July 2015 - Haskell-Cafe

Where else is EclipseFP available since SourceForge seems to be partially down?
by KC 20 Jul '15

20 Jul '15

Where else is EclipseFP available since SourceForge seems to be partially down? -- -- Sent from an expensive device which will be obsolete in a few months! :D Casey

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Add IsNumeric (similar to IsString and IsList)
by Michael Baikov 19 Jul '15

19 Jul '15

Hi Cafe! Now that we have OverloadedList and OverloadedStrings extensions it might make sense to add something similar for numerals - right now if you want to be able to specify some items as numbers you have to implement Num instance, unfortunately not everything that can be represented as number can have sensible operations required by Num: negation makes no sense for set of natural numbers for example, multiplication for some applications of currency values which results in Num instance full of (*) = error "No multipication for Foos", (+) = error "No addition for Foos". I don't have any statistics from hackage or github, but in codebase I'm working with there are 3 such instances. Proposal: add OverloadedNumerals language pragma, IsNumeric typeclass and some methods to make that class useful with behavior similar to Lists and Strings.

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Re: [Haskell-cafe] The evil GADTs extension in ghci 7.8.4 (maybe in other versions too?)
by Edward Z. Yang 19 Jul '15

19 Jul '15

GHC used to always generalize let-bindings, but our experience with GADTs lead us to decide that let should not be generalized with GADTs. So, it's not like we /wanted/ MonoLocalBinds, but that having them makes the GADT machinery simpler. This blog post gives more details on the matter: https://ghc.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7 Edward Excerpts from Ki Yung Ahn's message of 2015-06-04 20:37:27 -0700: > Such order dependent could be very confusing for the users. I thought I > turned off certain feature but some other extension turning it on is > strange. Wouldn't it be better to decouple GADT and MonoLocalBinds? > > 2015년 06월 04일 20:31에 Edward Z. Yang 이(가) 쓴 글: > > This is because -XGADTs implies -XMonoLocalBinds. > > > > Edward > > > > Excerpts from Ki Yung Ahn's message of 2015-06-04 20:29:50 -0700: > >> \y -> let x = (\z -> y) in x x > >> > >> is a perfectly fine there whose type is a -> a. > >> (1) With no options, ghci infers its type correctly. > >> (2) However, with -XGADTs, type check fails and raises occurs check. > >> (3) We can remedy this by supplying some additional options > >> (4) Howver, if you put -XGADTs option at the end, it fails again :( > >> > >> > >> kyagrd@kyahp:~$ ghci > >> GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help > >> Loading package ghc-prim ... linking ... done. > >> Loading package integer-gmp ... linking ... done. > >> Loading package base ... linking ... done. > >> Prelude> :t \y -> let x = (\z -> y) in x x > >> \y -> let x = (\z -> y) in x x :: t -> t > >> Prelude> :q > >> Leaving GHCi. > >> > >> > >> kyagrd@kyahp:~$ ghci -XGADTs > >> GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help > >> Loading package ghc-prim ... linking ... done. > >> Loading package integer-gmp ... linking ... done. > >> Loading package base ... linking ... done. > >> Prelude> :t \y -> let x = (\z -> y) in x x > >> > >> <interactive>:1:30: > >> Occurs check: cannot construct the infinite type: t0 ~ t0 -> t > >> Relevant bindings include > >> x :: t0 -> t (bound at <interactive>:1:11) > >> y :: t (bound at <interactive>:1:2) > >> In the first argument of ‘x’, namely ‘x’ > >> In the expression: x x > >> Prelude> :q > >> Leaving GHCi. > >> > >> > >> ~$ ghci -XGADTs -XNoMonoLocalBinds -XNoMonomorphismRestriction > >> GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help > >> Loading package ghc-prim ... linking ... done. > >> Loading package integer-gmp ... linking ... done. > >> Loading package base ... linking ... done. > >> Prelude> :t \y -> let x = (\z -> y) in x x > >> \y -> let x = (\z -> y) in x x :: t -> t > >> Prelude> :q > >> Leaving GHCi. > >> > >> > >> ~$ ghci -XNoMonoLocalBinds -XNoMonomorphismRestriction -XGADTs > >> GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help > >> Loading package ghc-prim ... linking ... done. > >> Loading package integer-gmp ... linking ... done. > >> Loading package base ... linking ... done. > >> Prelude> :t \y -> let x = (\z -> y) in x x > >> > >> <interactive>:1:30: > >> Occurs check: cannot construct the infinite type: t0 ~ t0 -> t > >> Relevant bindings include > >> x :: t0 -> t (bound at <interactive>:1:11) > >> y :: t (bound at <interactive>:1:2) > >> In the first argument of ‘x’, namely ‘x’ > >> > > _______________________________________________ > > Glasgow-haskell-users mailing list > > Glasgow-haskell-users(a)haskell.org > > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users > > >

