- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] typed tagless-final interpretation examples broken with recent ghc
by oleg＠okmij.org 12 Oct '14

12 Oct '14

Mathijs Kwik wrote more than two months ago, on Aug 2 2014: > I've been reading up on typed tagless-final interpretations [1] and the > accompanying examples. > > It seems the stuff about CPS transformations does no longer compile, > probably because of stricter typing rules. I tried both ghc 7.6 and ghc > 7.8, which both give a different kind of error message. GHC 7.6 has introduced the ambiguity check, which has been causing quite a bit of pain. For an example, please see http://ghc.haskell.org/trac/ghc/ticket/9587 The example in that ticket is a simplified version of the tagless final CPS transformations. >From experience, the best solution to that sort of problems is to re-write the code. Often the result is simpler. This has been the case for CPS transformations. The new version of the tagless-final CPS transformations is notably shorter http://okmij.org/ftp/tagless-final/course/CPS.hs and it should work in any GHC with type families, including GHC 7.8.3.

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Current state-of-the-art in getting a dev env set up (windows 8)
by Alistair Bayley 12 Oct '14

12 Oct '14

What is the current advice for getting a haskell dev env set up on windows? There's a lot of options, and it can be hard to quickly determine which are current and which are deprecated/bit-rotted. For example, apparently cabal sandbox is preferred over cabal-dev these days. I've read: http://onoffswitch.net/started-haskell/ http://stackoverflow.com/questions/304614/haskell-on-windows-setup http://coldwa.st/e/blog/2013-08-20-Cabal-sandbox.html http://bob.ippoli.to/archives/2013/01/11/getting-started-with-haskell/ I've installed the latest haskell platform (2014.2), and am using Sublime Text 2 as my editor. SublimeHaskell is installed, but apparently it needs aeson, haskell-src-ext, and haddock. cabal install aeson indicates that it will break unordered-containers-0.2.4.0 and case-insensitive-1.1.0.3, so I must use --force-reinstalls to get it. I'm doing this in a sandbox, in case it is a terrible idea. https://github.com/SublimeHaskell/SublimeHaskell Not yet sure how I'm going to get the sandbox install of these tools to work with SublimeHaskell. Presumably some PATH magic? hdevtools looks cool and seems like it can be used from SublimeHaskell, although a windows install is from a fork. Will install ghc-mod too. What else am I missing? Thanks, Alistair

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Map with incremental serialization
by Britt Mathis 11 Oct '14

11 Oct '14

Hello everyone, I'm not sure if something like this already exists so I figured I would ask here first before I reinvent the wheel. What I need is a data structure that behaves like Data.Map, but is serialized to disk incrementally so I only have the overhead of writing what has changed every time.

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Pretty Printing Libraries with an Emphasis on Consistency
by Oliver Charles 11 Oct '14

11 Oct '14

Hi all, It seems that all of the pretty printing libraries on Hackage at the moment have an emphasis on being compact, but not necessarily consistent. By consistent, I mean that similar blocks of code are indented in the same fashion, even when not strictly necessary. Take for example, the start of "Pretty Printing with Lazy Dequeues". Here, the example is: if True then if True then True else True else if False then False else False I call this inconsistent, because the two sides of the top-level if have been formatted under different rules. A consistent formatting of this would be: if True then if True then True else True else if False then False else False The above can obviously be reached if you *always* format `if` in this style, but that's not what I'm after. If it's possible to format both sides on one-line, then that should be preferred. So an equally desirable output (that meets my "consistency" requirement) is: if True then if True then True else True else if True then True else True It doesn't seem that there is a library on Hackage that lets one express such a pretty printer. It seems that a pretty printer of this nature would need to have some sort of backtracking ability - I can imagine trying to print both sides of the if statement on one-line, and if this fails, try printing *both* on multiple lines. Curious to hear what people have to say on this matter, - ocharles

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Chaining a map over lists with lenses
by Nicola Gigante 10 Oct '14

10 Oct '14

Hi! I’m sure the answer to this question is under my eyes but I can’t understand it only from the docs. Suppose a data type like this: data Type1 = Type1 { _myField :: [Type2] } makeLenses ‘’Type1 data Type2 = Type2 { _otherField :: String } Then with t^.myField I get access to the [Type2] list. Now, i would like to go through each element of the list, apply the otherField lens to each element, and get the list of the results. Of course this means: map (view otherField) (t^.myField) but I’m sure there’s some combinator to do the same thing by chaining lenses/prisms/traversals/whatever, right? E.g. something like t^.myField.something.otherField I’m still trying to understand the full picture about lenses, but afaik what I need is a prism, because a list is a sum type, right? Thank you for your help, Nicola

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Need help testing hGetContents modification
by David Feuer 10 Oct '14

10 Oct '14

I uploaded a change to hGetContents as D327 on Phabricator to address TRAC #9236. In particular, this generates an error when attempting to read from a closed handle instead of silently returning an empty string. I would love to get some help testing this against code that uses lazy I/O heavily, *especially* with pipes, sockets, and other such weirdness, if such things exist in the wild. I don't usually read haskell-cafe, so please send responses to my address as well as the list. Thanks a lot, David Feuer

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ICFP 2015 Call for Workshop and Co-located Event Proposals
by David Van Horn 09 Oct '14

09 Oct '14

CALL FOR WORKSHOP AND CO-LOCATED EVENT PROPOSALS ICFP 2015 20th ACM SIGPLAN International Conference on Functional Programming August 30 - September 5, 2015 Vancouver, Canada http://icfpconference.org/icfp2015/ The 120th ACM SIGPLAN International Conference on Functional Programming will be held in Vancouver, British Columbia, Canada on August 30-September 5, 2015. ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. Proposals are invited for workshops (and other co-located events, such as tutorials) to be affiliated with ICFP 2015 and sponsored by SIGPLAN. These events should be less formal and more focused than ICFP itself, include sessions that enable interaction among the attendees, and foster the exchange of new ideas. The preference is for one-day events, but other schedules can also be considered. The workshops are scheduled to occur on August 30 (the day before ICFP) and September 3-5 (the three days after ICFP). ---------------------------------------------------------------------- Submission details Deadline for submission: November 16, 2014 Notification of acceptance: December 15, 2014 Prospective organizers of workshops or other co-located events are invited to submit a completed workshop proposal form in plain text format to the ICFP 2015 workshop co-chairs (Tom Schrijvers and Nicolas Wu), via email to icfp2015-workshops(a)cs.kuleuven.be by November 16, 2014. (For proposals of co-located events other than workshops, please fill in the workshop proposal form and just leave blank any sections that do not apply.) Please note that this is a firm deadline. Organizers will be notified if their event proposal is accepted by December 15, 2014, and if successful, depending on the event, they will be asked to produce a final report after the event has taken place that is suitable for publication in SIGPLAN Notices. The proposal form is available at: http://www.icfpconference.org/icfp2015/icfp15-workshops-form.txt Further information about SIGPLAN sponsorship is available at: http://www.sigplan.org/Resources/Guidelines/Workshops ---------------------------------------------------------------------- Selection committee The proposals will be evaluated by a committee comprising the following members of the ICFP 2015 organizing committee, together with the members of the SIGPLAN executive committee. Workshop Co-Chair: Tom Schrijvers (KU Leuven) Workshop Co-Chair: Nicolas Wu (University of Oxford) General Chair: Kathleen Fisher (Tufts University) Program Chair: John Reppy (University of Chicago) ---------------------------------------------------------------------- Further information Any queries should be addressed to the workshop co-chairs (Tom Schrijvers and Nicolas Wu), via email to icfp2015-workshops(a)cs.kuleuven.be.

