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PPDP'19 Final CFP
by Ekaterina Komendantskaya 12 Apr '19

12 Apr '19

FINAL CALL FOR PAPERS -- PPDP 2019 21st International Symposium on Principles and Practice of Declarative Programming 7–9 October 2019, Porto, Portugal Collocated with FM'19 http://ppdp2019.macs.hw.ac.uk ====================================================================== Important Dates --------------- Title and abstract registration 26 April 2019 (AoE) Paper submission 3 May 2019 (AoE) Rebuttal period (48 hours) 3 June 2019 (AoE) Author notification 14 June 2019 Final paper version 15 July 2019 Conference 7–9 October 2019 About PPDP ---------- The PPDP 2019 symposium brings together researchers from the declarative programming communities, including those working in the functional, logic, answer-set, and constraint handling programming paradigms. The goal is to stimulate research in the use of logical formalisms and methods for analyzing, performing, specifying, and reasoning about computations, including mechanisms for concurrency, security, static analysis, and verification. Invited Speakers ---------------- Amal Ahmed Northeastern University, USA Title: TBA Naoki Kobayashi The University of Tokyo, Japan Title: 10 Years of the Higher-Order Model Checking Project Scope ----- Submissions are invited on all topics related to declarative programming, from principles to practice, from foundations to applications. Topics of interest include, but are not limited to - Language Design: domain-specific languages; interoperability; concurrency, parallelism and distribution; modules; probabilistic languages; functional languages; reactive languages; database languages; knowledge representation languages; languages with objects; language extensions for tabulation; metaprogramming. - Implementations: abstract machines; interpreters; compilation; compile-time and run-time optimization; memory management. - Foundations: types; logical frameworks; monads and effects; semantics. - Analysis and Transformation: partial evaluation; abstract interpretation; control flow; data flow; information flow; termination analysis; resource analysis; type inference and type checking; verification; validation; debugging; testing. - Tools and Applications: programming and proof environments; verification tools; case studies in proof assistants or interactive theorem provers; certification; novel applications of declarative programming inside and outside of CS; declarative programming pearls; practical experience reports and industrial application; education. For further information, please visit: http://ppdp2019.macs.hw.ac.uk

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Newtype, Data.Text and instance declarations...
by Guy-Laurent Subri 12 Apr '19

12 Apr '19

Hi all, There are a few things that I don't understand involving Data.Text and instance declarations... I'm building a small web app to practise Haskell and am using the SQLite.Simple library. I have defined a few types and functions like so: ''' data BookmarkDB = BookmarkDB Connection Table newtype Table = Table Text instance ToField Table where toField = SQLText initDB :: FilePath -> Table -> IO BookmarkDB initDB fp table = do let query = Query ("CREATE TABLE IF NOT EXISTS :table " <> " (url TEXT, tags TEXT, date INTEGER)") conn <- open fp executeNamed conn query [":table" := table] return $ BookmarkDB conn table ''' 1. My first question is about the instance declaration. Why would ''' instance ToField Table where toField = SQLText ''' return this error ? ''' Couldn't match type ‘Table’ with ‘Text’ Expected type: Table -> SQLData Actual type: Text -> SQLData • In the expression: SQLText In an equation for ‘toField’: toField = SQLText In the instance declaration for ‘ToField Table’ ''' I've noticed that if I write ''' instance ToField Table where toField (Table t) = SQLText t ''' I have no error and it compiles. I don't get it, what is 'toField' supposed to receive except a Table? 2. My second question is concerning the initDB function. If I write it that way, the code compiles, but I get an error during execution: bkmrk: SQLite3 returned ErrorError while attempting to perform prepare "CREATE TABLE IF NOT EXISTS :table (url TEXT, tags TEXT, date INTEGER)": near ":table": syntax error I don't understand what is wrong with the code and I doubt that my query is wrong because everything works with the last code snippet I wrote (see question #3). 3. Trying to get around the error I've noticed a few things. If I try to write this instead: ''' initDB :: FilePath -> Table -> IO BookmarkDB initDB fp table = do let query = Query ("CREATE TABLE IF NOT EXISTS " <> table <> " (url TEXT, tags TEXT, date INTEGER)") conn <- open fp execute_ conn query return $ BookmarkDB conn table ''' it doesn't compile because table is of type Table and not Data.Text.Internal.Text, which I get, but I don't know what is the Haskell way of resolving this. I've also noticed that this will help: ''' initDB :: FilePath -> Table -> IO BookmarkDB initDB fp t@(Table table) = do let query = Query ("CREATE TABLE IF NOT EXISTS " <> table <> " (url TEXT, tags TEXT, date INTEGER)") conn <- open fp execute_ conn query return $ BookmarkDB conn t ''' But I *really* don't get why! The most confusing part for me is that the last initDB function will do exactly what I want it to do, but I am unable to understand why it works...Why am I able to concatenate the 'table' now and not before and why is 't' of a different type than 'table'? All of this is very confusing and I find myself unable to reason about it. Sorry if this is a big/messy question. I hope everything is clear enough to answer... Thank you all!

