February 2019 - Haskell-Cafe

Extension and parameterized classes
by Pierre_van_der_Laar (Functional Account) 14 Feb '19

14 Feb '19

Dear Haskell experts, I working on a large Haskell project on Model Based Testing (see https://github.com/TorXakis/TorXakis) and I would like to get feedback on a design issue I have related to extension and parameterized classes. I will describe a small simplified example to illustrate the issue and I appreciate any feedback. I have a class that extends another class as follows: class Base a => Extend a where -- | Constructor from Base fromBase :: Base b => b -> a I can define an instance of Extend that hides the Base implementation as follows: {-# LANGUAGE ExistentialQuantification #-} data BaseHidden = forall a . Base a => BaseHidden { _base :: a , ... -- additional extension info } instance Extend BaseHidden where fromBase b = BaseHidden b ... -- additional extension initialization I can define a parameterized data class of which the parameter is the type of the Base class as follows: data BaseExpose a = BaseExpose { _base :: a , ... -- additional extension info } With this parameterized data class, I can make the following fromBase' function fromBase' :: Base a => a -> BaseExpose a fromBase' a = BaseExpose a ... -- additional extension initialization My question is can I make BaseExpose an instance of Extend? I am aware that the signature of fromBase states that for all 'b' of Base an Extend of type 'a' must be able to be made. Yet, in my case 'a' is a parameterized class 'BaseExpose x' and since the type of Extend is provided (it is a constructor) I think it is acceptable that 'BaseExpose b' is returned as type. I have looked at some extensions, such as InstanceSigs, yet I found no solution. Does anybody on this mailing list have an answer or solution for me? Also experience in a large project related to maintenance for the two alternatives (hiding or exposing types) is appreciated! Thanks in advance, Pierre van de Laar ________________________________ The information contained in this message may be confidential and legally protected under applicable law. The message is intended solely for the addressee(s). If you are not the intended recipient, you are hereby notified that any use, forwarding, dissemination, or reproduction of this message is strictly prohibited and may be unlawful. If you are not the intended recipient, please contact the sender by return e-mail and destroy all copies of the original message.

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Source code comprehension - Looking for ideas
by Németh Boldizsár 14 Feb '19

14 Feb '19

Dear Haskellers, I'm involved in extending the CodeCompass (https://github.com/Ericsson/CodeCompass) source code comprehension tool with a Haskell plugin. I'm looking for ideas on views or searches (both graphical and textual) that can help programmers to understand complex Haskell programs (using compile-time information). The basic examples that I will implement is finding the definition of a name and finding usages of a name. I'm also interested in similar tools or libraries if there is any. BR, Boldizsár

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Call for Submissions: ICFP Student Research Competition
by Sam Tobin-Hochstadt 13 Feb '19

13 Feb '19

ICFP 2019 Student Research Competition Call for Submissions ICFP invites students to participate in the Student Research Competition in order to present their research and get feedback from prominent members of the programming language research community. Please submit your extended abstracts through the submission website. ### Important dates Submissions due: 14 Jun 2019 (Friday) https://icfp19src.hotcrp.com Notification: 28 Jun 2019 (Friday) Conference: 18 August (Sunday) - 23 August (Friday) Each submission (referred to as "abstract" below) should include the student author’s name and e-mail address; institutional affiliation; research advisor’s name; ACM student member number; category (undergraduate or graduate); research title; and an extended abstract addressing the following: * Problem and Motivation: Clearly state the problem being addressed and explain the reasons for seeking a solution to this problem. * Background and Related Work: Describe the specialized (but pertinent) background necessary to appreciate the work in the context of ICFP areas of interest. Include references to the literature where appropriate, and briefly explain where your work departs from that done by others. * Approach and Uniqueness: Describe your approach in addressing the problem and clearly state how your approach is novel. * Results and Contributions: Clearly show how the results of your work contribute to programming language design and implementation in particular and to computer science in general; explain the significance of those results. * Submissions must be original research that is not already published at ICFP or another conference or journal. One of the goals of the SRC is to give students feedback on ongoing, unpublished work. Furthermore, the abstract must be authored solely by the student. If the work is collaborative with others and*or part of a larger group project, the abstract should make clear what the student’s role was and should focus on that portion of the work. * Formatting: Submissions must be in PDF format, printable in black and white on US Letter sized paper, and interpretable by common PDF tools. All submissions must adhere to the "ACM Small" template that is available (in both LaTeX and Word formats) from https://www.acm.org/publications/authors/submissions. For authors using LaTeX, a lighter-weight package, including only the essential files, is available from http://sigplan.org/Resources/Author/#acmart-format. The submission must not exceed 3 pages in PDF format. Reference lists do not count towards the 3-page limit. Further information is available at the ICFP SRC website: https://icfp19.sigplan.org/track/icfp-2019-Student-Research-Competition ICFP Student Research Competition Chair: William J. Bowman (University of British Columbia)