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Combine type family return value
by Dmitry Bogatov 09 Oct '14

09 Oct '14

Hello! Following code is my attempt to comprehend type families and data kinds. I want to annotate boolean formula with variables it contains, so I get function `value` checked at compile time. `value` should return Bool, if no variables are present in formula, list of variables otherwise. Of course, `value` can just return Either [Var] Bool, but I want to get Type-level programming-fu. {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE DataKinds #-} data Nat = Zero | Succ Nat type family Plus (x :: Nat) (y :: Nat) :: Nat type instance Plus Zero x = x type instance Plus (Succ x) y = Plus x (Succ y) type family Multi (x :: Nat) (y :: Nat) :: Nat type instance Multi Zero x = Zero type instance Multi (Succ x) y = Plus y (Multi x y) type One = Succ Zero type Two = Succ One type Three = Succ Two type Power2 a = Power Two a type family Power (n :: Nat) (k :: Nat) :: Nat type instance Power n Zero = One type instance Power n (Succ x) = Multi n (Power n x) data Var = Pv | Qv | Rv type family PEncode (v :: Var) :: Nat type instance PEncode Pv = Power2 Zero type instance PEncode Qv = Power2 One type instance PEncode Rv = Power2 Two -- type instance PDecode (v :: Nat) :: * data Formula (n :: Nat) where P :: Formula (PEncode Pv) Q :: Formula (PEncode Qv) R :: Formula (PEncode Rv) Const :: Bool -> Formula Zero Conj :: Formula n -> Formula m -> Formula (Plus n m) type family FormulaValue (x :: Nat) :: * type instance FormulaValue Zero = Bool type instance FormulaValue (Succ n) = [Var] value :: Formula n -> FormulaValue n value (Const v) = v value P = [Pv] value Q = [Pv] value R = [Pv] value (Conj x y) = {- I have no idea -} deriving instance Show (Formula n) main = print $ Conj P Q -- Best regards, Dmitry Bogatov <KAction(a)gnu.org>, Free Software supporter, esperantisto and netiquette guardian. GPG: 54B7F00D

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ANN: FunGEn game engine 0.4.6 released, now on github
by Simon Michael 09 Oct '14

09 Oct '14

FunGEn 0.4.3 was uninstallable from hackage for a long time (and no-one noticed). Now, thanks to gelisam's bug report and some further cleanup work, I'm happy to announce that FunGEn 0.4.6 is available! I've also moved it from darcs hub to github. FunGEn (Functional Game Engine) is a BSD-licensed, cross-platform, OpenGL/GLUT-based, non-FRP game engine/framework written in Haskell. Created by Andre Furtado in 2002, it's the oldest Haskell game engine, and with very few dependencies and two example games, it's one of the easiest ways to get started with Haskell game development. It provides: - Initialization, updating, removing, rendering and grouping routines for game objects - Definition of a game background (or map), including texture-based maps and tile maps - Reading and interpretation of the player's keyboard and mouse input - Collision detection - Time-based functions and predefined game actions - Loading and displaying of 24-bit bitmap files - Some debugging and game performance evaluation facilities If you'd like to take over this package and run with it, or co-maintain, contact me. Best! -Simon

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`zip` doesn't work with infinite `Seq`s
by Matteo Ferrando 09 Oct '14

09 Oct '14

Hello, We are writing a compiler[1] for a course and found that the `zip` function included in the `Data.Sequence` module, `zip :: Seq a -> Seq b -> Seq (a, b)` would hang on the following code: > -- using the `zip` from `Data.Sequence` > zip ys (fomList $ repeat x) We checked the implementation[2] in the source of `Data.Sequence` and found the following: > zip :: Seq a -> Seq b -> Seq (a, b) > zip = zipWith (,) > -- Here `zipWith` assumes *non-infinite `Seq`s* > zipWith :: (a -> b -> c) -> Seq a -> Seq b -> Seq c > zipWith f xs ys > | length xs <= length ys = zipWith' f xs ys > | otherwise = zipWith' (flip f) ys xs > -- Function not exported by `Data.Seq`, assumes `length xs <= length ys` > zipWith' :: (a -> b -> c) -> Seq a -> Seq b -> Seq c > zipWith' f xs ys = snd (mapAccumL k ys xs) > where > k kys x = case viewl kys of > (z :< zs) -> (zs, f x z) EmptyL -> error "zipWith': unexpected EmptyL" In the lazy reading of the documentation we did, we didn't find any warning of using infinite `Seq`s for zips. (Maybe there are warings that we didn't see). But looking at the code of `zip` in the `Prelude`: > zip :: [a] -> [b] -> [(a,b)] > zip (a:as) (b:bs) = (a,b) : zip as bs > zip _ _ = [] We see that we could just *pattern-match* both heads, instead of making assumptions. Maybe this should be better explained in the documentation[3] of `zip` for `Seq`s: > zip :: Seq a -> Seq b -> Seq (a, b) > O(min(n1,n2)). zip takes two sequences and returns a sequence of > corresponding pairs. If one input is short, excess elements are discarded > from the right end of the longer sequence. Or just change the implementation for it to work with infinite `Seq`s. For those of you who are curious, we ended up using the following code to fix the *infinite `Seq`s problem*: > -- using the `zip` from `Prelude` > zip (toList ys) (repeat x) [1] https://github.com/chamini2/sapphire [2] http://hackage.haskell.org/package/containers-0.5.5.1/docs/src/Data-Sequenc… [3] http://hackage.haskell.org/package/containers-0.5.5.1/docs/Data-Sequence.ht…

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Re: [Haskell-cafe] type families with non-type indices
by oleg＠okmij.org 09 Oct '14

09 Oct '14

Doug McIlroy wrote: > An example of such a family is integers mod p. One wants to define the > operations once for all p, so that a type for any particular p can be > introduced by a one-line declaration. That was precisely the motivating example for the 2004 Haskell Workshop paper Implicit configurations -- or, type classes reflect the values of types ftp://ftp.deas.harvard.edu/techreports/tr-15-04.pdf (The Technical Report is the extended version of the paper) which discusses several solutions to the problem.

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Haskell Weekly News: Issue 309
by Daniel Santa Cruz 09 Oct '14