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Re: [Haskell-cafe] Type signature for function where function parameter is used twice?
by Ivan Lazar Miljenovic 19 Jul '15

19 Jul '15

(Re-CC'ing Cafe) I think what's happening here is that when you do "apply f1 f1", the two c's and d's don't actually match with each other. But when you do it with applyBoth, they *have* to match each other. As a comparison, compare: :t (read, read) (read, read) :: (Read a, Read a1) => (String -> a, String -> a1) and :t Control.Monad.join read Control.Monad.join (,) read :: Read a => (String -> a, String -> a) I'm not sure how you could define such a function to do what you're requesting; my attempt is this: applyBoth :: (forall a b a2 b2. arr a b -> arr a2 b2) -> D arr a b -> D arr a2 b2 applyBoth f = apply f f but whilst ghci accepts it, I couldn't actually seem to use it (lots of errors about being unable to match types). On 19 July 2015 at 17:15, Clinton Mead <clintonmead(a)gmail.com> wrote: > Thanks for your reply Ivan > > Here's a full code example ( http://ideone.com/1K3Yhw ): > > --- > > data D c a b = D (c a b) (c b a) > > apply :: (c a b -> c a2 b2) -> (c b a -> c b2 a2) -> D c a b -> D c a2 b2 > apply f1 f2 (D x y) = D (f1 x) (f2 y) > > applyBoth f = apply f f > > f :: Int -> String > f = show > > g :: String -> Int > g = read > > h :: Int -> Int > h = (*2) > > join :: (a -> b) -> (c -> d) -> ((a, c) -> (b, d)) > join f g (x, y) = (f x, g y) > > x1 = D f g > > f1 = join h > > y1 = apply f1 f1 x1 -- This compiles > y2 = apply g g x1 where g = f1 -- Also works > > z1 = applyBoth f1 x1 -- This fails > > main = return () > > --- > > You see, with "y1" and "y2", a different "f1" instantiation (is that the > right word) of "f1" is chosen. Even when I let "g = f1", each "g" passed to > apply is different, the first operates on "Int -> String", the second, > "String -> Int". > > On the face of it, if: > > f x = g x x > > Then well, you should be able to replace "g x x" with "f x" wherever you see > it. > > But it seems in this case this doesn't work. Surely Haskell doesn't require > me to "repeat myself", and there's a way to write "applyBoth", instead of > having to write "apply f f" (note the repetition of f) all the time? > > I assume there's some type signature I can give "applyBoth f" so it can be > used just like "apply f f" but I can't work out what it is. > > On Sun, Jul 19, 2015 at 10:23 AM, Ivan Lazar Miljenovic > <ivan.miljenovic(a)gmail.com> wrote: >> >> On 19 July 2015 at 10:13, Clinton Mead <clintonmead(a)gmail.com> wrote: >> > Lets say I've got the following data type: >> > >> > data D c a b = D (c a b) (c b a) >> > >> > And I define a function to manipulate it: >> > >> > apply :: (c a b -> c a2 b2) -> (c b a -> c b2 a2) -> D c a b -> D c a2 >> > b2 >> > apply f1 f2 (D x y) = D (f1 x) (f2 y) >> > >> > This is all fine. But I want a shorter function if (f1 = f2). So I >> > write: >> > >> > applyBoth f = apply f f >> > >> > I originally thought that if "apply f f" is valid, then logically >> > "applyBoth >> > f" should also be valid. But it seems that type inference results in >> > applyBoth only working for functions "c a a -> c a2 a2". >> >> applyBoth f = apply f f >> >> In apply, (f1 :: c a b -> c a2 b2) and (f2 :: c b a -> c b2 a2). >> >> So for "apply f f" to typecheck, we must have that a ~ b and a2 ~ b2 >> (as the above two type signatures must match since f1 = f2). >> >> So we must have that (f :: c a a -> c a2 a2). >> >> > >> > Is there a way to type "applyBoth" so it works for all functions that >> > would >> > work simply by repeating them twice in "apply"? >> >> I'm not sure what you mean; what else would make sense? Can you >> provide an example? >> >> -- >> Ivan Lazar Miljenovic >> Ivan.Miljenovic(a)gmail.com >> http://IvanMiljenovic.wordpress.com > > -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Type signature for function where function parameter is used twice?
by Clinton Mead 19 Jul '15