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Re: [Haskell-cafe] ANN:DBFunctor-0.1.1.0 - Functional Data Management / ETL Data Processing in Haskell
by Olaf Klinke 13 Feb '19

13 Feb '19

Apologies Nikos, apparently clicking on the sender in the mail archives replaces the address shown with "haskell-cafe(a)haskell.org", a behaviour I was unaware of. Olaf > Dear Nikos, > > (not CC'ing the list yet because I don't know whether my remarks miss the design goals of DBFunctor) > > As a data scientist I have some questions regarding the design of DBFunctor. > > 1. > Why is RTable, or rather RDataType a closed type? Was this forced by compatibility to other databases? There might be RTabular Types whose elements are hard to represent as RDataType, or the conversion deletes some of the data's semantics. What is the essence that makes the operations in the Julius language work? For example, the inner join could have the more general type > MonadPlus m => (a -> b -> Bool) -> m a -> m b -> m (a,b) > > 2. > Is there a way of saying that a column may not contain NULL values? > > 3. > What about the efficiency of the operations? I see no complexities stated in the documentation of Etl.Julius. > > 4. > I like the feature of named operations (:=>). But can we bind a sub-operation to a Haskell variable without leaving Julius? After all, takeNamedResult can lead to exceptions because the user may reference a name that has not been defined. With haskell variables, the compiler prevents this. Why not make the Julius language a monad, so that one can use do-notation for building the result? E.g. > > type JuliusExpr = JuliusLang RTable > instance Monad JuliusLang where ... > > Kind regards, > Olaf

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Third Call for Papers: PACMPL issue ICFP 2019
by Sam Tobin-Hochstadt 13 Feb '19