09 Oct '14

Welcome to issue 309 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from September 28 to October 4, 2014 Quotes of the Week * Uh... I'ma have to read the IRC logs... No good quotes @remember'd Top Reddit Stories * Announcing needle: ASCII-fied arrow notation Domain: scrambledeggsontoast.github.io, Score: 103, Comments: 119 Original: [1] http://goo.gl/wCPh8K On Reddit: [2] http://goo.gl/UwTJSt * How we might abolish Cabal Hell, part 1 Domain: well-typed.com, Score: 95, Comments: 69 Original: [3] http://goo.gl/N19hPq On Reddit: [4] http://goo.gl/CQQRF0 * After some failed attempts to learn Parsec I came across this incredibly patient tutorial and now I'm writing parsers! Domain: github.com, Score: 79, Comments: 17 Original: [5] http://goo.gl/msqtkb On Reddit: [6] http://goo.gl/p3EgqQ * Neil Mitchell's Haskell Blog: Why Traversable/Foldable should not be in the Prelude Domain: neilmitchell.blogspot.co.il, Score: 71, Comments: 289 Original: [7] http://goo.gl/sykNse On Reddit: [8] http://goo.gl/4wIxBX * Tutorial: Implementing highly efficient data structures with Monoids and Fingertrees Domain: codementor.io, Score: 47, Comments: 37 Original: [9] http://goo.gl/WB7Jwg On Reddit: [10] http://goo.gl/jGrLX1 * There is No Haskell Topology Domain: immanence.org, Score: 45, Comments: 32 Original: [11] http://goo.gl/JAoEDw On Reddit: [12] http://goo.gl/wuuF4i * Introduction to Low Level Haskell Optimization - Dan Doel Domain: youtu.be, Score: 44, Comments: 2 Original: [13] http://goo.gl/vYS1Hy On Reddit: [14] http://goo.gl/dG9Pn5 * Turn an optparse-applicative program into a CGI program! Domain: github.com, Score: 39, Comments: 10 Original: [15] http://goo.gl/sjCqsb On Reddit: [16] http://goo.gl/BXxeno * Yesod 1.4 released Domain: yesodweb.com, Score: 36, Comments: 1 Original: [17] http://goo.gl/T5Vqe8 On Reddit: [18] http://goo.gl/977ey0 * Updating Auto-Update Domain: yesodweb.com, Score: 36, Comments: 20 Original: [19] http://goo.gl/OwOrGZ On Reddit: [20] http://goo.gl/J2zr5n * Type-safe Routing for Spock Web Framework released Domain: github.com, Score: 32, Comments: 39 Original: [21] http://goo.gl/QZS9gU On Reddit: [22] http://goo.gl/BokvJ8 * Lattice-Based Data Structures for Deterministic Parallel and Distributed Programming (related to CRDTs; implementation in Haskell) Domain: composition.al, Score: 30, Comments: 9 Original: [23] http://goo.gl/mVQqqX On Reddit: [24] http://goo.gl/tZ0XrW * A non-recursive sorting algorithm Domain: haskellexists.blogspot.com, Score: 27, Comments: 36 Original: [25] http://goo.gl/jD0z0K On Reddit: [26] http://goo.gl/3HpTBy * Dijkstra monads Domain: why-lambda.blogspot.co.uk, Score: 25, Comments: 4 Original: [27] http://goo.gl/kx1Q6w On Reddit: [28] http://goo.gl/C86igH * Improving Haskell-related documentation Domain: self.haskell, Score: 23, Comments: 62 Original: [29] http://goo.gl/rQeuUT On Reddit: [30] http://goo.gl/rQeuUT * Haskell, Monads and Purity • via Hacker News Domain: jelv.is, Score: 23, Comments: 11 Original: [31] http://goo.gl/vm7juH On Reddit: [32] http://goo.gl/1P5OgF * FP Complete is hiring: Systems engineer for cloud and cluster work Domain: fpcomplete.com, Score: 21, Comments: 11 Original: [33] http://goo.gl/MLT5Ud On Reddit: [34] http://goo.gl/Ql8uIX * The future of the haskell2010/haskell98 packages for GHC 7.10 Domain: haskell.org, Score: 21, Comments: 22 Original: [35] http://goo.gl/94bnK3 On Reddit: [36] http://goo.gl/0c80JU * ANN: Nomyx V0.7, the only game where you can change the rules Domain: self.haskell, Score: 20, Comments: 7 Original: [37] http://goo.gl/1gGtqb On Reddit: [38] http://goo.gl/1gGtqb * Let's Build a Browser Engine in Haskell: part 5 Domain: hrothen.github.io, Score: 19, Comments: 6 Original: [39] http://goo.gl/zzSF2D On Reddit: [40] http://goo.gl/9xzqdR Top StackOverflow Questions * How fundamentaly different push-pull and arrowized FRP are? votes: 10, answers: 0 Read on SO: [41] http://goo.gl/XbbbVW * What are hashes (#) used for in Haskell's library's source? votes: 10, answers: 1 Read on SO: [42] http://goo.gl/mvNNlb * What is the inverse of a promise? votes: 9, answers: 3 Read on SO: [43] http://goo.gl/usQIiW * Is there a default polymorphic unit type haskell votes: 7, answers: 1 Read on SO: [44] http://goo.gl/5Ozy9G * nvcc + c2hs on OS X 10.9.5 votes: 7, answers: 1 Read on SO: [45] http://goo.gl/0XMNFu * How to sort a list using partial order in Haskell? votes: 7, answers: 2 Read on SO: [46] http://goo.gl/GLMiwA * Relationship between forward and backward map in Isomorphism (Lens package) votes: 6, answers: 1 Read on SO: [47] http://goo.gl/icIdIR * How to implement a generic neural network efficiently in Haskell? votes: 6, answers: 0 Read on SO: [48] http://goo.gl/HIHY8e * How do I unify two or more Signals in elerea? votes: 6, answers: 1 Read on SO: [49] http://goo.gl/jacb0r * Ambiguous type variable fixed with type equality constraint votes: 6, answers: 1 Read on SO: [50] http://goo.gl/JADmBj Until next time, [51]+Daniel Santa Cruz References 1. http://scrambledeggsontoast.github.io/2014/09/28/needle-announce/ 2. http://www.reddit.com/r/haskell/comments/2hpzpu/announcing_needle_asciified… 3. http://www.well-typed.com/blog/2014/09/how-we-might-abolish-cabal-hell-part… 4. http://www.reddit.com/r/haskell/comments/2hvm59/how_we_might_abolish_cabal_… 5. https://github.com/JakeWheat/intro_to_parsing 6. http://www.reddit.com/r/haskell/comments/2ia5u2/after_some_failed_attempts_… 7. http://neilmitchell.blogspot.co.il/2014/10/why-traversablefoldable-should-n… 8. http://www.reddit.com/r/haskell/comments/2hzqii/neil_mitchells_haskell_blog… 9. https://www.codementor.io/haskell-tutorial/monoids-fingertrees-implement-ab… 10. http://www.reddit.com/r/haskell/comments/2i3ttm/tutorial_implementing_highl… 11. http://immanence.org/post/there-is-no-haskell-topology/ 12. http://www.reddit.com/r/haskell/comments/2i85k7/there_is_no_haskell_topolog… 13. http://youtu.be/McFNkLPTOSY 14. http://www.reddit.com/r/haskell/comments/2hy6cp/introduction_to_low_level_h… 15. https://github.com/maxpow4h/webcloud#webcloud 16. http://www.reddit.com/r/haskell/comments/2huhrg/turn_an_optparseapplicative… 17. http://www.yesodweb.com/blog/2014/09/announcing-yesod-1-4 18. http://www.reddit.com/r/haskell/comments/2hvoxb/yesod_14_released/ 19. http://www.yesodweb.com/blog/2014/10/updating-auto-update 20. http://www.reddit.com/r/haskell/comments/2i5d7m/updating_autoupdate/ 21. https://github.com/agrafix/Spock 22. http://www.reddit.com/r/haskell/comments/2i2zfw/typesafe_routing_for_spock_… 23. http://composition.al/blog/2014/09/29/my-thesis-defense-talk/ 24. http://www.reddit.com/r/haskell/comments/2i17zj/latticebased_data_structure… 25. http://haskellexists.blogspot.com/2014/09/a-non-recursive-sorting-algorithm… 26. http://www.reddit.com/r/haskell/comments/2hsea9/a_nonrecursive_sorting_algo… 27. http://why-lambda.blogspot.co.uk/2014/09/dijkstra-monads.html 28. http://www.reddit.com/r/haskell/comments/2hzu5b/dijkstra_monads/ 29. http://www.reddit.com/r/haskell/comments/2i1z9u/improving_haskellrelated_do… 30. http://www.reddit.com/r/haskell/comments/2i1z9u/improving_haskellrelated_do… 31. http://jelv.is/blog/Haskell-Monads-and-Purity/ 32. http://www.reddit.com/r/haskell/comments/2i8tnl/haskell_monads_and_purity_v… 33. https://www.fpcomplete.com/page/systems-engineer 34. http://www.reddit.com/r/haskell/comments/2hz6ue/fp_complete_is_hiring_syste… 35. http://www.haskell.org/pipermail/glasgow-haskell-users/2014-September/02528… 36. http://www.reddit.com/r/haskell/comments/2i0bhr/the_future_of_the_haskell20… 37. http://www.reddit.com/r/haskell/comments/2hw529/ann_nomyx_v07_the_only_game… 38. http://www.reddit.com/r/haskell/comments/2hw529/ann_nomyx_v07_the_only_game… 39. http://hrothen.github.io/2014/10/01/lets-build-a-browser-engine-in-haskell-… 40. http://www.reddit.com/r/haskell/comments/2i0wnr/lets_build_a_browser_engine… 41. http://stackoverflow.com/questions/26164135/how-fundamentaly-different-push… 42. http://stackoverflow.com/questions/26169990/what-are-hashes-used-for-in-has… 43. http://stackoverflow.com/questions/26084439/what-is-the-inverse-of-a-promise 44. http://stackoverflow.com/questions/26097797/is-there-a-default-polymorphic-… 45. http://stackoverflow.com/questions/26125870/nvcc-c2hs-on-os-x-10-9-5 46. http://stackoverflow.com/questions/26158233/how-to-sort-a-list-using-partia… 47. http://stackoverflow.com/questions/26082221/relationship-between-forward-an… 48. http://stackoverflow.com/questions/26092063/how-to-implement-a-generic-neur… 49. http://stackoverflow.com/questions/26104735/how-do-i-unify-two-or-more-sign… 50. http://stackoverflow.com/questions/26146983/ambiguous-type-variable-fixed-w… 51. https://plus.google.com/105107667630152149014/about

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Fingerprinting Haskell Objects
by Ozgun Ataman 09 Oct '14

09 Oct '14

Hello everybody, I have a little question I wanted to run by the folks here. I've run into it several times over the past few years and would love to lock down a good answer. What's the best way to "fingerprint" a Haskell object into, say, ByteString, so that this fingerprint can be used as the "lookup key" in a database (for example) and be trusted that it will remain constant over time even as the underlying libraries evolve? Here's a simple example: - Say I'm building a manual index on top of a key-value store (redis, dynamodb, etc.) - I want my keys to be arbitrary tuples (or similar records) that may contain various fields in them - I would like to avoid ad-hoc, hand-written MyTuple -> ByteString and ByteString -> MyTuple conversions. However, Generic derivations, template-haskell, etc. are acceptable - Notice how your fingerprint, which is used as a lookup key in the database, has to remain stationary. If it changes even by a single bit over time for the same MyTuple, the key-value store will NOT be able to find the index associated with MyTuple at this later time Here are some ideas (and related concepts) I've considered and used over the years: - Hand-write a "Prism' MyTuple ByteString". This works, but is tedious and error-prone. - Use Serialize/Binary and trust that the encode/decode pair will produce results consistently in 5 years (dangerous territory!) - Use SafeCopy, which is great for ensuring timeless decoding of the *value* in the index, but can we be sure that fingerprint (MyTuple -> ByteString) conversion is persistent? What if SafeCopy authors one day decide to encode tuples differently? They would write the migrations to transparently handle legacy code for *values*, but not for *keys*. Also notice here how migrations help with the ByteString -> MyTuple leg, but do not ensure MyTuple -> ByteString produces the same ByteString over time. - Hashable would've been nice, but there is NO guarantee of persistent results, even across multiple runs of the same code What would be your preferred solution? Thank you, Oz