19 Jul '15

Lets say I've got the following data type: data D c a b = D (c a b) (c b a) And I define a function to manipulate it: apply :: (c a b -> c a2 b2) -> (c b a -> c b2 a2) -> D c a b -> D c a2 b2 apply f1 f2 (D x y) = D (f1 x) (f2 y) This is all fine. But I want a shorter function if (f1 = f2). So I write: applyBoth f = apply f f I originally thought that if "apply f f" is valid, then logically "applyBoth f" should also be valid. But it seems that type inference results in applyBoth only working for functions "c a a -> c a2 a2". Is there a way to type "applyBoth" so it works for all functions that would work simply by repeating them twice in "apply"?

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ICFP 2015 Call for Participation
by David Van Horn 18 Jul '15

18 Jul '15

[ Early registration ends 3 August. ] ===================================================================== Call for Participation ICFP 2015 20th ACM SIGPLAN International Conference on Functional Programming and affiliated events August 30 - September 5, 2015 Vancouver, British Columbia, Canada http://icfpconference.org/icfp2015/ ===================================================================== ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. A full week dedicated to functional programming: 1 conference, 1 symposium, 11 workshops, tutorials, programming contest results, student research competition, and mentoring workshop * Program: http://icfpconference.org/icfp2015/program.html * Accepted Papers: http://icfpconference.org/icfp2015/accepted.html * Affiliated Events: http://icfpconference.org/icfp2015/affiliated.html * Local arrangements (including travel and accommodation): http://icfpconference.org/icfp2015/local.html * Registration is available via: https://regmaster4.com/2015conf/ICFP15/register.php Early registration is due 3 August, 2015. * Programming contest, 7-10 August, 2015: http://icfpcontest.org/ * Follow @icfp_conference on twitter for the latest news: http://twitter.com/icfp_conference There are several events affiliated with ICFP: Sunday, August 30 Haskell Implementors Workshop Workshop on Higher-order Programming with Effects Workshop on Generic Programming Programming Languages Mentoring Workshop Ally Skills Tutorial Monday, August 31 – Wednesday, September 2 ICFP Thursday, September 3 Workshop on Functional High-Performance Computing Haskell Symposium – Day 1 ML Family Workshop Commercial Users of Functional Programming – Day 1 Friday, September 4 Erlang Workshop Haskell Symposium – Day 2 OCaml Workshop Commercial Users of Functional Programming – Day 2 Scheme and Functional Programming Workshop Saturday, September 5 Functional Art, Music, Modeling and Design Commercial Users of Functional Programming – Day 3 Conference Organizers General Chair: Kathleen Fisher, Tufts University Program Chair: John Reppy, University of Chicago Local Arrangements Chair: Ronald Garcia, University of British Columbia Industrial Relations Chair: Anil Madhavapeddy, University of Cambridge Workshop Co-Chairs: Tom Schrijvers, KU Leuven Nicolas Wu, University of Bristol Programming Contest Chair: Joe Kiniry, Galois Student Research Competition Chair: Andrew Kennedy, Microsoft Research Mentoring Workshop Co-Chairs: Ronald Garcia, University of British Columbia Stephanie Weirich, University of Pennsylvania Publicity Chair: David Van Horn, University of Maryland Video Chair: Iavor Diatchki, Galois Student Volunteer Co-Chairs: Felipe Bañados Schwerter, University of British Columbia Gabriel Scherer, INRIA Mobile App Chair: Reid Holmes, University of Waterloo Industrial partners: Platinum partners Jane Street Capital Gold partners Anonymous donor Ahrefs Google Mozilla Research Oracle Labs Silver partners Bloomberg Tsuru Capital Galois The University of Chicago Bronze partners Erlang Solutions FireEye IntelliFactory PivotCloud Systor Vest =====================================================================