13 Feb '19

PACMPL Volume 3, Issue ICFP 2019 Call for Papers accepted papers to be invited for presentation at The 24th ACM SIGPLAN International Conference on Functional Programming Berlin, Germany http://icfp19.sigplan.org/ ### Important dates Submissions due: 1 March 2019 (Friday) Anywhere on Earth https://icfp19.hotcrp.com Author response: 16 April (Tuesday) - 18 Apri (Friday) 14:00 UTC Notification: 3 May (Friday) Final copy due: 22 June (Saturday) Conference: 18 August (Sunday) - 23 August (Friday) ### About PACMPL Proceedings of the ACM on Programming Languages (PACMPL <https://pacmpl.acm.org/>) is a Gold Open Access journal publishing research on all aspects of programming languages, from design to implementation and from mathematical formalisms to empirical studies. Each issue of the journal is devoted to a particular subject area within programming languages and will be announced through publicized Calls for Papers, like this one. ### Scope [PACMPL](https://pacmpl.acm.org/) issue ICFP 2019 seeks original papers on the art and science of functional programming. Submissions are invited on all topics from principles to practice, from foundations to features, and from abstraction to application. The scope includes all languages that encourage functional programming, including both purely applicative and imperative languages, as well as languages with objects, concurrency, or parallelism. Topics of interest include (but are not limited to): * *Language Design*: concurrency, parallelism, and distribution; modules; components and composition; metaprogramming; type systems; interoperability; domain-specific languages; and relations to imperative, object-oriented, or logic programming. * *Implementation*: abstract machines; virtual machines; interpretation; compilation; compile-time and run-time optimization; garbage collection and memory management; multi-threading; exploiting parallel hardware; interfaces to foreign functions, services, components, or low-level machine resources. * *Software-Development Techniques*: algorithms and data structures; design patterns; specification; verification; validation; proof assistants; debugging; testing; tracing; profiling. * *Foundations*: formal semantics; lambda calculus; rewriting; type theory; monads; continuations; control; state; effects; program verification; dependent types. * *Analysis and Transformation*: control-flow; data-flow; abstract interpretation; partial evaluation; program calculation. * *Applications*: symbolic computing; formal-methods tools; artificial intelligence; systems programming; distributed-systems and web programming; hardware design; databases; XML processing; scientific and numerical computing; graphical user interfaces; multimedia and 3D graphics programming; scripting; system administration; security. * *Education*: teaching introductory programming; parallel programming; mathematical proof; algebra. Submissions will be evaluated according to their relevance, correctness, significance, originality, and clarity. Each submission should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. The technical content should be accessible to a broad audience. PACMPL issue ICFP 2019 also welcomes submissions in two separate categories — Functional Pearls and Experience Reports — that must be marked as such at the time of submission and that need not report original research results. Detailed guidelines on both categories are given at the end of this call. Please contact the principal editor if you have questions or are concerned about the appropriateness of a topic. ### Preparation of submissions **Deadline**: The deadline for submissions is **Friday, March 1, 2019**, Anywhere on Earth (<https://en.wikipedia.org/wiki/Anywhere_on_Earth>). This deadline will be strictly enforced. **Formatting**: Submissions must be in PDF format, printable in black and white on US Letter sized paper, and interpretable by common PDF tools. All submissions must adhere to the "ACM Small" template that is available (in both LaTeX and Word formats) from <https://www.acm.org/publications/authors/submissions>. For authors using LaTeX, a lighter-weight package, including only the essential files, is available from <http://sigplan.org/Resources/Author/#acmart-format>. There is a limit of **25 pages for a full paper or Functional Pearl** and **12 pages for an Experience Report**; in either case, the bibliography will not be counted against these limits. Submissions that exceed the page limits or, for other reasons, do not meet the requirements for formatting, will be summarily rejected. Supplementary material can and should be **separately** submitted (see below). See also PACMPL's Information and Guidelines for Authors at <https://pacmpl.acm.org/authors.cfm>. **Submission**: Submissions will be accepted at <https://icfp19.hotcrp.com/> Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. **Author Response Period**: Authors will have a 72-hour period, starting at 14:00 UTC on **Tuesday, April 16, 2019**, to read reviews and respond to them. **Supplementary Material**: Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. This supplementary material should **not** be submitted as part of the main document; instead, it should be uploaded as a **separate** PDF document or tarball. Supplementary material should be uploaded **at submission time**, not by providing a URL in the paper that points to an external repository. Authors are free to upload both anonymized and non-anonymized supplementary material. Anonymized supplementary material will be visible to reviewers immediately; non-anonymized supplementary material will be revealed to reviewers only after they have submitted their review of the paper and learned the identity of the author(s). **Authorship Policies**: All submissions are expected to comply with the ACM Policies for Authorship that are detailed at <https://www.acm.org/publications/authors/information-for-authors>. **Republication