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newbie question about "Functional dependencies conflict between instance declarations:".....
by Nicholls, Mark 05 Jul '13

05 Jul '13

Hello, I largely don't know what I'm doing or even trying to do, it is a voyage into the unknown....but....if I go... > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE FunctionalDependencies #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE UndecidableInstances #-} > class Foo x y | x -> y, y -> x > instance Foo Integer Integer That seems to work....and my head seems to say...your created some sort of binary relation between 2 types...and made <Integer,Integer> a member of it... Something like that anyway.... Then I go.... > data Bar > instance Foo Bar x Error!....but I'm think I understand this....I can't claim that <Bar,x> is a member of Foo and <Integer,Integer> is member of Foo and preserve my functional dependencies, because <Bar,Integer> is now a member of Foo.. Bad programmer....... So how I naively go.... > class NotAnInteger a > instance (NotAnInteger x) => Foo Bar x I haven't declared integer to be "NotAnInteger"....so (in a closed world)....this would seem to exclude the contradiction....but... Functional dependencies conflict between instance declarations: instance Foo Integer Integer -- Defined at liam1.lhs:7:12 instance NotAnInteger x => Foo Bar x -- Defined at liam1.lhs:13:12 So i) I clearly don't understand something about the type system. ii) I don't know how to restrict type variables in instance declarations....i.e. how do I use the notion of "Foo" across different combinations of types, without them colliding. CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

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Product-Categories
by Lyndon Maydwell 05 Jul '13

05 Jul '13

Hi Café. I'm wracking my brain trying to figure out a simple, reasonably general, implementation for a category instance for pairs of categories. So far I've looked at [1], which seems great, but doesn't use the built-in category instance, and [2], which I'm just not sure about. Ideally I'd like to be able to express something like - instance (Category a, Category b) => Category (Product a b) where id = Product id id Product o1 o2 . Product i1 i2 = Product (o1 . i1) (o2 . i2) However, it all falls apart when I actually try to define anything. Is this possible? If not, why not? As far as I can tell the issue boils down to not being able to translate "Category i o" to "Product (Fst i) (Fst o) (Snd i) (Snd o)" without breaking the kind expectation of the category instance. Please help me, I'm having a bad brain day :-) [1] - http://twanvl.nl/blog/haskell/categories-over-pairs-of-types [2] - http://hackage.haskell.org/packages/archive/categories/1.0.6/doc/html/Contr…

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Automatically test different versions of single dependency
by Daniel Díaz Casanueva 05 Jul '13

05 Jul '13

Hello everyone. I would like to check what dependencies is one of my packages compatible with. For example, say I have a package called "foo" that depends on package "bar". Most likely, "foo" does not build with each version of "bar". What I want to do is try to build "foo" with each single version of "bar" (not manually). What is the best approach to this?

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getting exported original names using the GHC API
by Pasqualino "Titto" Assini 05 Jul '13

05 Jul '13

Say that I have a module like: module Test(L.find) where import Data.List as L Using the GHC API, I extract the module exports (using GHC.modInfoExports) and I get a list of names. Now, I want to get the name of the export with the namespace stated in the source ("L.find") but I seem to be able to get it only as "Data.List.find". I have tried different variations of functions on Name or OccName but nothing seems to work. Obviously, I could just parse the module and get the info that way, but I wondered if it could be done via the API. Thanks, titto -- Dr. Pasqualino "Titto" Assini http://quid2.org/

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Call for Demo Proposals: Workshop on Functional Art, Music, Modeling and Design
by Brent Yorgey 04 Jul '13