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Looking for a Google Hangouts tutor/advisor on Haskell
by Steven Arnold 09 Oct '14

09 Oct '14

I am looking for a Haskell developer who can review my code, suggest improvements, and generally help me become a better Haskell programmer more quickly. As such, you should be very proficient with Haskell yourself. Maybe you do Haskell work professionally; maybe you have contributed to a couple open-source projects, maybe you're a well-known Haskell blogger, writer or teacher. I am a beginner, although I have been programming professionally for many years. I do have a basic foundation in functional programming languages, including Haskell, but most of my background is with dynamic languages like Ruby and Python. I'm thinking we would meet once a week on a Google Hangout (or other technology) for an hour or so and I'd show you my code and have you review it. (Take it away and mark it up if you want.) I'd ask questions and we can discuss them. You can tell me about anything you think I might need to know to be more effective. I might also email questions to you or ask for more time than described above. You would help me as your schedule allows. (I understand you no doubt have a day job.) I am willing to pay for your time. Please let me know your hourly rate. I can compensate you with Paypal, a check....whatever works. Please feel free to contact me off-list if you're interested in helping. Thanks for taking the time to read this. Best Regards, steven

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Jop opening: web developers (with a taste for Haskell!) at Anchor
by Andrew Cowie 09 Oct '14

09 Oct '14

Web Developers and Web Designers ================================ The engineering department at Anchor is building the analytics capabilities, internal systems, cloud infrastructure, deployment tooling, and operations practices that will be needed to take the company to the next level. These things all need front-ends. It goes without saying that the web is the platform for bringing new products to market, but we're a hosting company; most of our staff are operations-focused and our customers are too. What they really want are better tools to manipulate infrastructure. And even though Real Sysadmins™ prefer command-line tools (we build those too) there's still the need to cleanly present and visualize information, and the web is the place to do that. Haskell is the working language for internal development; the benefits of type safety, resilience when refactoring, and stability over time added to the power of functional programming has already paid dividends. An open question is how far we can push strongly-typed functional programming out to the edge. JavaScript is unavoidable browser-side, but does it need to be written by hand? The plethora of alt-js implementations clearly say “no!”, but _they_ all seem to force you into contortions. If you're going to be working in something other than JavaScript, then why not a real language? Having the codebase be common on both sides of the network would really be something; are haste or ghcjs up to the challenge? It's an exciting time. We have every intention of changing the hosting industry to make developing applications — and operating them at scale — easier for people facing massive growth. Interested in helping? Requirements ------------ A solid knowledge of HTML, JavaScript, and CSS is a prerequisite, obviously. Experience with — and hatred of — a number of different server-side frameworks (hack 'n slash PHP, enterprisey JSP, hipster RoR, unmaintainable PyThings, etc) is necessary to give you an appropriately jaded outlook on just how hard the web problem is. Understanding (or at least strong opinions about) how to build sites where content and presentation, design and implementation are separated is paramount. Can your designers make it beautiful without needing to run the entire stack? Can your systems developers improve performance without needing to run the website to find out if it works? Exposure to the harsh realities of IT operations and experience of the thrill of Linux systems administration will stand you in good stead. Knowledge of the history, progress, and problems in configuration management will be helpful. Familiarity with open source development practices, and the ability to tolerate other people refactoring your code is something we'll want you to demonstrate. Experience programming in Haskell would be a definite asset. We need to see a track record of working well in the open (regardless of language or project). Good documentation is as important as good code, and command of written English is necessary both to collaborate with your peers and to describe and promote your results. A Bachelor's in science, engineering, or computational biology will be well thought of. Your specific field of study doesn't matter, but how you've used computing to solve problems in your chosen field does. A degree in engineering will be a distinct plus. An undergraduate degree in computer science will not be held against you (much). Applying -------- Anchor Systems, based in Sydney Australia, has successfully been providing managed hosting and infrastructure to clients around the world for almost 15 years. The critical difference in the service we offer our customers often boils down to providing operations expertise and infrastructure configuration balancing the short term needs to get a new installation running with the long term imperatives of security, scalability, and flexibility. If you're interested in applying, send a PDF copy of your CV to <jobs(a)anchor.com.au>, along with the URL to your GitHub account. We also invite you to include a pointer to something that shows you in a good light: a mailing list thread where you argued a contentious position, a piece of particularly thorny code in which you found an elegant solution to a problem, or even an essay or article you've written. We don't do international relocations (sorry everyone who wants to work remotely from Hawaii; I all want to work remotely from Hawaii too), but we are willing to sponsor work visas for new hires. So whether you've just moved to Australia or have been here for generations, give us a shout next time you're in Sydney. AfC Sydney -- Andrew Frederick Cowie Head of Engineering Anchor Systems https://twitter.com/afcowie https://github.com/anchor/ https://www.anchor.com.au/

1 0

cabal sandbox: Can't load .so/.DLL for...
by Joel Hermanns 08 Oct '14

08 Oct '14

Hi all, I have some trouble with cabal sandboxes. I used `cabal sandbox add-source` to add a library. When I add the library as a dependency for my project and try to build I’m always getting <command line>: can't load .so/.DLL for: /Users/joel/workspace/haskell/bookingsystem/.cabal-sandbox/lib/x86_64-osx-ghc-7.8.3/google-maps-0.1.0.0/libHSgoogle-maps-0.1.0.0-ghc7.8.3.dylib (dlopen(/Users/joel/workspace/haskell/bookingsystem/.cabal-sandbox/lib/x86_64-osx-ghc-7.8.3/google-maps-0.1.0.0/libHSgoogle-maps-0.1.0.0-ghc7.8.3.dylib, 9): Symbol not found: _googlezmmapszm0zi1zi0zi0_WebziGoogleziMapsziInternal_queryAPI_closure Referenced from: /Users/joel/workspace/haskell/bookingsystem/.cabal-sandbox/lib/x86_64-osx-ghc-7.8.3/google-maps-0.1.0.0/libHSgoogle-maps-0.1.0.0-ghc7.8.3.dylib Expected in: flat namespace in /Users/joel/workspace/haskell/bookingsystem/.cabal-sandbox/lib/x86_64-osx-ghc-7.8.3/google-maps-0.1.0.0/libHSgoogle-maps-0.1.0.0-ghc7.8.3.dylib) I couldn’t find a solution so far. Anyone can help me? I’m using ghc-7.8.3 on OS X 10.9.5 and cabal-install version 1.20.0.3 using version 1.20.0.1 of the Cabal library. Thanks, Joel

2 3

type families with non-type indices
by Doug McIlroy 08 Oct '14

08 Oct '14

Is there an analog or variant of type families that is indexed by values rather than by types? An example of such a family is integers mod p. One wants to define the operations once for all p, so that a type for any particular p can be introduced by a one-line declaration. Of course this can be done with p being a run-time parameter: data Modp = Modp Integer Int instance Num (Modp) where (Modp x p) + (Modp y q) = if p==q then Modp ((x+y) 'mod' p) p else error "unequal moduli in Modp operation" But it would be better to catch the error at compile time and not have to check at every operation. Doug

2 1

IO Exceptions through monad transformers stack
by Nicola Gigante 08 Oct '14

08 Oct '14

Hi, I’m not sure if this is the right list for “base” questions like this, if there are any more specific lists please let me know. I’m writing a parser and it works in a stack of monad transformers defined in this way: type Parser = StateT ParserState (ExceptT ParserError IO) The base monad is IO because the parse could have the need to load other files included by the file that’s being parsed. The ExceptT transformer make it easy for the lexer and the parser to signal errors. At the end I have a runParser function that compose runExceptT and evalStateT and provides me with a nice Either that contains the parsed result or a value of type ParserError. The ParserError data type is the following: data ParserError = LexingError <fields> | ParsingError <fields> Currently, whenever the parser encounter an error I simply do throwError $ ParsingError something…, for example. What I would like to do is to report in this way also the possible IO errors that can occur, by adding a IOError constructor to the datatype above and having runParser return such a value if any operation on the underlying IO monad throws an exception. How can I do something like that? The root of the problem is that I still have to grasp how the IO exceptions relate to all the other ExceptT, ErrorT, MonadError, types and how all the functions like throwError, catchError, catch, try, and so on work together, in a stack of transformers. Any clarification or pointer would be very appreciated Thank you very much, Nicola