Policies**: Each submission must adhere to SIGPLAN's republication policy, as explained on the web at <http://www.sigplan.org/Resources/Policies/Republication>. **Resubmitted Papers**: Authors who submit a revised version of a paper that has previously been rejected by another conference have the option to attach an annotated copy of the reviews of their previous submission(s), explaining how they have addressed these previous reviews in the present submission. If a reviewer identifies him/herself as a reviewer of this previous submission and wishes to see how his/her comments have been addressed, the principal editor will communicate to this reviewer the annotated copy of his/her previous review. Otherwise, no reviewer will read the annotated copies of the previous reviews. ### Review Process This section outlines the two-stage process with lightweight double-blind reviewing that will be used to select papers for PACMPL issue ICFP 2019. We anticipate that there will be a need to clarify and expand on this process, and we will maintain a list of frequently asked questions and answers on the conference website to address common concerns. **PACMPL issue ICFP 2019 will employ a two-stage review process.** The first stage in the review process will assess submitted papers using the criteria stated above and will allow for feedback and input on initial reviews through the author response period mentioned previously. At the review meeting, a set of papers will be conditionally accepted and all other papers will be rejected. Authors will be notified of these decisions on **May 3, 2019**. Authors of conditionally accepted papers will be provided with committee reviews (just as in previous conferences) along with a set of mandatory revisions. After four weeks (May 31, 2019), the authors will provide a second submission. The second and final reviewing phase assesses whether the mandatory revisions have been adequately addressed by the authors and thereby determines the final accept/reject status of the paper. The intent and expectation is that the mandatory revisions can be addressed within four weeks and hence that conditionally accepted papers will in general be accepted in the second phase. The second submission should clearly identify how the mandatory revisions were addressed. To that end, the second submission must be accompanied by a cover letter mapping each mandatory revision request to specific parts of the paper. The cover letter will facilitate a quick second review, allowing for confirmation of final acceptance within two weeks. Conversely, the absence of a cover letter will be grounds for the paper’s rejection. **PACMPL issue ICFP 2019 will employ a lightweight double-blind reviewing process.** To facilitate this, submitted papers must adhere to two rules: 1. **author names and institutions must be omitted**, and 2. **references to authors' own related work should be in the third person** (e.g., not "We build on our previous work ..." but rather "We build on the work of ..."). The purpose of this process is to help the reviewers come to an initial judgement about the paper without bias, not to make it impossible for them to discover the authors if they were to try. Nothing should be done in the name of anonymity that weakens the submission or makes the job of reviewing the paper more difficult (e.g., important background references should not be omitted or anonymized). In addition, authors should feel free to disseminate their ideas or draft versions of their paper as they normally would. For instance, authors may post drafts of their papers on the web or give talks on their research ideas. ### Information for Authors of Accepted Papers * As a condition of acceptance, final versions of all papers must adhere to the new ACM Small format. The page limit for the final versions of papers will be increased by two pages to help authors respond to reviewer comments and mandatory revisions: **27 pages plus bibliography for a regular paper or Functional Pearl, 14 pages plus bibliography for an Experience Report**. * Authors of accepted submissions will be required to agree to one of the three ACM licensing options: open access on payment of a fee (**recommended**, and SIGPLAN can cover the cost as described next); copyright transfer to ACM; or retaining copyright but granting ACM exclusive publication rights. Further information about ACM author rights is available from <http://authors.acm.org>. * PACMPL is a Gold Open Access journal. It will be archived in ACM’s Digital Library, but no membership or fee is required for access. Gold Open Access has been made possible by generous funding through ACM SIGPLAN, which will cover all open access costs in the event authors cannot. Authors who can cover the costs may do so by paying an Article Processing Charge (APC). PACMPL, SIGPLAN, and ACM Headquarters are committed to exploring routes to making Gold Open Access publication both affordable and sustainable. * ACM offers authors a range of copyright options, one of which is Creative Commons CC-BY publication; this is the option recommended by the PACMPL editorial board. A reasoned argument in favour of this option can be found in the article [Why CC-BY?](https://oaspa.org/why-cc-by/) published by OASPA, the Open Access Scholarly Publishers Association. * We intend that the papers will be freely available for download from the ACM Digital Library in perpetuity via the OpenTOC mechanism. * ACM Author-Izer is a unique service that enables ACM authors to generate and post links on either their home page or institutional repository for visitors to download the definitive version of their articles from the ACM Digital Library at no charge. Downloads through Author-Izer links are captured in official ACM statistics, improving the accuracy of usage and impact measurements. Consistently linking to the definitive version of an ACM article should reduce user confusion