04 Jul '13

Hi all, Below is the call for demonstration proposals for FARM 2013. Please forward to anyone who might be interested -- apologies if you receive multiple copies! Brent Yorgey publicity chair for FARM 2013 ====================================================================== FARM 2013: Call for Demo Proposals ACM SIGPLAN Workshop on Functional Art, Music, Modeling and Design Boston, Massachusetts, USA 28th September, 2013 (directly after ICFP) http://www.cis.upenn.edu/~byorgey/farm13/ ====================================================================== Do you enjoy writing beautiful code to produce beautiful artifacts? Have something cool to show off at the intersection of functional programming and visual art, music, sound, modeling, visualization, or design? FARM 2013 is seeking proposals for 10-20 minute demonstrations to be given during the workshop. For example, a demonstration could consist of a short tutorial, an exhibition of some work, or even a livecoding performance. Slots for demonstrations will be shorter than slots for accepted papers, and will not be published as part of the formal proceedings, but can be a great way to show off interesting work and get feedback from other workshop participants. A demonstration slot could be a particularly good way to get feedback on work-in-progress. Scope ----- The functional programming community is largely interested in writing beautiful programs. This workshop is intended to gather researchers and practitioners interested in writing beautiful programs that generate beautiful artifacts. Such artifacts may include visual art, music, 3D sculptures, animations, GUIs, video games, physical models, architectural models, choreographies for dance, poetry, and even physical objects such as VLSI layouts, GPU configurations, or mechanical engineering designs. The framework used need not be purely functional (“mostly functional” is fine); may be based on abstractions such as higher-order functions, monads, arrows, or streams; and may be manifested as a domain specific language or tool. Theoretical foundations, language design, implementation issues, and applications are all within the scope of the workshop. Important dates --------------- - Demonstration proposals due: Wednesday, 31 July 2013, 18:00 EDT (UTC-4) - Author notification: Monday, 5 August 2013 - Workshop: Saturday, 28 September 2013 (the day after ICFP) A demo proposal should consist of a 1 page abstract, in PDF format, explaining the proposed content of the demonstration and why it would be of interest to the attendees of FARM. Proposals will be judged on interest and relevance to the stated goals and themes of the workshop. Submissions can be made via EasyChair at https://www.easychair.org/conferences/?conf=farm2013. Organizers ---------- Workshop Chair: Paul Hudak Program Chair: Conal Elliott Publicity Chair: Brent Yorgey Program Committee ----------------- Daniel Cukier (University of São Paulo) Conal Elliott (Tabula) (chair) Kathleen Fisher (Tufts University) Richard Gabriel (IBM Research) George Giorgidze (University of Tübingen) Paul Hudak (Yale University) José Pedro Magalhães (University of Oxford) Alex McLean (University of Leeds) John Peterson (Western State Colorado University) Michael Sperber (Active Group) Henning Thielemann Brent Yorgey (University of Pennsylvania)

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How to use Template Haskell based on code that generated by another Template?
by Magicloud Magiclouds 04 Jul '13

04 Jul '13

I have a yesod project, which generated, say, UserPassword in module Model. Then I wrote my template code which generate a piece of code to use UserPassword. I imported Model in my code. Then I got Illegal variable name: `UserPassword' When splicing a TH declaration: -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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bug in Data.ByteString.Lazy or in me?
by John MacFarlane 04 Jul '13

04 Jul '13

On a 64-bit Windows 8 server EC2 instance, with Haskell Platform freshly installed from the package installer, GHCI> :m + Data.ByteString.Lazy GHCI> Data.ByteString.Lazy.hGetContents stdin gives me an immediate error "hGetBufSome: resource exhausted (Not enough space)", while GHCI> :m + Data.ByteString GHCI> Data.ByteString.hGetContents stdin waits for user input, as expected. On 32-bit Windows, both work as expected. Can anyone explain this? Is this a bug in bytestring? John

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Haskell Weekly News: Issue 272
by Daniel Santa Cruz 04 Jul '13

