August 2018 - Haskell-Cafe

OT: compiler writing like company from EU like country having made more than 3e6 sales?
by Marc Weber 26 Aug '18

26 Aug '18

Eventually its possible to get 150K of funding from EU to move 'knowledge' into Kosovo/Albania. I've been missing a simple compiler with some features for a long time. This might be a chance to get it done. By features I mean - discus like region typing - ur like mixing of server/client lanugage - quick and dirty way to implement type level type deducations. Eg matrix * matrix => new type so that you also can type databases and the like. I know that Haskell is touring complete in this regard, but it also should be fast. - macro programming Reply to this thread or private message if you think you own a company which have been working in hosting or package management or the like and is located within Europe including Switzerland. Marc Weber

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Blog post about modelling state machines with free categories
by Marcin Szamotulski 25 Aug '18

25 Aug '18

Hello, I wrote a blog post about modelling state machines with free categories: https://marcinszamotulski.me/posts/finite-state-machines.html Best regards, Marcin Sent with ProtonMail Secure Email.

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Alternative instance for non-backtracking parsers
by Olaf Klinke 24 Aug '18

24 Aug '18

Dear Cafe, can anyone explain the use of an Alternative instance for a parser monad where the parser (p <|> q) does not backtrack automatically when p fails? Consider megaparsec for example. When p is a parser that does not backtrack automatically, then (p <|> q) never succeeds with q. While I do understand that fine-grained control of backtracking is desirable for performance reasons in general, it escapes me why such an Alternative instance could be useful. See also "A note on backtracking" in the parser-combinators package documentation [1]. Minimal code example and context below. Thanks, Olaf [1] http://hackage.haskell.org/package/parser-combinators-1.0.0/docs/Control-Ap… import Text.Megaparsec import Text.Megaparsec.Char import Data.Void p :: Parsec Void String Bool p = do char '-' string "True" return True q :: Parsec Void String Bool q = do char '-' string "False" return False >>> parseTest (p <|> q) "-True" True >>> parseTest (p <|> q) "-False" 1:2: unexpected "Fals" expecting "True" -- Since q can parse "-False", I would expect (p <|> q) to parse "-False", too. Context: Of course the example above can be mitigated by moving the char '-' outside the alternative. But we might not have control over whether the two parsers share a common prefix. I am trying to write some code generic in the parser monad and I am having difficulties making my code work for both backtracking and non-backtracking parsers. If possible, I'd like to keep my parser type class devoid of a 'try' combinator, which would be the identity function for parser monads that always backtrack.

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Cobertura from .tix/.mix
by Paul 24 Aug '18

24 Aug '18

Hello, Cafe! Is any tool to generate Cobertura from .tix/.mix files? As I understand they are serialized Mix and Tix types from HPC related libraries, but better to avoid coding and to use ready tool :)

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GHC gets simpler!?
by Jeffrey Brown 24 Aug '18

24 Aug '18

I just flipped through the list of accepted GHC proposals[1]. I feel like a kid in a candy shop. I am particularly excited to see the large fraction of proposals that look like either simplifications or homogenizations (making the language more uniform): * Make Constraint not apart from Type * Allow signatures on pattern synonym constructors * Explicit foralls ("Permit ``forall ...`` to bind type variables after the word ``instance`` ...") * Remove the * kind syntax * Allow ScopedTypeVariables to refer to types * Treat kind and type vars identically with `forall` * Embrace (Type :: Type) ("This proposal expands the meaning of -XPolyKinds to include -XTypeInType, deprecates -XTypeInType, and advocates for using Type in place of *.") At various times, I have tried to explore the GHC extensions thoroughly -- well enough to know which ones will let you do what, so that I can learn the right one in detail when I need to. I have always turned back; the space seems too big to cover, and only growing. But now I see that GHC hosts competing forces of simplification and complication. This might be a squishy question, but I'm interested in your feelings regarding how navigable the space of extensions is, and whether you think balkanization is a problem, and how you expect those qualities to evolve. [1] https://github.com/ghc-proposals/ghc-proposals/pulls?page=2&q=label%3AAccep… -- Jeff Brown | Jeffrey Benjamin Brown Website <https://msu.edu/~brown202/> | Facebook <https://www.facebook.com/mejeff.younotjeff> | LinkedIn <https://www.linkedin.com/in/jeffreybenjaminbrown>(spammy, so I often miss messages here) | Github <https://github.com/jeffreybenjaminbrown>

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How to express constraint succeeded?
by Paul 23 Aug '18

23 Aug '18

Hello, Dear Cafe! I have expression 'True ~ 'True in right hand side of type level function. Is some "constant" for the same?