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ANN: islink 0.1.0.0: check if an HTML element is a link (useful for web scraping)
by Marios Titas 07 Oct '14

07 Oct '14

Hello everybody, I'd like to announce the first public release of islink. It's library that basically provides a list of combinations of HTML tag names and attributes that correspond to links to external resources. This includes things like ("a", "href"), ("img", "src"), ("script", "src") etc. It also comes with a convenience function to check if a particular pair (tag, attribute) corresponds to a link. This can be useful for web scraping. Here's an example how to use it to extract all (external) links from an HTML document (with the help of hxt): {-# LANGUAGE Arrows #-} import Text.Html.IsLink import Text.XML.HXT.Core -- returns a list of tuples containing the tag name, attribute name, -- attribute value of all links getAllLinks :: FilePath -> IO [(String, String, String)] getAllLinks path = runX $ doc >>> multi getLink where doc = readDocument [withParseHTML yes, withWarnings no] path getLink :: ArrowXml a => a XmlTree (String, String, String) getLink = proc node -> do tag <- getName -< node attrbNode <- getAttrl -< node attrb <- getName -< attrbNode val <- xshow getChildren -< attrbNode isLinkA -< (tag, attrb, val) where isLinkA = isLink `guardsP` this isLink (tag, attrb, _) = isLinkAttr tag attrb

1 0

New mailing list: haskell-wai
by Michael Snoyman 07 Oct '14

07 Oct '14

After some discussion[1] on a Github issue, it became apparent that a number of people using WAI[2] could definitely benefit from having a shared discussion area. One obvious area of collaboration is discussing more efficient route dispatch algorithms, but there are likely many other topics worth discussing. Since none of the existing mailing lists seem appropriate for this, I've created a new mailing list: https://groups.google.com/d/forum/haskell-wai If you're using WAI- especially using it to write a web framework or coding applications directly against it, but even just indirectly via an existing framework- I'd highly recommend you join the discussion. [1] https://github.com/philopon/apiary-benchmark/pull/2 [2] http://hackage.haskell.org/package/wai

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Re: [Haskell-cafe] Need help with scrap-your-boilerplate
by Roman Cheplyaka 06 Oct '14

06 Oct '14

On 06/10/14 21:24, Michael Sperber wrote: > Would using a different generic-programming framework help me do this > without the unsafe stuff? Yes. For example, using my traverse-with-class library: {-# LANGUAGE TemplateHaskell, MultiParamTypeClasses, FlexibleInstances, ConstraintKinds, UndecidableInstances, ImplicitParams, OverlappingInstances #-} import Data.Generics.Traversable import Data.Generics.Traversable.TH import Data.Proxy data Foo a = Foo a deriveGTraversable ''Foo -- define a class around our generic operation class Bar b where bar :: b -> b -- default case instance Bar a where bar x = x -- Foo-specific case instance Bar (Foo a) where bar x = x -- deep traversal recursiveBar :: (Bar a, GTraversable (Rec Bar) a) => a -> a recursiveBar x = let ?c = Proxy :: Proxy Bar -- traversal with the Bar dictionary in everywhere bar x Roman

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New Functional Programming Job Opportunities
by Functional Jobs 06 Oct '14

06 Oct '14

Here are some functional programming job opportunities that were posted recently: Full Stack Functional Web Engineer at Front Row Education http://functionaljobs.com/jobs/8745-full-stack-functional-web-engineer-at-f… Senior Software Engineer at McGraw-Hill Education http://functionaljobs.com/jobs/8744-senior-software-engineer-at-mcgraw-hill… Cheers, Sean Murphy FunctionalJobs.com

1 0

Cryptographically secure random numbers?
by Bardur Arantsson 06 Oct '14

06 Oct '14

Hi all, I'm in a situation where I have to generate cryptographically secure random UUIDs, i.e. ones that cannot feasibly be guessed/predicted. Does anyone know what options there are in this space? Ideally, I'd like to fulfill as many of these points as possible: - Few big/complicated dependencies. - Avoid native C if possible (ditto for non-base dependencies). - Avoid RDRAND if possible. - Non-opaque UUID type. - Cross-platform if possible, but the main platform would be Linux. (Generating a UUID from a random stream would be trivial, so obviously a library for secure random numbers would be a-okay too.) Any suggestions?

3 3

Wreq: use basic auth and also change the response timeout
by Carlo Hamalainen 06 Oct '14

06 Oct '14

Hi, I am using Wreq with basic authentication. I also want to change the response timeout. The following code compiles but the user authentication fails: import Network.HTTP.Client (defaultManagerSettings, managerResponseTimeout) opts = defaults & manager .~ Left (defaultManagerSettings { managerResponseTimeout = Just 3000000000 } ) & auth .~ basicAuth "user" "pass" Perhaps I'm not using the lens onto the manager correctly? If I use just opts = defaults & auth .~ basicAuth "user" "pass" then my program runs fine. But I need the longer timeout for some large downloads. Thanks, -- Carlo Hamalainen http://carlo-hamalainen.net

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ANN: Aivika 1.4 - simulation library
by David Sorokin 05 Oct '14

05 Oct '14

Hi Cafe, I'm glad to announce version 1.4 of my simulation library Aivika. The main improvement is that the main package aivika[1] contains a new module now, which is destined for simplifying an export of simulation results. Also the module can print in a text form the results of simulation. It recognizes queues, servers, arrival timers as well as references, variables and ordinary simulation variables. All this can help you to work on the simulation model directly from terminal if you like it, although, as before, my library provides powerful charting components that can automatically plot charts, save the CSV files, gather statistics, build histograms and unite them all in a single HTML file that can be then observed in your favorite Internet browser. I believe that all this must simplify much the process of modeling, simulation and analysis. At least, the deviation charts (using the rule of 3 sigma) cannot be underestimated as a tool of sensitivity analysis for the Monte-Carlo simulation. Another improvement is that the main package aivika can be compiled to JavaScript with help of Haste. I didn't check yet how it works in the browser but the package itself can be installed with help of haste-inst. I think this is a very interesting direction as it allows creating online flight simulators using Aivika. I hope you will find this information helpful. Best regards, David Sorokin [1] http://hackage.haskell.org/package/aivika

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Unable to build a mingw cross compiler.
by Sven Bartscher 05 Oct '14

05 Oct '14

Greetings, I'm trying to build GHC to cross compile to windows. GCC and friends are in /usr/bin/x86_64-w64-mingw32-*, libraries and headers are in /usr/x86_64-w64-mingw32/{include,lib}. I configured with: ./configure --target=x86_64-w64-mingw32 \ --with-gcc=/usr/bin/x86_64-w64-mingw32-gcc # Needed because # otherwise /usr/bin/gcc # would be used Building fails with the following error: utils/hsc2hs/dist/build/Main.o: In function `s2nI_info': (.text+0x5c5): undefined reference to `GetModuleFileNameW' collect2: error: ld returned 1 exit status (The GHC invocation line is included in the attached file) So it seems that GHC either uses the wrong linker or invokes it with the wrong search path. However, the configure script detects the right linker (/usr/bin/x86_64-w64-mingw32). Is this a bug, or a feature? System data: OS: Debian jessie Linux/GNU i386 Installed GHC (for bootstrapping): version 7.6.3 GHC I'm trying to build: version 7.8.3 Regards Sven

2 1

Printing telnet stream to wx widget with conduit
by Tilmann 05 Oct '14

05 Oct '14

Hi, I have this little program that works 'sometimes'. It is supposed to connect to a server via telnet and print the incoming text to a wx textwidget. Usually I get an error message: ChessGui: <socket: 22>: hGetBufSome: illegal operation (handle is closed), but I acutally saw it working a few times... The whole thing is a bit out of my Haskell-league, so I´m a bit lost now. Any help is very appreciated! Best regards, Tilmann module Main where import Control.Concurrent (forkIO, killThread) import Control.Monad.IO.Class (MonadIO, liftIO) import Control.Monad.Trans.Resource import Data.Conduit import qualified Data.Conduit.Binary as CB import qualified Data.ByteString.Char8 as BS import Network (connectTo, PortID (..)) import System.IO import Graphics.UI.WX import Graphics.UI.WX.Types import Graphics.UI.WXCore.WxcDefs main = start gui gui = do f <- frame [] t <- textCtrlEx f (wxTE_MULTILINE .+. wxTE_RICH2) [font := fontFixed] e <- entry f [] set f [layout := boxed "console" (grid 5 5 [[floatLeft $ expand $ hstretch $ widget t] ,[expand $ hstretch $ widget e]])] telnet "freechess.org" 5000 t telnet :: String -> Int -> TextCtrl() -> IO () telnet host port t = runResourceT $ do (releaseSock, hsock) <- allocate (connectTo host $ PortNumber $ fromIntegral port) hClose liftIO $ mapM_ (`hSetBuffering` LineBuffering) [ hsock ] liftIO $ forkIO $ CB.sourceHandle hsock $$ (sink' t) return () sink' :: TextCtrl () -> Sink BS.ByteString IO () sink' textCtrl = do mstr <- await case mstr of Nothing -> return () Just str -> do text' <- liftIO $ (get textCtrl text) liftIO $ set textCtrl [text := text' ++ BS.unpack str ] sink' textCtrl