over article versioning. After an article has been published and assigned to the appropriate ACM Author Profile pages, authors should visit <http://www.acm.org/publications/acm-author-izer-service> to learn how to create links for free downloads from the ACM DL. * At least one author of each accepted submissions will be expected to attend and present their paper at the conference. The schedule for presentations will be determined and shared with authors after the full program has been selected. Presentations will be videotaped and released online if the presenter consents. * The official publication date is the date the papers are made available in the ACM Digital Library. This date may be up to *two weeks prior* to the first day of the conference. The official publication date affects the deadline for any patent filings related to published work. ### Artifact Evaluation Authors of papers that are conditionally accepted in the first phase of the review process will be encouraged (but not required) to submit supporting materials for Artifact Evaluation. These items will then be reviewed by an Artifact Evaluation Committee, separate from the paper Review Committee, whose task is to assess how the artifacts support the work described in the associated paper. Papers that go through the Artifact Evaluation process successfully will receive a seal of approval printed on the papers themselves. Authors of accepted papers will be encouraged to make the supporting materials publicly available upon publication of the papers, for example, by including them as "source materials" in the ACM Digital Library. An additional seal will mark papers whose artifacts are made available, as outlined in the ACM guidelines for artifact badging. Participation in Artifact Evaluation is voluntary and will not influence the final decision regarding paper acceptance. ### Special categories of papers In addition to research papers, PACMPL issue ICFP solicits two kinds of papers that do not require original research contributions: Functional Pearls, which are full papers, and Experience Reports, which are limited to half the length of a full paper. Authors submitting such papers should consider the following guidelines. #### Functional Pearls A Functional Pearl is an elegant essay about something related to functional programming. Examples include, but are not limited to: * a new and thought-provoking way of looking at an old idea * an instructive example of program calculation or proof * a nifty presentation of an old or new data structure * an interesting application of functional programming techniques * a novel use or exposition of functional programming in the classroom While pearls often demonstrate an idea through the development of a short program, there is no requirement or expectation that they do so. Thus, they encompass the notions of theoretical and educational pearls. Functional Pearls are valued as highly and judged as rigorously as ordinary papers, but using somewhat different criteria. In particular, a pearl is not required to report original research, but, it should be concise, instructive, and entertaining. A pearl is likely to be rejected if its readers get bored, if the material gets too complicated, if too much specialized knowledge is needed, or if the writing is inelegant. The key to writing a good pearl is polishing. A submission that is intended to be treated as a pearl must be marked as such on the submission web page, and should contain the words "Functional Pearl" somewhere in its title or subtitle. These steps will alert reviewers to use the appropriate evaluation criteria. Pearls will be combined with ordinary papers, however, for the purpose of computing the conference's acceptance rate. #### Experience Reports The purpose of an Experience Report is to help create a body of published, refereed, citable evidence that functional programming really works — or to describe what obstacles prevent it from working. Possible topics for an Experience Report include, but are not limited to: * insights gained from real-world projects using functional programming * comparison of functional programming with conventional programming in the context of an industrial project or a university curriculum * project-management, business, or legal issues encountered when using functional programming in a real-world project * curricular issues encountered when using functional programming in education * real-world constraints that created special challenges for an implementation of a functional language or for functional programming in general An Experience Report is distinguished from a normal PACMPL issue ICFP paper by its title, by its length, and by the criteria used to evaluate it. * Both in the papers and in any citations, the title of each accepted Experience Report must end with the words "(Experience Report)" in parentheses. The acceptance rate for Experience Reports will be computed and reported separately from the rate for ordinary papers. * Experience Report submissions can be at most 12 pages long, excluding bibliography. * Each accepted Experience Report will be presented at the conference, but depending on the number of Experience Reports and regular papers accepted, authors of Experience reports may be asked to give shorter talks. * Because the purpose of Experience Reports is to enable our community to accumulate a body of evidence about the efficacy of functional programming, an acceptable Experience Report need not add to the body of knowledge of the functional-programming community by presenting novel results or conclusions. It is sufficient if the Report states a clear thesis and provides supporting evidence. The thesis must be relevant to ICFP, but it need not be novel. The review committee will accept or reject Experience Reports based on whether they judge the evidence to be convincing. Anecdotal evidence will be acceptable provided it is well argued and the