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[CfP] Functional High-Performance and Numerical Computing 2019
by Marco Zocca 11 Apr '19

11 Apr '19

===================================================== Call for Papers (Extended Abstracts and Full Papers) ACM SIGPLAN Workshop on Functional High-Performance and Numerical Computing (FHPNC), Berlin, August 18, 2019 Satellite event of the 24th ACM SIGPLAN International Conference on Functional Programming (ICFP 2019) 18 - 23 August, 2019 ===================================================== https://icfp19.sigplan.org/home/FHPNC-2019#Call-for-Papers The ACM SIGPLAN International Workshop on Functional High-Performance and Numerical Computing aims to bring together researchers and practitioners exploring or employing the use of functional or declarative programming languages or techniques in scientific computing, and specifically in the domains of high-performance computing and numerical programming. The purpose of the meeting is to enable sharing of results, experiences, and novel ideas about how high-level, declarative techniques can help make high-performance, distributed/parallel, or numerically-intensive code dealing with computationally challenging problems easier to write, read, maintain, or portable to new hardware architectures. Areas of interest include, but are not limited to: * relevant compiler technologies * runtime systems (including fault tolerance mechanisms and those supporting distributed or parallel computation) * domain-specific languages (embedded or standalone) * type systems * formal methods * software libraries (e.g. for exact or interval arithmetic). ## Submission details Submissions should fall into one of two categories: * Regular research papers (up to 12 pages) * Extended abstracts (1 - 2 pages) The bibliography will not be counted against the page limits for either category. Regular research papers are expected to present novel and interesting research results, and will be included in the formal proceedings. Extended abstracts should report work in progress that the authors would like to present at the workshop; they will be evaluated primarily for relevance and interest. Extended abstracts will be distributed to workshop attendees but will not be published in the formal proceedings. We welcome submissions from PC members (with the exception of the PC Chair), but these submissions will be held to a higher standard. Submission is handled through the HotCRP site. All submissions should be in portable document format (PDF) and formatted using the ACM SIGPLAN style guidelines. Submissions written with LaTeX are required to use the ‘acmart’ format and the two-column ‘sigplan’ subformat (not to be confused with the one-column ‘acmlarge’ subformat!). Extended Abstracts must be submitted with the label ‘Extended abstract’ clearly in the title. ## Publication The proceedings of FHPNC 2019 will be published in the ACM Digital Library. ## Related links * Author Information and LaTeX templates : http://www.sigplan.org/Resources/Author/ * Attendee Code of Conduct: http://www.sigplan.org/Resources/Policies/CodeOfConduct/

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Possible ghc bug
by Michel Haber 10 Apr '19