04 Jul '13

Welcome to issue 272 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 23 to 29, 2013. Quotes of the Week * kmc: johnw: I'm rejoining this channel after months away just to tell you how incredibly wrong you are that monads are the "core concept" of Haskell * dmwit: semigroups are monoids with an identity crisis * olsner: I think doing eta something means to make it more or less pointless. But I don't recall which eta does what * mm_freak: Oleg knows what's at the end of foldr f z [1..]. * copumpkin: it's like grasping desperately for the rope of induction as you fall down the bottomless pit of coinduction Top Reddit Stories * From zero to cooperative threads in 33 lines of Haskell code Domain: haskellforall.com, Score: 78, Comments: 39 On Reddit: [1] http://goo.gl/rtkrO Original: [2] http://goo.gl/wgyAC * RSS reader written in Haskell and Ur/Web Domain: bazqux.com, Score: 66, Comments: 38 On Reddit: [3] http://goo.gl/sUw1U Original: [4] http://goo.gl/gdh5G * FP Haskell Center -- full video demo -- app development, automatic deployment, Git and GitHub integration Domain: youtube.com, Score: 64, Comments: 68 On Reddit: [5] http://goo.gl/gBi5l Original: [6] http://goo.gl/TlGL9 * Building LLVM using Shake Domain: neilmitchell.blogspot.ca, Score: 54, Comments: 6 On Reddit: [7] http://goo.gl/38rWZ Original: [8] http://goo.gl/tA5tM * Hasochism: The Pleasure and Pain of Dependently Typed Haskell Programming [pdf] Domain: personal.cis.strath.ac.uk, Score: 52, Comments: 8 On Reddit: [9] http://goo.gl/6dDsL Original: [10] http://goo.gl/ZZPZL * GHC Core by example, episode 1 - Hello Core! Domain: alpmestan.com, Score: 46, Comments: 3 On Reddit: [11] http://goo.gl/iL5ho Original: [12] http://goo.gl/mUiO5 * HPCwire: Lustre Founder Spots Haskell on HPC Horizon Domain: hpcwire.com, Score: 36, Comments: 2 On Reddit: [13] http://goo.gl/h2dmv Original: [14] http://goo.gl/1WLf8 * Adventures in Three Monads - Logic, Prompt, Failure Domain: web.mit.edu, Score: 33, Comments: 8 On Reddit: [15] http://goo.gl/JEOfw Original: [16] http://goo.gl/AI5wV * Darcs' source code is some of the cleanest Haskell I've read in a while Domain: github.com, Score: 32, Comments: 13 On Reddit: [17] http://goo.gl/TWWMh Original: [18] http://goo.gl/fZGE8 * I accidentally the entire heap: debugging Haskell programs with space leaks Domain: bitba.se, Score: 32, Comments: 6 On Reddit: [19] http://goo.gl/9Vtzy Original: [20] http://goo.gl/HCS5N * GHCJS introduction – Concurrent Haskell in the browser Domain: weblog.luite.com, Score: 30, Comments: 4 On Reddit: [21] http://goo.gl/Fk8Xd Original: [22] http://goo.gl/8CUVT * Functional Reactive Web Interfaces with GHCJS and sodium Domain: weblog.luite.com, Score: 29, Comments: 12 On Reddit: [23] http://goo.gl/5QKn0 Original: [24] http://goo.gl/VpAOv * Who Ya Gonna Call? Getting Dirty with Cmm, the GHC backend, and writing new PrimOps with Carter Schonwald (at NY-Haskell) Domain: vimeo.com, Score: 26, Comments: 14 On Reddit: [25] http://goo.gl/wUVcs Original: [26] http://goo.gl/cZaGd * "Business system" using functional programming? Domain: self.haskell, Score: 24, Comments: 44 On Reddit: [27] http://goo.gl/QcE5P Original: [28] http://goo.gl/QcE5P * The Resource Applicative Domain: haskellforall.com, Score: 23, Comments: 16 On Reddit: [29] http://goo.gl/G7TIs Original: [30] http://goo.gl/4e0Gm Top StackOverflow Questions * What's the conceptual difference between Machines and Conduits (or other similar libraries)? votes: 13, answers: 1 Read on SO: [31] http://goo.gl/hrNMl * “inject” progress logging/tracing in haskell computation? votes: 13, answers: 2 Read on SO: [32] http://goo.gl/RdEKU * Difference between free monads and fixpoints of functors? votes: 12, answers: 0 Read on SO: [33] http://goo.gl/O3oBi * Can Haskell optimize function calls the same way Clang / GCC does? votes: 12, answers: 3 Read on SO: [34] http://goo.gl/Sgf88 * Trying to figure out `random` function in Haskell votes: 10, answers: 3 Read on SO: [35] http://goo.gl/kcHsi * What does !