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Strange error message with PolyKinds
by Nikolay Amiantov 17 Jul '15

17 Jul '15

Hi Cafe, I've bumped into a strange error message -- it looks like an obvious bug, but maybe there's something that I'm just not aware of: GHCi, version 7.10.1: http://www.haskell.org/ghc/ :? for help ghci> type Test a = a ghci> type Huh = Test <interactive>:3:1: Type synonym ‘Test’ should have 1 argument, but has been given none In the type declaration for ‘Huh’ So far, so good: ghci> :set -XPolyKinds ghci> type Test a = a ghci> type Huh = Test <interactive>:6:1: Type synonym ‘Test’ should have 2 arguments, but has been given 1 In the type declaration for ‘Huh’ (Notice the number of arguments reported in the second case) -- Nikolay.

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OCL 2015: ** Deadline Extension ** Submit Your Paper Until July 26, 2015
by Achim D. Brucker 17 Jul '15

17 Jul '15

(Apologies for duplicates) If you are working on the foundations, methods, or tools for OCL or textual modelling, you should now finalise your submission for the OCL workshop! *** The submission deadline has been extended to July 26th, 2015! *** CALL FOR PAPERS 15th International Workshop on OCL and Textual Modeling Tools and Textual Model Transformations Co-located with ACM/IEEE 18th International Conference on Model Driven Engineering Languages and Systems (MODELS 2015) September 28th, 2015, Ottawa, Canada http://ocl2015.lri.fr Modeling started out with UML and its precursors as a graphical notation. Such visual representations enable direct intuitive capturing of reality, but some of their features are difficult to formalize and lack the level of precision required to create complete and unambiguous specifications. Limitations of the graphical notations encouraged the development of text-based modeling languages that either integrate with or replace graphical notations for modeling. Typical examples of such languages are OCL, textual MOF, Epsilon, and Alloy. Textual modeling languages have their roots in formal language paradigms like logic, programming and databases. The goal of this workshop is to create a forum where researchers and practitioners interested in building models using OCL or other kinds of textual languages can directly interact, report advances, share results, identify tools for language development, and discuss appropriate standards. In particular, the workshop will encourage discussions for achieving synergy from different modeling language concepts and modeling language use. The close interaction will enable researchers and practitioners to identify common interests and options for potential cooperation. Topics of interest include (but are not limited to) =================================================== - Mappings between textual modeling languages and other languages/formalisms - Algorithms, evaluation strategies and optimizations in the context of textual modeling languages for -- validation, verification, and testing, -- model transformation and code generation, -- meta-modeling and DSLs, and -- query and constraint specifications - Alternative graphical/textual notations for textual modeling languages - Evolution, transformation and simplification of textual modeling expressions - Libraries, templates and patterns for textual modeling languages - Tools that support textual modeling languages (e.g., verification of OCL formulae, runtime monitoring of invariants) - Complexity results for textual modeling languages - Quality models and benchmarks for comparing and evaluating textual modeling tools and algorithms - Successful applications of textual modeling languages - Case studies on industrial applications of textual modeling languages - Experience reports -- usage of textual modeling languages and tools in complex domains, -- usability of textual modeling languages and tools for end-users - Empirical studies about the benefits and drawbacks of textual modeling languages - Innovative textual modeling tools - Comparison, evaluation and integration of modeling languages - Correlation between modeling languages and modeling tasks This year, we particularly encourage submissions describing tools that support - in a very broad sense - textual modeling languages (if you have implemented OCL.js to run OCL in a web browser, this is the right workshop to present your work) as well as textual model transformations. Venue ===== The workshop will be organized as a part of MODELS 2015 Conference in Ottawa, Canada. It continues the series of OCL workshops held at UML/MODELS conferences: York (2000), Toronto (2001), San Francisco (2003), Lisbon (2004), Montego Bay (2005), Genova (2006), Nashville (2007), Toulouse (2008), Denver (2009), Oslo (2010), Zurich (2011, at the TOOLs conference), 2012 in Innsbruck, 2013 in Miami, and 2014 in Valencia, Spain. Similar to its predecessors, the workshop addresses both people from academia and industry. The aim is to provide a forum for addressing integration of OCL and other textual modeling languages, as well as tools for textual modeling, and for disseminating good practice and discussing the new requirements for textual modeling. Workshop Format =============== The workshop will include short (about 15 min) presentations, parallel sessions of working groups, and sum-up discussions. Submissions =========== Three types of papers will be considered: * short papers (between 6 and 8 pages) describing ideas, * tool papers (between 6 and 8 pages), and * full papers (between 12 and 16 pages) in LNCS format. Submissions should be uploaded to EasyChair (https://easychair.org/conferences/?conf=ocl20150) The program committee will review the submissions (minimum 2 reviews per paper, usually 3 reviews) and select papers according to their relevance and interest for discussions that will take place at the workshop. Accepted papers will be published online in a pre-conference edition of CEUR (http://www.ceur-ws.org) Important Dates =============== Submission of papers: July 26, 2015 (extended) Notification: August 21, 2015 Workshop date: September 28, 2015 Organizers ========== Achim D. Brucker, SAP SE, Germany Marina Egea, Indra Sistemas S.A., Spain Martin Gogolla, University of Bremen, Germany Frederic Tuong, Univ. Paris-Sud - IRT SystemX - LRI, France Programme Committee =================== Mira Balaban, Ben-Gurion University of the Negev, Israel Tricia Balfe, Nomos Software, Ireland Achim D. Brucker, SAP SE, Germany Fabian Buettner, Inria - Ecole des Mines de Nantes, France Jordi Cabot, Inria - Ecole des Mines de Nantes, France Dan Chiorean, Babes-Bolyai University, Romania Robert Clariso, Universitat Oberta de Catalunya, Spain Tony Clark, Middlesex University, UK Manuel Clavel, IMDEA Software Institute, Spain Carolina Dania, IMDEA Software Institute, Spain Birgit Demuth, Technische Universitat Dresden, Germany Marina Egea, Indra Sistemas S.A., Spain Geri Georg, Colorado State University, USA Martin Gogolla, University of Bremen, Germany Shahar Maoz, Tel Aviv University, Israel Istvan Rath, Budapest University of Technology and Economics, Hungary Bernhard Rumpe, RWTH Aachen, Germany Frederic Tuong, Univ. Paris-Sud - IRT SystemX - LRI, France Claas Wilke, Technische Universitat Dresden, Germany Edward Willink, Willink Transformations Ltd., UK Burkhart Wolff, Univ. Paris-Sud - LRI, France Steffen Zschaler, King's College, UK -- Dr. Achim D. Brucker, SAP SE, Vincenz-Priessnitz-Str. 1, D-76131 Karlsruhe Phone: +49 6227 7-52595, http://www.brucker.ch/