author explains what efforts were made to gather as much evidence as possible. Typically, more convincing evidence is obtained from papers which show how functional programming was used than from papers which only say that functional programming was used. The most convincing evidence often includes comparisons of situations before and after the introduction or discontinuation of functional programming. Evidence drawn from a single person's experience may be sufficient, but more weight will be given to evidence drawn from the experience of groups of people. An Experience Report should be short and to the point: it should make a claim about how well functional programming worked on a particular project and why, and produce evidence to substantiate this claim. If functional programming worked in this case in the same ways it has worked for others, the paper need only summarize the results — the main part of the paper should discuss how well it worked and in what context. Most readers will not want to know all the details of the project and its implementation, but the paper should characterize the project and its context well enough so that readers can judge to what degree this experience is relevant to their own projects. The paper should take care to highlight any unusual aspects of the project. Specifics about the project are more valuable than generalities about functional programming; for example, it is more valuable to say that the team delivered its software a month ahead of schedule than it is to say that functional programming made the team more productive. If the paper not only describes experience but also presents new technical results, or if the experience refutes cherished beliefs of the functional-programming community, it may be better to submit it as a full paper, which will be judged by the usual criteria of novelty, originality, and relevance. The principal editor will be happy to advise on any concerns about which category to submit to. ### ICFP Organizers General Chair: Derek Dreyer (MPI-SWS, Germany) Artifact Evaluation Co-Chairs: Simon Marlow (Facebook, UK) Industrial Relations Chair: Alan Jeffrey (Mozilla Research, USA) Programming Contest Organiser: Ilya Sergey (Yale-NUS College, Singapore) Publicity and Web Chair: Sam Tobin-Hochstadt (Indiana University, USA) Student Research Competition Chair: William J. Bowman (University of British Columbia, Canada) Workshops Co-Chair: Christophe Scholliers (Universiteit Gent, Belgium) Jennifer Hackett (University of Nottingham, UK) Conference Manager: Annabel Satin (P.C.K.) ### PACMPL Volume 3, Issue ICFP 2019 Principal Editor: François Pottier (Inria, France) Review Committee: Lennart Beringer (Princeton University, United States) Joachim Breitner (DFINITY Foundation, Germany) Laura M. Castro (University of A Coruña, Spain) Ezgi Çiçek (Facebook London, United Kingdom) Pierre-Evariste Dagand (LIP6/CNRS, France) Christos Dimoulas (Northwestern University, United States) Jacques-Henri Jourdan (CNRS, LRI, Université Paris-Sud, France) Andrew Kennedy (Facebook London, United Kingdom) Daan Leijen (Microsoft Research, United States) Kazutaka Matsuda (Tohoku University, Japan) Bruno C. d. S. Oliveira (University of Hong Kong, China) Klaus Ostermann (University of Tübingen, Germany) Jennifer Paykin (Galois, United States) Frank Pfenning (Carnegie Mellon University, USA) Mike Rainey (Indiana University, USA) Chung-chieh Shan (Indiana University, USA) Sam Staton (University of Oxford, UK) Pierre-Yves Strub (Ecole Polytechnique, France) German Vidal (Universitat Politecnica de Valencia, Spain) External Review Committee: Michael D. Adams (University of Utah, USA) Robert Atkey (University of Strathclyde, IK) Sheng Chen (University of Louisiana at Lafayette, USA) James Cheney (University of Edinburgh, UK) Adam Chlipala (Massachusetts Institute of Technology, USA) Evelyne Contejean (LRI, Université Paris-Sud, France) Germán Andrés Delbianco (IRIF, Université Paris Diderot, France) Dominique Devriese (Vrije Universiteit Brussel, Belgium) Richard A. Eisenberg (Bryn Mawr College, USA) Conal Elliott (Target, USA) Sebastian Erdweg (Delft University of Technology, Netherlands) Michael Greenberg (Pomona College, USA) Adrien Guatto (IRIF, Université Paris Diderot, France) Jennifer Hackett (University of Nottingham, UK) Troels Henriksen (University of Copenhagen, Denmark) Chung-Kil Hur (Seoul National University, Republic of Korea) Roberto Ierusalimschy (PUC-Rio, Brazil) Ranjit Jhala (University of California, San Diego, USA) Ralf Jung (MPI-SWS, Germany) Ohad Kammar (University of Oxford, UK) Oleg Kiselyov (Tohoku University, Japan) Hsiang-Shang ‘Josh’ Ko (National Institute of Informatics, Japan) Ondřej Lhoták (University of Waterloo, Canada) Dan Licata (Wesleyan University, USA) Geoffrey Mainland (Drexel University, USA) Simon Marlow (Facebook, UK) Akimasa Morihata (University of Tokyo, Japan) Shin-Cheng Mu (Academia Sinica, Taiwan) Guillaume Munch-Maccagnoni (Inria, France) Kim Nguyễn (University of Paris-Sud, France) Ulf Norell (Gothenburg University, Sweden) Atsushi Ohori (Tohoku University, Japan) Rex Page (University of Oklahoma, USA) Zoe Paraskevopoulou (Princeton University, USA) Nadia Polikarpova (University of California, San Diego, USA) Jonathan Protzenko (Microsoft Research, USA) Tiark Rompf (Purdue University, USA) Andreas Rossberg (Dfinity, Germany) KC Sivaramakrishnan (University of Cambridge, UI) Nicholas Smallbone (Chalmers University of Technology, Sweden) Matthieu Sozeau (Inria, France) Sandro Stucki (Chalmers | University of Gothenburg, Sweden) Don Syme (Microsoft, UK) Zachary Tatlock (University of Washington, USA) Sam Tobin-Hochstadt (Indiana University, USA) Takeshi Tsukada (University of Tokyo, Japan) Tarmo Uustalu (Reykjavik University, Iceland) Benoit Valiron (LRI, CentraleSupelec, Univ. Paris Saclay, France) Daniel Winograd-Cort (University of Pennsylvania, USA) Nicolas Wu (University of Bristol, UK)