( ) means in data constructor? votes: 9, answers: 2 Read on SO: [36] http://goo.gl/ysnNL * Anything prevents optimizing tail-recursion? votes: 9, answers: 1 Read on SO: [37] http://goo.gl/LaTY2 * Where can I find a good introduction to Haskell optimization passes? votes: 9, answers: 1 Read on SO: [38] http://goo.gl/wMYQH * Passing void * through haskell votes: 9, answers: 1 Read on SO: [39] http://goo.gl/Fiu5Q * Call main function with arguments votes: 9, answers: 2 Read on SO: [40] http://goo.gl/omwDh Until next time, [41]+Daniel Santa Cruz References 1. http://www.haskellforall.com/2013/06/from-zero-to-cooperative-threads-in-33… 2. http://www.reddit.com/r/haskell/comments/1h0wf7/from_zero_to_cooperative_th… 3. https://bazqux.com/ 4. http://www.reddit.com/r/haskell/comments/1ha5dd/rss_reader_written_in_haske… 5. http://www.youtube.com/watch?v=cyyDmQKcHMs 6. http://www.reddit.com/r/haskell/comments/1h7693/fp_haskell_center_full_vide… 7. http://neilmitchell.blogspot.ca/2013/06/building-llvm-using-shake.html 8. http://www.reddit.com/r/haskell/comments/1gxaee/building_llvm_using_shake/ 9. https://personal.cis.strath.ac.uk/conor.mcbride/pub/hasochism.pdf 10. http://www.reddit.com/r/haskell/comments/1gz03w/hasochism_the_pleasure_and_… 11. http://alpmestan.com/2013/06/27/ghc-core-by-example-episode-1/ 12. http://www.reddit.com/r/haskell/comments/1h5ml0/ghc_core_by_example_episode… 13. http://www.hpcwire.com/hpcwire/2013-06-24/lustre_founder_spots_haskell_on_h… 14. http://www.reddit.com/r/haskell/comments/1h24rv/hpcwire_lustre_founder_spot… 15. http://web.mit.edu/~ezyang/Public/threemonads.pdf 16. http://www.reddit.com/r/haskell/comments/1h1etb/adventures_in_three_monads_… 17. https://github.com/asperous/darcs_source_code 18. http://www.reddit.com/r/haskell/comments/1h4rak/darcs_source_code_is_some_o… 19. https://www.bitba.se/i-accidentally-the-entire-heap/ 20. http://www.reddit.com/r/haskell/comments/1h75fa/i_accidentally_the_entire_h… 21. http://weblog.luite.com/wordpress/?p=14&date=2013-06-03 22. http://www.reddit.com/r/haskell/comments/1h4s12/ghcjs_introduction_concurre… 23. http://weblog.luite.com/wordpress/?p=127 24. http://www.reddit.com/r/haskell/comments/1h795v/functional_reactive_web_int… 25. https://vimeo.com/69025829 26. http://www.reddit.com/r/haskell/comments/1gzmw1/who_ya_gonna_call_getting_d… 27. http://www.reddit.com/r/haskell/comments/1hb4ho/business_system_using_funct… 28. http://www.reddit.com/r/haskell/comments/1hb4ho/business_system_using_funct… 29. http://www.haskellforall.com/2013/06/the-resource-applicative.html 30. http://www.reddit.com/r/haskell/comments/1haums/the_resource_applicative/ 31. http://stackoverflow.com/questions/17281556/whats-the-conceptual-difference… 32. http://stackoverflow.com/questions/17290276/inject-progress-logging-tracing… 33. http://stackoverflow.com/questions/17307416/difference-between-free-monads-… 34. http://stackoverflow.com/questions/17344869/can-haskell-optimize-function-c… 35. http://stackoverflow.com/questions/17288614/trying-to-figure-out-random-fun… 36. http://stackoverflow.com/questions/17336774/what-does-means-in-data-constru… 37. http://stackoverflow.com/questions/17346161/anything-prevents-optimizing-ta… 38. http://stackoverflow.com/questions/17354005/where-can-i-find-a-good-introdu… 39. http://stackoverflow.com/questions/17366776/passing-void-through-haskell 40. http://stackoverflow.com/questions/17374356/call-main-function-with-argumen… 41. https://plus.google.com/105107667630152149014/about

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Custom Setup.hs and Paths module
by Daniel Díaz Casanueva 04 Jul '13

04 Jul '13

Hi everyone. I am writing a package where I am using the Paths module that cabal generates automatically. After adding the Paths module to the "other-modules" section in my cabal file everything worked just fine, until I wanted to write a custom Setup.hs. This Setup.hs just writes a couple of files in the system and then calls "defaultMain". The thing is that now "cabal install" does not find the Paths module, so the package is broken. Any thoughts? I need your help, people! Thank you, Daniel Díaz.

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ghci & ghc -> JS (Emscripten)
by B B 04 Jul '13

04 Jul '13

Hi! Does anybody tried, or is there anywhere a project, of online ghc or ghci (compiled to JavaScript with Emscripten)?

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