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very general directed hypergraphs over text
by Jeffrey Brown 17 Jul '15

17 Jul '15

I'm writing a graph class that generalizes the traditional mathematical graph: Nodes can be statements or relationships, and relationships ("edges") can involve any number of nodes, including other relationships. [1] describes the motivation in a little more detail. [2] explains the idea behind the major types -- Graph, Node, Stmt, and Rel. Of the .hs files in the project, at this time I believe only the primary one, Dwt.hs [3], deserves your attention. It is only about 100 lines of code. I am interested in any kind of feedback at all, no matter how minute in detail or expansive in scope. [4] lists what I already think might be likely problems and solutions. In particular I wonder whether I should interpret these two patterns: "case ni of Rel -> .. Stmt -> .." and "if (Maybe.isJust nMb) ... else error ..." as signs that I'm not completely solving the problem. I suspect it might be helpful to make the graph operations Maybe or Either, and use <|> on them -- but while I can monkey those words, I don't really know what I'm saying. [1] https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/master/engl… [2] https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/master/engl… [3] https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/master/src/… [4] https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/master/engl… Thank you, Jeff

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Call for Contributions: Haskell Implementors' Workshop 2015
by Josef Svenningsson 17 Jul '15

17 Jul '15

Call for Contributions ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2015 Vancouver, Canada, 30 August, 2015 Co-located with ICFP 2015 http://www.icfpconference.org/icfp2015/ Important dates --------------- Proposal Deadline: Monday, 3 August, 2015 (midnight, anywhere on earth) Notification: Monday, 10 August, 2015 Workshop: Sunday, 30 August, 2015 The 7th Haskell Implementors' Workshop is to be held alongside ICFP 2015 this year in Vancouver. It is a forum for people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure, to share their work and discuss future directions and collaborations with others. Talks and/or demos are proposed by submitting an abstract, and selected by a small program committee. There will be no published proceedings; the workshop will be informal and interactive, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. Attendance figures clearly reflect the growth of the Haskell user community and a constant interest in implementation aspects. Scope and target audience ------------------------- It is important to distinguish the Haskell Implementors' Workshop from the Haskell Symposium which is also co-located with ICFP 2015. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors' Workshop will have no proceedings -- although we will aim to make talk videos, slides and presented data available with the consent of the speakers. In the Haskell Implementors' Workshop, we hope to study the underlying technology. We want to bring together anyone interested in the nitty-gritty details behind turning plain-text source code into a deployed product. Having said that, members of the wider Haskell community are more than welcome to attend the workshop -- we need your feedback to keep the Haskell ecosystem thriving. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimisation and benchmarking * Virtual machines and run-time systems * Libraries and tools for development or deployment Talks ----- At this stage we would like to invite proposals from potential speakers for talks and demonstrations. We are aiming for 20 minute talks with 10 minutes for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 300 words. Submissions should be made via HotCRP. The website is: https://icfp-hiw15.hotcrp.com/ We will also have a lightning talks session which will be organised on the day. These talks will be 5-10 minutes, depending on available time. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Organisers ---------- * Richard Eisenberg (University of Pennsylvania) * Edward Kmett (S&P Capital IQ) * Hans-Wolfgang Loidl (Heriot-Watt University) * Trevor L. McDonell (Indiana University) * Deian Stefan (Stanford University) * Josef Svenningsson - chair (Chalmers University of Technology) Contact ------- * Josef Svenningsson <josef.svenningsson at chalmers.se>

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