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CFP: 4th International Workshop on Real World Domain Specific Languages
by Simon Thompson 23 Aug '18

23 Aug '18

Apologies for any duplicates! CALL FOR PAPERS 4th International Workshop on Real World Domain Specific Languages https://sites.google.com/site/realworlddsl In conjunction with The International Symposium on Code Generation and Optimisation 2019. http://cgo.org/cgo2019/ Washington DC, USA, 16th February, 2019 As the use of computers proliferates, the complexity and variety of systems continues to grow. As a result, it is becoming increasingly inflexible to "hard wire” behaviours into software. Software developers can enable more control over their software configurations by exploiting Domain Specific Languages (DSLs). Such DSLs provide a systematic way to structure the underlying computational components: to coin a phrase, a DSL is a library with syntax. There is an enormous variety of DSLs for a very wide range of domains. Most DSLs are highly idiosyncratic, reflecting both the specific natures of their application domains and their designers’ own preferences. This workshop will bring together constructors of DSLs for “real world” domains; that is, DSLs intended primarily to aid in building software to solve real world problems rather than to explore the more theoretical aspects of language design and implementation. We are looking for submissions that present the motivation, design, implementation, use and evaluation of such DSLs. Previous workshops were RWSDL’16 (Barcelona), RWDSL’17 (Austin) and RWDSL’18 (Vienna), all collocated with CGO and published in the ACM Digital Library. Key Dates: Paper submission deadline: 9th November 2018 Author notification: 7th December 2018 Final manuscript due: 11th January 2019 Workshop: 16th February 2019 Submission Instructions: The EasyChair submission page for this workshop is: https://easychair.org/conferences/?conf=rwdsl19 Accepted submissions will be published in the ACM Digital Library within its International Conference Proceedings Series. Submissions should be 8-10 pages in ACM double-column format. Authors should follow the information for formatting ACM SIGPLAN conference papers, which can be found at http://www.acm.org/publications/proceedings-template. Full submission details are on the workshop web page. Program Chairs Rob Stewart, Heriot-Watt University, UK Greg Michaelson, Heriot-Watt University, UK PC Members Allin Cottrell, Wake Forest University, USA Nina Dethlefs, University of Hull, UK Andy Gill, X – The Moonshot Factory, USA Kevin Hammond, University of St Andrews, UK Patrick Maier, University of Glasgow, UK Mathijs Schuts, Phillips Healthcare, Netherlands Simon Thompson, University of Kent, UK Phil Trinder, University of Glasgow, UK Contact Please email inquiries concerning the workshop to: R.Stewart(a)hw.ac.uk

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Access violation when stack haddock haskell-src-exts since LTS 12.0
by Yuji Yamamoto 23 Aug '18

23 Aug '18

I always get an " Access violation in generated code when reading 0x28900000" error when executing > stack haddock haskell-src-exts since LTS 12.0. I'm not sure to which project I should submit a bug report. GHC? Haddock? Or Stack? And how can I make a smaller case to reproduce the same error? I attached a log when executing with --verbose --cabal-verbose for your info. -- 山本悠滋 twitter: @igrep GitHub: https://github.com/igrep GitLab: https://gitlab.com/igrep Facebook: http://www.facebook.com/igrep Google+: https://plus.google.com/u/0/+YujiYamamoto_igrep

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ANN: leancheck-v0.7.2, enumerative QuickCheck-like testing
by Rudy Matela 22 Aug '18

22 Aug '18

Hello Haskell-Café, LeanCheck is a simple enumerative property-based testing library for Haskell. It is highly portable, and even works on Hugs98 (v2006-09). It does not pull any dependencies other than what's bundled with GHC. Check it out on Hackage and GitHub: https://hackage.haskell.org/package/leancheck https://github.com/rudymatela/leancheck A simple example follows: > import Test.LeanCheck > import Data.List (sort, union) > check $ \xs -> sort (sort xs) == sort (xs::[Int]) +++ OK, passed 200 tests. > check $ \xs ys -> xs `union` ys == ys `union` (xs::[Int]) *** Failed! Falsifiable (after 4 tests): [] [0,0] LeanCheck works on all types that are instances of the Listable typeclass. A TH automatic deriver is provided with LeanCheck so you don't need to learn LeanCheck's DSL to test with your types. I recently released 0.7.2, there are several additions since last announcement here. Notable changes include: * minor fixes for loading on Hugs98; * functions to compute statistics about generated values; and * a family of functions to test invariants of typeclass instances: > data MyTy = ... > instance ... MyTy where ... > import Test.LeanCheck.Utils > check $ (okEqOrd :: MyTy -> MyTy -> MyTy -> Bool) +++ OK, passed 200 tests. > check $ (okNum :: MyTy -> MyTy -> MyTy -> Bool) +++ OK, passed 200 tests. LeanCheck is available on Hackage or Stackage, and can be installed using: $ cabal install leancheck As far as I am aware, there are no bindings to use LeanCheck with tasty, test-framework or hspec yet, but contributions are welcome. I may create bindings for those if I ever get the time. Best Regards, Rudy

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how do FFI calls show up in profiling info?
by Johannes Waldmann 22 Aug '18

22 Aug '18

Dear Cafe, how would FFI calls show up in profiles? I am using hmatrix-glpk. The relevant FFI call is in https://hackage.haskell.org/package/hmatrix-glpk-0.19.0.0/docs/src/Numeric.… I am compiling with profiling (for executables and libraries), and I am running with +RTS -P . The .prof file does not contain c_simplex_sparse (the C function) It does mention simplexSparse (the Haskell function that calls it) but with a suspiciously low time. - J.W.

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