/blog/2012/01/conduit-changes-0-2 10. http://www.reddit.com/r/haskell/comments/p1iu0/exciting_changes_coming_to_c… 11. http://disciple-devel.blogspot.com/2012/02/vectorisation-without-replicatio… 12. http://www.reddit.com/r/haskell/comments/p7f6u/vectorisation_without_replic… 13. http://www.reddit.com/r/haskell/comments/pavb1/if_software_purity_in_functi… 14. http://www.reddit.com/r/haskell/comments/pavb1/if_software_purity_in_functi… 15. http://justtesting.org/released-dataarrayaccelerate-0900-the-haskell 16. http://www.reddit.com/r/haskell/comments/p5sij/released_dataarrayaccelerate… 17. http://www.typesandotherdistractions.com/2012/02/fun-with-xpolykinds-polyki… 18. http://www.reddit.com/r/haskell/comments/p8djh/fun_with_xpolykinds_polykind… 19. http://www.happstack.com/docs/crashcourse/AcidState.html 20. http://www.reddit.com/r/haskell/comments/p75ko/introduction_to_using_acidst… 21. http://blog.jasonknight.us/2012/02/how-to-install-ghc-741-in-ubuntu-from.ht… 22. http://www.reddit.com/r/haskell/comments/pabfi/cant_wait_for_the_haskell_pl… 23. http://mainisusuallyafunction.blogspot.com/2012/02/generating-random-functi… 24. http://www.reddit.com/r/haskell/comments/p89fq/generating_random_functions/ 25. http://www.reddit.com/r/haskell/comments/p5lv0/how_to_deal_with_undesired_v… 26. http://www.reddit.com/r/haskell/comments/p5lv0/how_to_deal_with_undesired_v… 27. http://pcapriotti.wordpress.com/2012/02/02/an-introduction-to-guarded-pipes/ 28. http://www.reddit.com/r/haskell/comments/p8g55/guarded_pipes_or_how_to_writ… 29. http://uu-computerscience.github.com/uhc-js/ 30. http://www.reddit.com/r/haskell/comments/p1nqh/uhc_javascript_backend_proje… 31. http://stackoverflow.com/questions/9055214/what-are-the-rules-about-concurr… 32. http://stackoverflow.com/questions/9074261/functions-dont-just-have-types-t… 33. http://stackoverflow.com/questions/9088821/defining-polymorphic-constants-i… 34. http://stackoverflow.com/questions/9145656/do-guarded-pipes-behave-the-same… 35. http://stackoverflow.com/questions/9122825/what-is-the-purpose-of-the-ident… 36. http://stackoverflow.com/questions/9050725/call-cc-implementation 37. http://stackoverflow.com/questions/9074284/understanding-bind-function-in-h… 38. http://stackoverflow.com/questions/9098247/how-to-show-that-a-monad-is-a-fu… 39. http://stackoverflow.com/questions/9054731/avoiding-lift-with-monad-transfo… 40. http://stackoverflow.com/questions/9078168/container-element-types

1 0

Contributing to http-conduit
by Myles C. Maxfield 08 Feb '12

08 Feb '12

To: Michael Snoyman, author and maintainer of http-conduit CC: haskell-cafe Hello! I am interested in contributing to the http-conduit library. I've been using it for a little while and reading through its source, but have felt that it could be improved with two features: - Allowing the caller to know the final URL that ultimately resulted in the HTTP Source. Because httpRaw is not exported, the caller can't even re-implement the redirect-following code themselves. Ideally, the caller would be able to know not only the final URL, but also the entire chain of URLs that led to the final request. I was thinking that it would be even cooler if the caller could be notified of these redirects as they happen in another thread. There are a couple ways to implement this that I have been thinking about: - A straightforward way would be to add a [W.Ascii] to the type of Response, and getResponse can fill in this extra field. getResponse already knows about the Request so it can tell if the response should be gunzipped. - It would be nice for the caller to be able to know in real time what URLs the request is being redirected to. A possible way to do this would be for the 'http' function to take an extra argument of type (Maybe (Control.Concurrent.Chan W.Ascii)) which httpRaw can push URLs into. If the caller doesn't want to use this variable, they can simply pass Nothing. Otherwise, the caller can create an IO thread which reads the Chan until some termination condition is met (Perhaps this will change the type of the extra argument to (Maybe (Chan (Maybe W.Ascii)))). I like this solution, though I can see how it could be considered too heavyweight. - Making the redirection aware of cookies. There are redirects around the web where the first URL returns a Set-Cookie header and a 3xx code which redirects to another site that expects the cookie that the first HTTP transaction set. I propose to add an (IORef to a Data.Set of Cookies) to the Manager datatype, letting the Manager act as a cookie store as well as a repository of available TCP connections. httpRaw could deal with the cookie store. Network.HTTP.Types does not declare a Cookie datatype, so I would probably be adding one. I would probably take it directly from Network.HTTP.Cookie. I'd be happy to do both of these things, but I'm hoping for your input on how to go about this endeavor. Are these features even good to be pursuing? Should I be going about this entirely differently? Thanks, Myles C. Maxfield P.S. I'm curious about the lack of Network.URI throughout Network.HTTP.Conduit. Is there a particular design decision that led you to use raw ascii strings?