10 Apr '19

Hello Cafe, I was trying to load a module containing this function in ghci: "add x y = wrap $ unwrap x + unwrap y" with the following extensions activated: ConstraintKinds DataKinds DeriveFunctor DuplicateRecordFields FlexibleContexts FlexibleInstances GADTs KindSignatures MultiParamTypeClasses PolyKinds TypeFamilies TypeOperators AllowAmbiguousTypes And it loaded without problem. So then I tested its type with ":t add", which gave: add :: (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 -> b1 Then I added this signature to the function in the module. This caused ghci to refuse to load it and give the following error: src/Exercises.hs:55:11: error: • Could not deduce (Newtype Integer b1) arising from a use of ‘wrap’ from the context: (Num a, Newtype a b1, Newtype a b2, Newtype a b3) bound by the type signature for: add :: forall a b1 b2 b3. (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 -> b1 at src/Exercises.hs:54:1-74 • In the expression: wrap $ unwrap x + unwrap y In an equation for ‘add’: add x y = wrap $ unwrap x + unwrap y | 55 | add x y = wrap $ unwrap x + unwrap y | ^^^^^^^^^^^^^^^^^^^^^^^^^^ src/Exercises.hs:55:18: error: • Could not deduce (Newtype Integer b2) arising from a use of ‘unwrap’ from the context: (Num a, Newtype a b1, Newtype a b2, Newtype a b3) bound by the type signature for: add :: forall a b1 b2 b3. (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 -> b1 at src/Exercises.hs:54:1-74 • In the first argument of ‘(+)’, namely ‘unwrap x’ In the second argument of ‘($)’, namely ‘unwrap x + unwrap y’ In the expression: wrap $ unwrap x + unwrap y | 55 | add x y = wrap $ unwrap x + unwrap y | ^^^^^^^^ src/Exercises.hs:55:29: error: • Could not deduce (Newtype Integer b3) arising from a use of ‘unwrap’ from the context: (Num a, Newtype a b1, Newtype a b2, Newtype a b3) bound by the type signature for: add :: forall a b1 b2 b3. (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 -> b1 at src/Exercises.hs:54:1-74 • In the second argument of ‘(+)’, namely ‘unwrap y’ In the second argument of ‘($)’, namely ‘unwrap x + unwrap y’ In the expression: wrap $ unwrap x + unwrap y | 55 | add x y = wrap $ unwrap x + unwrap y | ^^^^^^^^ Failed, no modules loaded. This does not make sense to me, since I only used the signature that ghci itself gave me. Is this a bug? if not, could someone please explain this behaviour to me? Thanks, Michel

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ANN: gitlab-haskell
by Rob Stewart 10 Apr '19

10 Apr '19

This new gitlab-haskell library provides read/write access to data on a GitLab server, using the GitLab web API. The library provides access to: branches, commits, groups, issues, jobs, members, merge requests, pipelines, projects, repositories, repository files and users. JSON responses are parsed into typed Haskell values for convenience. https://hackage.haskell.org/package/gitlab-haskell-0.1.0.0 -- Rob Stewart

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Re: [Haskell-cafe] streaming with logs / metadata
by Olaf Klinke 10 Apr '19