2 2

makeFields without lenses
by Nicola Gigante 05 Oct '14

05 Oct '14

Good morning to everyone. I really love what makeFields from Control.Lens.TH does: generating a typeclasse HasXXX for each field _typeXXX and instances of the class for each type that has a record field with the same name. However, it generates lenses, which at this time I don’t use in my little project. I'll learn to use them well sooner or later, I promise! In the meantime, is there some module out there than provides a template haskell function that do the same thing but generating simple accessor functions instead of lenses? Or can I configure the parameters of makeFieldsWith to generate simple functions instead of lenses? Another related question: if I use makeFields in more than one module, and different records have fields with the same name, the generated typeclasses will have the same name but will be different, because declared twice in different modules, am I right? Is there a way to use a unique typeclass in the whole project for every generated field of the same name? Thank you very much, Nicola

5 6

Several default implementations for instance methods
by Andreas Abel 04 Oct '14

04 Oct '14

Consider the following class for an overloaded pretty printer. For atomic data (numeric types, String etc.) one would implement prettyShow, for complex data either pretty or prettyPrec. > import Text.PrettyPrint > > class Pretty a where > pretty :: a -> Doc > prettyShow :: a -> String > prettyPrec :: Int -> a -> Doc Implementing one of these methods should be enough, giving default implementations for the other two. It is easy to get prettyShow and prettyPrec from pretty. > prettyShow = render . pretty > prettyPrec = const . pretty However, to define pretty from one of the others, I need two default implementations. > pretty = text . prettyShow > pretty = prettyPrec 0 Is there a way to get this to work? Workarounds (not entirely satisfactory): Technically, one could define a cycle of default implementations. Alternative 1: > pretty = prettyPrec 0 > prettyShow = render . pretty > prettyPrec _ = text . prettyShow Problem: Here, if pretty is defined, > prettyPrec _ = text . render . pretty instead of just > prettyPrec _ = pretty and (text . render) is not the identity (destroys inner document structure). Alternative 2: > pretty = text . prettyShow > prettyShow = render . prettyPrec 0 > prettyPrec _ = pretty Problem: Here, if prettyPrec is defined, > pretty = text . render . prettyPrec 0 instead of just > pretty = prettyPrec 0 I guess alternative 2 is worse than alternative 1, as one would usually define prettyPrec to get pretty, and not the otherway round. But none of these two alternatives really does the job. -- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel(a)gu.se http://www2.tcs.ifi.lmu.de/~abel/

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Call for Contributions - Haskell Communities and Activities Report, November 2014 edition
by Mihai Maruseac 04 Oct '14

04 Oct '14

Dear all, We would like to collect contributions for the 27th edition of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 1 Nov 2014 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format) ============================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough --- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell us, so that we can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the previous edition --- you will find interesting projects described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Mihai Maruseac and Alejandro Serrano Mena FAQ: Q: What format should I write in? A: The required format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/11-2014/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2014/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should soon receive your old entry as a template (provided we have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include Haskell code? A: Yes. Please use lhs2tex syntax (http://www.andres-loeh.de/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: Should I send files in .zip archives or similar? A: No, plain file attachements are the way. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also simply ask us. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell us that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. Q: Will I get confirmation if I send an entry? How do I know whether my email has even reached its destination, and not ended up in a spam folder? A: Prior to publication of the final report, we will send a draft to all contributors, for possible corrections. So if you do not hear from us within two weeks after the deadline, it is safer to send another mail and check whether your first one was received. -- Mihai Maruseac (MM) "If you don't know, the thing to do is not to get scared, but to learn." -- Atlas Shrugged.

1 0

ANN: hxt-css 0.1.0.0, a CSS selector engine for HXT
by Marios Titas 03 Oct '14

03 Oct '14

Hello everybody, I'd like to announce the first public release of hxt-css, a CSS selector engine for the Haskell XML Toolbox (HXT). Its main design goals are: * support as many CSS selectors as possible: it currently supports all CSS 3 selectors except the ones that do not make sense outside a web browser (e.g. such as :hover). For example, it supports weird things like div > span + p:not(:nth-of-type(3n-1)) * try to be 100% correct: in all tests I ran, the output of hxt-css was identical to that of firefox & chrome. * follow the conventions of other hxt packages: for example, error reporting is done the same way as hxt-xpath. Note, there is already a similar package in hackage called HandsomeSoup. - Marios

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u3502350@connect.hku.hk has indicated you're a friend. Accept?
by u3502350＠connect.hku.hk 03 Oct '14

03 Oct '14

Hi, u3502350(a)connect.hku.hk wants to follow you. ****** Is u3502350(a)connect.hku.hk you friend? ****** If Yes please follow the link below: http://invites.flipmailer.com/signup_e.html?fullname=&email=haskell-caf… If No please follow the link below: http://invites.flipmailer.com/signup_e.html?fullname=&email=haskell-caf… Follow the link below to remove yourself from all such emails http://invites.flipmailer.com/uns_inviter.jsp?email=haskell-cafe@haskell.or…

1 0

Need help with scrap-your-boilerplate
by Michael Sperber 02 Oct '14

02 Oct '14

I'm a casual Haskell user, and am trying to use scrap-your-boilerplate to write a transformation - and failing. The rub is that the base function is polymorphic, boiling down to this: data Foo a = Foo a bar :: Foo a -> Foo a bar x = x Now, I'm trying to use SYB like so: foo :: Typeable a => a -> a foo = mkT bar ... but I get: Could not deduce (Typeable a0) arising from a use of ‘mkT’ from the context (Typeable a) bound by the type signature for foo :: Typeable a => a -> a at foo.hs:... The type variable ‘a0’ is ambiguous Note: there are several potential instances: Is there any way I could make this work? Any help would be much appreciated! -- Regards, Mike

4 3

Add linker flags to GHC 6.12.3 using extra-gcc-opts
by Shaun Jackman 02 Oct '14

02 Oct '14

Hi. I get the following error message when linking with GHC. ``` $ ghc -o hello-hs hello-hs.hs /projects/btl/linuxbrew/bin/ld: cannot find -lgmp collect2: ld returned 1 exit status ``` It's fixed by adding `-L/projects/btl/linuxbrew/lib` to the command line. ``` $ ghc -L/projects/btl/linuxbrew/lib -o hello-hs hello-hs.hs ``` I'd like to add the above `-L` option to the default linking arguments of GHC. I thought that I could do this by modifying the file `extra-gcc-opts`. I changed it from ` -fwrapv` to ` -fwrapv -L/projects/btl/linuxbrew/lib`, but it has no effect. I confirmed this by changing it to ` -fBROKEN`, and again no effect. The file location is `/projects/btl/linuxbrew/Cellar/ghc612/6.12.3/lib/ghc-6.12.3/extra-gcc-opts`. How do I change the default gcc link options of GHC 6.12.3? It would be difficult for me to upgrade to GHC 7.8.3 because the binary package does not run on CentOS 5.11. Yes, I know it's ancient. It's an HPC cluster, and I have no control over it. So, likewise, I'm not able to install `libgmp-devel` in `/usr`, which would also solve this issue. Thanks for your help. Cheers, Shaun http://sjackman.ca

1 0

GHC 7.10 GHC-API Changes and proposed changes.
by Alan & Kim Zimmerman 02 Oct '14