5 57

Loading a texture in OpenGL
by Clark Gaebel 08 Feb '12

08 Feb '12

Using the OpenGL package on Hackage, how do I load a texture from an array? In the red book[1], I see their code using glGenTextures and glBindTexture, but I can't find these in the documentation. Are there different functions I should be calling? [1] http://glprogramming.com/red/chapter09.html

6 14

-XPolyKinds and undefined arguments
by Douglas McClean 08 Feb '12

08 Feb '12

There are all sorts of useful functions that would otherwise require explicit type applications which we work around by passing undefined and ignoring its value. (See http://www.haskell.org/pipermail/libraries/2010-October/014742.html for a prior discussion of why this is, how the use of undefined for something that is perfectly well defined smells, and various suggestions about one day changing it.) Examples: Data.Typeable.typeOf, Foreign.Storable.sizeOf, etc. It seems to me that the new -XPolyKinds extension might provide a good long-term solution to statically insuring that arguments like this are always undefined and are never inspected. If we had a library definition: -- a kind-indexed family of empty types, Proxy :: AnyK -> * data Proxy a then we could write things that resemble type applications such as "sizeOf @Int64". Unlike the current approach, we'd be assured that nobody tries to use the value of such a proxy in any meaningful way, since you can't pattern match it or pass a "Proxy t" in a context expecting a "t". We'd have formal documentation of which parameters are in this value-is-unused-and-only-type-matters category (which might help us erase more often?). This is essentially the same Proxy type as in section 2.5 of the "Giving Haskell a Promotion" paper, except that by making it empty instead of isomorphic to () we end up with a type that only has one value (bottom) instead of two (Proxy and bottom), which should be a desirable property if we are trying to erase uses and forbid pattern matches and such, I think. Adding a syntax extension (or even some TH?) to make something like "@Int64" desugar to "undefined :: Proxy Int64" would eliminate the clutter/smell from the undefineds. Poly kinds allow us to use the same mechanism where the "undefined" parameter is a phantom type at a kind other than *. It seems to work nicely, and it cleaned up some of my code by unifying various proxy types I had made specialized at kinds like Bool. How would you compare this to some of the proposals in the 2010 thread? Would people still prefer their fantasy-Haskell-of-the-future to include full-blown support for explicit type applications instead? -Doug

2 1

FP activities and researchers in Warwickshire
by Ivan Perez 08 Feb '12

08 Feb '12

Hello, I recently moved to Kenilworth, Warwickshire, UK, and I'd like to know if there are meetings, talks, or any FP-related activities going on around here. I contacted somebody at Warwick University but, from what I understood, their Formal Methods group doesn't exist as such any longer and they don't carry out any coordinated, regular events regarding FP anymore. Cheers, Ivan

2 1

Evaluating parallel computations in order of finishing (approximately)
by Victor Miller 08 Feb '12

08 Feb '12

Suppose that we have a list [a] of computations that we want to evaluate in parallel. I would like to have something (probably a monad) which would return the list in order (roughly) of finishing: Say the monad is M. It would be something like the state monad, in that it would be implemented by a state transformer function. In this case the state would the set of computations to be evaluated. we might have a function include :: [a] -> M a () which would say that the monad's responsibility would be to evaluate all the members of a in parallel. We might also have a function strategy :: Strategy -> M a () which would indicate the parallel strategy to be used. The key thing would be function, completed, which produces a list of all the computations to be evaluated as a list roughly in order of completion. That is, if, inside the M monad we finished the do with completed then we would have a value M a [a] which would be the list in order of completion. Since everything is lazy we could ask for the head of the list, and it would be the first value whose computation finished. Does such a thing exist? Victor

3 2

network-2.3.0.10 compiled for ghc 7.4.1 windows
by Alberto G. Corona 08 Feb '12

08 Feb '12

Hi, After some pains, I compiled the package Network. sa_family_t is needed in network.Socket.Internal.asc and MinGW seems that it has no such variable defined. I share here the package with the compiled binaries included: http://skydrive.live.com/?cid=2F229EB748D4CBA3&id=2F229EB748D4CBA3%21105

3 8

Domain Specific Languages and better Error messages
by Victor Miller 08 Feb '12

08 Feb '12

This week, at work, I've been attending a workshop which is concerned with writing (actually rewriting) a domain specific language that we've been using. Although I've only known Haskell for four months, I can see that it's infected my brain, since I kept seeing Haskell solutions to all sorts of problems that were coming up. This DSL was written by two demon programmers in C. They've made many ad-hoc choices which look rather ugly to me. Even worse, they've invented a very primitive type system which would only be checked at run time. This looks particularly bad since the applications that we'd be running are very large parallel computations. I would like to make a try at writing an alternative based on Haskell, even though there's probably little chance that I could get it adopted (since no-one else here is familiar with functional programs, etc.). However, there is one issue that I'd like to address. I was looking at http://donsbot.wordpress.com/2007/03/10/practical-haskell-shell-scripting-w… gave a very nice presentation of a domain specific language for shell scripts. One point that was made that commands were implemented as functions, and privileged commands (i.e. those which needed root privileges) were produced a result with a separate type, so that type checking would give a compile time error if you didn't execute it via the priv combinator. But, the error messages given by ghc would be rather incomprehensible to someone who wasn't familiar with Haskell (and maybe even to someone who was!). Has anyone looked into something that would filter the error messages of put out by GHC and rewrite them into something more domain specific? Victor

1 0