10 Apr '19

>> Anything beyond the Functor instance for (Either e), though, models exceptions that abort the computation as soon as the first Left arises. You seem to use the type in a different way which may be one reason why you do not find your functions implemented elsewhere. For logging or warnings, most Haskellers use something like a writer monad transformer. The underlying monad, >> >> Writer w a = (w,a) > > That's the thing I was saying destroys streaming. The problem is that > it returns ([log], [a]), instead of [Either log a], so the > interleaving has been lost. > Evan, When you say interleaving, do you think of lists like the following? [Left warning1,Right value 1,Left warning 2,Right value2,...] The above is isomorphic to [(warning1,value1),(warning2,value2),...] I don't think that the writer monad destroys streaming. Below is a somewhat contrived, but ordinary use case. {-- begin example --} import Control.Monad.Writer -- from mtl type Log = [String] -- some Monoid half :: Int -> Writer Log Int half x = case x `quotRem` 2 of (y,0) -> return y (y,1) -> writer (y,["Warning: "++(show x)++" was uneven."]) stream :: [Int] stream = 4:8:5:3:7:2: error "broken stream" (ys,warnings) = runWriter (mapM half stream) {-- end example --} ghci> -- load the example above ghci> ys [2,4,2,1,3,1*** Exception: broken stream ghci> mapM_ putStrLn warnings Warning: 5 was uneven. Warning: 3 was uneven. Warning: 7 was uneven. *** Exception: broken stream You see that there is some output before the error is reached, proving that both ys and warnings could be consumed in a streaming manner. Here it is important that the Log monoid's `mappend` function is not strict in the second argument. It would not, for example, work with Log = Data.Sequence.Seq String With mtl you could easily throw in a State monad component that keeps track of the position in the stream, so that warning messages can hint at the place where the warning originated. Olaf

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[Announcement] Copilot 3.0, a hard realtime C generator and runtime verification framework
by Frank Dedden 09 Apr '19

09 Apr '19

Dear all, We are very pleased to announce the release of Copilot 3.0, a stream-based DSL for writing and monitoring embedded C programs, with an emphasis on correctness and hard realtime requirements. Copilot is typically used as a runtime verification framework, where programs serve as specifications to the system we want to monitor. Additional libraries provide utility functions for temporal logic (LTL, PTLTL and MTL), clocks and voting algorithms. Over the years, Copilot has been used at the Safety Critical Avionics Systems Branch of NASA Langley Research Center for monitoring test flights of drones. This new release contains a number of additional features that make writing programs easier: * The extended language with support for monitoring and generating structs and arrays in a type-safe manner. * A completely rewritten C99 code generator to support the new features. * Significant improvements on the quality and readability of the output code, which help in traceability and certification of the code. * Internal simplifications for a more future proof codebase. Copilot is available on hackage [1]. For more information, including documentation, examples and links to the source code, please visit the webpage [2]. Kind regards, The Copilot developers: - Frank Dedden (maintainer) - Alwyn Goodloe (maintainer) - Ivan Perez [1] http://hackage.haskell.org/package/copilot [2] https://copilot-language.github.io

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OO "dot" notation and records...
by Nicholls, Mark 09 Apr '19

09 Apr '19

Is there something in Haskell like this? I could do with getting OO like intelisense going...i.e. you have some values, the types are inhered in my environment, and the available functions prompt the programmer. (I'm not a big Haskell user, but I'm trying to "sell" it as an alternative for F# and scala in an industrial context). 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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looking for yesod-auth example with LTS-13
by Johannes Waldmann 09 Apr '19

09 Apr '19

Dear Cafe, I am trying to update some legacy code, and I have trouble getting yesod-auth to work under LTS-13. It worked with LTS-10. I think the error in this demonstration project (not mine) is related to what I am seeing: https://github.com/TobyGoodwin/auth38/issues/1 My actual code (and error message) is here https://gitlab.imn.htwk-leipzig.de/autotool/all0/issues/565 but "auth38" is certainly easier to reproduce. I was unable to apply this idea: https://stackoverflow.com/questions/23188494/yesod-subsites-and-forms/29394… (which seems dated - the HandlerT type is gone?) Any ideas? Thanks - J.W.

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Package takeover intent - dyre
by Fraser Tweedale 09 Apr '19

09 Apr '19

The dyre[1] package is unmaintained for 5 years. I would like to contribute to it or take over maintainership, but have been unable to contact the maintainer (Cc'd). Pursuant to wiki[2] I am announcing my intention to take over the package. [1] https://hackage.haskell.org/package/dyre [2] https://wiki.haskell.org/Taking_over_a_package Thanks, Fraser

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