02 Oct '14

There are a number of changes in GHC 7.10 that will make it easier for tool writers to use the GHC API. These include 1. More parser entrypoints, to explicitly parse fragments [Andrew Gibiansky] The following parsers are now provided parseModule, parseImport, parseStatement, parseDeclaration, parseExpression, parseTypeSignature, parseFullStmt, parseStmt, parseIdentifier, parseType, parseHeader 2. No more landmines in the AST [Alan Zimmerman] In the past it was difficult to work with the GHC AST as any generic traversals had to carefully tiptoe around an assortment of panic and undefined expressions. These have now been removed, allowing standard traversal libraries to be used. There is a third change currently being discussed, and I would like feedback on it's desirability. 3. Introduce an annotation structure to the ParsedSource to record the location of uncaptured keywords. At the moment the location of let / in / if / then / else / do etc is not captured in the AST. This makes it difficult to parse some source, transform the AST, and then output it again preserving the original layout. The current proposal, which can be seen at [1] and a proof of concept implementation at [2] returns a structure keyed to each AST element containing simply the specific SrcSpan's not already captured in the AST. This is the analogue of the Language.Haskell.Exts.Annotated.Syntax from haskell-src-exts, except a custom SrcSpanInfo structure is provided for each AST element constructor, and it is not embedded within the AST. [1] https://ghc.haskell.org/trac/ghc/wiki/GhcAstAnnotations [2] https://phabricator.haskell.org/D297 So, apart from HaRe, are there any other users who would be interested in using this? Regards Alan

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Haskell Weekly News: Issue 308
by Daniel Santa Cruz 02 Oct '14

02 Oct '14

1 0

Reflection functional dependency as an associated type?
by Brandon Moore 01 Oct '14

01 Oct '14

I was trying to make a bridge like this class MonadStateTF m where type StateOf m :: *instance (MonadState s m) => MonadStateTF m where type StateOF m = s to allow writing constraints on state monads without a dangling type parameter type NumMonad m = (MonadStateTF m, Num (StateOf m)) Unfortunately, the declaration type StateOF m = s complains that "s" isn't mentioned on the left hand side rather than somehow making use of the fundep. Brandon

2 1

Broken linkr in wiki for Ivo
by Evan Rowley 01 Oct '14

01 Oct '14

The theorem provers page has a link for Ivor at the bottom: http://www.haskell.org/haskellwiki/Libraries_and_tools/Theorem_provers This current link is dead: http://www.dcs.st-and.ac.uk/~eb/ivor.php This new link works: http://eb.host.cs.st-andrews.ac.uk/ivor.php -- -- - EJR

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A prototype concept for teaching type-class polymorphic functions
by Christopher Done 01 Oct '14

01 Oct '14

Hey there, For those teaching Haskell and making teaching tools, you might be interested in this mode of presentation of a function: http://chrisdone.com/fmap It's a little slow because the implementation is naive (it abuses my tryhaskell.org server), however the concept is: see what fmap really does by showing what you get when applying it to a variety of different types, including “strange” ones like Const and the function type. A given function has visually evident ways of applying to different data structures. The value changes and so does the type. Challenge: take this ball and run with it to illustrate return and >>= for a number of monads. Maybe, [], LogicT, Either, etc. Given that I've been using Haskell for 6 years and haven't yet written a monad tutorial, I don't want to break that clean sheet now. If that's too hard, there're plenty of simpler classes to experiment with. Some types require special handling to display in an intuitive way, like the ((->) r) example. But ignoring that, if you were sufficiently shrewd, you could automatically generate such an interactive page by taking a given typeclass, asking GHCi or the GHC API for all known instances of that type, which are Showable (or somehow representable) and then enumerating them in this kind of display. Others have suggested a "before and after" side-by-side look. So that's an option to consider. Technically you can get before/after with ‘id’ (a lesson in itself, the identity law!), but a two-column layout is an option. Ciao!

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Journal of Functional Programming - Call for PhD Abstracts
by Graham Hutton 01 Oct '14

01 Oct '14

============================================================ CALL FOR PHD ABSTRACTS Journal of Functional Programming Deadline: 31st October 2014 http://tinyurl.com/jfp-phd-abstracts ============================================================ PREAMBLE: Many students complete PhDs in functional programming each year, but there is currently no common location in which to promote and advertise the resulting work. The Journal of Functional Programming would like to change that! As a service to the community, JFP recently launched a new feature, in the form of a regular publication of abstracts from PhD dissertations that were completed during the previous year. The abstracts are made freely available on the JFP website, i.e. not behind any paywall, and do not require any transfer for copyright, merely a license from the author. The first round of published abstracts can be downloaded from http://journals.cambridge.org/jfp/phd_abstracts. Please submit dissertation abstracts according to the instructions below. A dissertation is eligible if parts of it have or could have appeared in JFP, that is, if it is in the general area of functional programming. JFP will not have these abstracts reviewed. We welcome submissions from both the PhD student and PhD advisor/ supervisor although we encourage them to coordinate. ============================================================ SUBMISSION: Please submit the following information to Graham Hutton <graham.hutton(a)nottingham.ac.uk> by 31st October 2014. o Dissertation title: (including any subtitle) o Student: (full name) o Awarding institution: (full name and country) o Date of PhD award: (month and year; depending on the institution, this may be the date of the viva, corrections being approved, graduation ceremony, or otherwise) o Advisor/supervisor: (full names) o Dissertation URL: (please provide a permanently accessible link to the dissertation if you have one, such as to an institutional repository or other public archive; links to personal web pages should be considered a last resort) o Dissertation abstract: (plain text, maximum 1000 words; you may use \emph{...} for emphasis, but we prefer no other markup or formatting in the abstract, but do get in touch if this causes significant problems) Please do not submit a copy of the dissertation itself, as this is not required. JFP reserves the right to decline to publish abstracts that are not deemed appropriate. ============================================================ PHD ABSTRACT EDITOR: Graham Hutton School of Computer Science University of Nottingham Nottingham NG8 1BB United Kingdom ============================================================ This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Re: [Haskell-cafe] Increasing Haskell modularity
by Gershom B 01 Oct '14

01 Oct '14

It seems to me the far easier choice, in ditching the feature “not worth the problem” is simply to eliminate orphan instances as in “Extension 1” described in Edward Yang’s blog post (http://blog.ezyang.com/2014/09/open-type-families-are-not-modular/) I’d like to open up a further discussion on this at some future point, as I’m convinced that there is no “legitimate” use of orphans in the Haskell ecosystem that cannot be substituted for with other safer and more straightforward techniques. -g On October 1, 2014 at 9:26:45 AM, Gesh (gesh(a)gesh.uni.cx) wrote: > Dear Café, > Edward Yang recently wrote about the non-modularity of the current semantics > for typeclasses[0] and open type families[1]. > Given these problems, we can either try to work around the problem, or > decide > whether these features are worth the problems. > The specific pair of proposals I am making are the following: > * Give up on global uniqueness of typeclass instances. > Oleg discusses adding local typclass instances without losing > coherence in his > reflection paper[2]. If we adopt his proposal, and weaken global > uniqueness of > instances to local uniqueness, we can then say that typeclass > instances in a > module are considered more local than imported instances, and therefore > override them. > Since instances will be locally unique, we don't have to pass around > dictionaries. Data types that expect globally unique instances, such > as Set, > will have to store dictionaries, however. > * Give up on non-associated open type families. > This I have researched less well, but it seems to me that these have > no use > case. Since one can make whatever instances one wishes for these type > families, one can't do anything useful with values of their instances. > In other words, values of type F a are morally of type (forall a. a). > Thus, the only way I can see these being useful is if anyone creating an > instance of the family must also provide functions that can work with > these > instances, which brings us back to the case of associated types. > > As far as I can see, these two features are useless and impeding the > drive to > make Haskell more modular. Therefore, my questions for you are twofold: > * Is there a reason to keep these features? > * If not, why don't we drop them? > > Awaiting your criticism and ire, > Gesh > > [0] - > http://blog.ezyang.com/2014/07/type-classes-confluence-coherence-global-uni… > [1] - http://blog.ezyang.com/2014/09/open-type-families-are-not-modular/ > [2] - http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.112.7819 > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Increasing Haskell modularity
by Gesh 01 Oct '14

01 Oct '14

Dear Café, Edward Yang recently wrote about the non-modularity of the current semantics for typeclasses[0] and open type families[1]. Given these problems, we can either try to work around the problem, or decide whether these features are worth the problems. The specific pair of proposals I am making are the following: * Give up on global uniqueness of typeclass instances. Oleg discusses adding local typclass instances without losing coherence in his reflection paper[2]. If we adopt his proposal, and weaken global uniqueness of instances to local uniqueness, we can then say that typeclass instances in a module are considered more local than imported instances, and therefore override them. Since instances will be locally unique, we don't have to pass around dictionaries. Data types that expect globally unique instances, such as Set, will have to store dictionaries, however. * Give up on non-associated open type families. This I have researched less well, but it seems to me that these have no use case. Since one can make whatever instances one wishes for these type families, one can't do anything useful with values of their instances. In other words, values of type F a are morally of type (forall a. a). Thus, the only way I can see these being useful is if anyone creating an instance of the family must also provide functions that can work with these instances, which brings us back to the case of associated types. As far as I can see, these two features are useless and impeding the drive to make Haskell more modular. Therefore, my questions for you are twofold: * Is there a reason to keep these features? * If not, why don't we drop them? Awaiting your criticism and ire, Gesh [0] - http://blog.ezyang.com/2014/07/type-classes-confluence-coherence-global-uni… [1] - http://blog.ezyang.com/2014/09/open-type-families-are-not-modular/ [2] - http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.112.7819

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Re: [Haskell-cafe] ANN: Nomyx 0.7 beta, the only game where you can change the rules
by Corentin Dupont 01 Oct '14

01 Oct '14

Hi again, we started the game, please join, don't be shy :) Some interresting rules have already been proposed by players: - we now have a list of friends. - we have bank accounts in a currency called the "Bling". - we'll soon have planets, and maybe spaceships. A lot of things can happen now (what friends allow to do? How to win money? how to trade with planets? etc.). www.nomyx.net Cheers! C On Tue, Sep 30, 2014 at 12:31 PM, Corentin Dupont <corentin.dupont(a)gmail.com > wrote: > > Hello everybody! > I released Nomyx <http://www.nomyx.net> V0.7, the only game where You can > change the rules! > Here is the website: www.nomyx.net > Here is a video introduction of the game: http://vimeo.com/58265498 > I created a tutorial to learn how to play: > http://www.corentindupont.info/blog/posts/2014-09-23-first-Nomyx-tutorial.h… > > Let's start a new game! I propose to call it "The Space Merchants". Dear > rulers, this part of the Galaxy is yours... > Please login to the game here: www.nomyx.net:8000/Nomyx. > The corresponding forum thread is here: > http://www.nomyx.net/forum/viewtopic.php?f=4&t=1525&p=1739 > Please also register on the mailing list to follow the game: > nomyx-game(a)googlegroups.com > > Some background: this is an implementation of a Nomic [1] game in Haskell > (I believe the first complete implementation of a Nomic game on a > computer). In a Nomyx game you can change the rules of the game itself > while playing it. The players can submit new rules or modify existing ones, > thus completely changing the behaviour of the game through time. The rules > are managed and interpreted by the computer. They must be written in the > Nomyx language <http://hackage.haskell.org/package/Nomyx-Language>, which > is a subset of Haskell. > > At the beginning, the initial rules are describing: > - how to add new rules and change existing ones. For example a unanimity > vote is necessary to have a new rule accepted. > - how to win the game. For example you win the game if you have 5 rules > accepted. > But of course even that can be changed! > > New in this beta release: > - the Nomyx language is now based on functional reactive programming > - the language also embeds a mechanism to separate effectful instructions > from effectless ones > - ability to fork an existing game into a new one > - ability to type-check rules before submitting them > - ability to create new games > > If you wanna help developing Nomyx, join us on the dev mailing-list: > dev-game(a)googlegroups.com <nomyx-game(a)googlegroups.com> > > > Cheers, > Corentin > > [1] www.nomic.net > >

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context deduction in GHC
by Doug McIlroy 30 Sep '14

30 Sep '14

Why does ghci 7.6.3 not like this example, which section 4.3.2 of the language definition says is valid Haskell? class Foo a class Foo a => Bar a instance (Eq a, Show a) => Foo [a] instance Num a => Bar [a] Ghci complains, "Could not deduce (Show a) arising from the superclasses of an instance declaration from the context (Num a)". Yet Show is a superclass of Num. Hugs accepts the code. Doug McIlroy

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ANN: Nomyx 0.7 beta, the only game where you can change the rules
by Corentin Dupont 30 Sep '14

30 Sep '14

Hello everybody! I released Nomyx <http://www.nomyx.net> V0.7, the only game where You can change the rules! Here is the website: www.nomyx.net Here is a video introduction of the game: http://vimeo.com/58265498 I created a tutorial to learn how to play: http://www.corentindupont.info/blog/posts/2014-09-23-first-Nomyx-tutorial.h… Let's start a new game! I propose to call it "The Space Merchants". Dear rulers, this part of the Galaxy is yours... Please login to the game here: www.nomyx.net:8000/Nomyx. The corresponding forum thread is here: http://www.nomyx.net/forum/viewtopic.php?f=4&t=1525&p=1739 Please also register on the mailing list to follow the game: nomyx-game(a)googlegroups.com Some background: this is an implementation of a Nomic [1] game in Haskell (I believe the first complete implementation of a Nomic game on a computer). In a Nomyx game you can change the rules of the game itself while playing it. The players can submit new rules or modify existing ones, thus completely changing the behaviour of the game through time. The rules are managed and interpreted by the computer. They must be written in the Nomyx language <http://hackage.haskell.org/package/Nomyx-Language>, which is a subset of Haskell. At the beginning, the initial rules are describing: - how to add new rules and change existing ones. For example a unanimity vote is necessary to have a new rule accepted. - how to win the game. For example you win the game if you have 5 rules accepted. But of course even that can be changed! New in this beta release: - the Nomyx language is now based on functional reactive programming - the language also embeds a mechanism to separate effectful instructions from effectless ones - ability to fork an existing game into a new one - ability to type-check rules before submitting them - ability to create new games If you wanna help developing Nomyx, join us on the dev mailing-list: dev-game(a)googlegroups.com <nomyx-game(a)googlegroups.com> Cheers, Corentin [1] www.nomic.net

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Cabal Testing Pain
by Mike Craig 30 Sep '14

30 Sep '14

Hey all, I'm trying to setup an executable cabal project, the options for cabal-based testing seem poor. I poked around for discussions of this but didn't find much. My apologies if I'm beating a dead horse. The current options, as I understand them, are 1. Split code into library, executable, and test-suite. This gets you single compilation but you have to list every source file in the library's exposed/other-modules sections. 2. Split code into executable and test-suite. You don't have to list the source files, but you get double compilation and you have to declare every dependency (and version!) twice. Am I missing a third (better) option, or is this the state of things? -- Mike Craig

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the complex package is hidden
by briand＠aracnet.com 28 Sep '14

28 Sep '14

Hi, surprisingly i can't seem to google an answer to this: Could not find module `Complex' It is a member of the hidden package `haskell98-2.0.0.2'. Use -v to see a list of the files searched for. it's been quite a while since I've used ghci, and I'm just about positive I've solved this problem before, but can't seem to figure it out. Thanks for any help.

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Deploying bash completion with cabal?
by Jacob Stanley 28 Sep '14

28 Sep '14

Hi All I'm looking for a good way to deploy a bash completion script somewhere appropriate for the given operating system. Has anyone tackled this before? I was considering packaging the script in to the install using cabal data files, then having an 'install--bash-completion' option that tries to detect the right place to install it depending on the OS. Cheers Jacob

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Separate compilation
by Lars Hupel 28 Sep '14

28 Sep '14

Hi all, I'm trying to compile a couple of file separately. The main purpose for that is that we're checking student-submitted exercises, and we want to compile them with -XSafe. The test suite doesn't require -XSafe. There are basically three files: * Exercise.hs -- uploaded by the student * Exercise_Interface.hs -- imports the 'Exercise' module and creates new bindings with appropriate types * Exercise_Test.hs -- imports the interface module and contains the test code and 'main' Here's how I try to compile these files: TEMP="$(mktemp -d -p /tmp ghcXXX)" ghc -c -XSafe -outputdir "$TEMP" Exercise.hs ghc -c -outputdir "$TEMP" -i"$TEMP" Exercise_Interface.hs ghc -o "$TEMP/runner" -outputdir "$TEMP" -i"$TEMP" Exercise_Test.hs This works except for the last step: GHC complains that it can't find the module 'Exercise_Interface'. Upon closer inspection with '-v', it turns out that it's actually looking for the .hs file, which I find odd. If I add '-c' to the last invocation, it works, but I don't get an executable. What am I doing wrong? Cheers Lars

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Arrow key functionality for REPL written in Haskell
by PENG, BO YA 28 Sep '14

28 Sep '14

Hi, I'm trying to implement the arrow key functionality for a systemf REPL to recall previously entered lines. I think I should save the previous lines in a list and then traverse through the list when an arrow key is pressed, but I'm no sure how to detect an arrow key and implement this functionality. Can anyone give a brief description about the implementation or point me to some other resources? Thanks! Emma

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