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role constraints
by Ben Franksen 13 Feb '19

13 Feb '19

I have a question about roles for type variables. I have a number of data types that more or less represent partial bijections. They take two type parameters that represent domain and range. The phantom type parameters are extremely useful: code that shuffles these things around will fail to type check if domains and ranges don't match up exactly. However, there are some situations where I know more about the semantics than the type checker and need to be able to coerce the phantom parameters. No problem, that's what coerce is for... Except, most of the code is polymorphic over the data type, requiring only a few class constraints. And this polymorphism forces the type checker to assume representational roles for the type parameters. Which means I can't use coerce. Instead I have to use unsafeCoerce which I would like to avoid. Is there a way to express, as a constraint, that a certain parameter has a phantom role? What I want to say is: you are only allowed to instantiate the type variable 'a :: * -> * -> *' with data type 'A' if 'x' and 'y' in 'A x y' have both role phantom. And what I get is that I can call coerce to modify the parameters 'x' and 'y'.

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[ANN] dejafu-2.0.0.0 - a library for unit-testing concurrent programs
by Michael Walker 13 Feb '19

13 Feb '19

I'm pleased to announce a new super-major release of [dejafu][1], a library for testing concurrent Haskell programs. While there are breaking changes, common use-cases shouldn't be affected too significantly (or not at all). There is a brief guide to the changes, and how to migrate if necessary, [on the website][2]. What's dejafu? -------------- dejafu is a unit-testing library for concurrent Haskell programs. Tests are deterministic, and work by systematically exploring the possible schedules of your concurrency-using test case, allowing you to confidently check your threaded code. [HUnit][3] and [Tasty][4] bindings are available. dejafu requires your test case to be written against the `MonadConc` typeclass from the [concurrency][5] package. This is a necessity, dejafu cannot peek inside your `IO` or `STM` actions, so it needs to be able to plug in an alternative implementation of the concurrency primitives for testing. There is some guidance for how to switch from `IO` code to `MonadConc` code [on the website][6]. If you really need `IO`, you can use `MonadIO` - but make sure it's deterministic enough to not invalidate your tests! Here's a small example reproducing a deadlock found in an earlier version of the [auto-update][7] library: ``` > :{ autocheck $ do auto <- mkAutoUpdate defaultUpdateSettings auto :} [fail] Successful [deadlock] S0--------S1-----------S0- [fail] Deterministic [deadlock] S0--------S1-----------S0- () S0--------S1--------p0-- ``` dejafu finds the deadlock, and gives a simplified execution trace for each distinct result. More in-depth traces showing exactly what each thread did are also available. This is using a version of auto-update modified to use the `MonadConc` typeclass. The source is in the [dejafu testsuite][8]. What's new? ----------- The highlights for this release are setup actions, teardown actions, and invariants: - **Setup actions** are for things which are not really a part of your test case, but which are needed for it (for example, setting up a test distributed system). As dejafu can run a single test case many times, repeating this work can be a significant overhead. By defining this as a setup action, dejafu can "snapshot" the state at the end of the action, and efficiently reload it in subsequent executions of the same test. - **Teardown actions** are for things you want to run after your test case completes, in all cases, even if the test deadlocks (for example). As dejafu controls the concurrent execution of the test case, inspecting shared state is possible even if the test case fails to complete. - **Invariants** are effect-free atomically-checked conditions over shared state which must always hold. If an invariant throws an exception, the test case is aborted, and any teardown action run. Here is an example of a setup action with an invariant: ``` > :{ autocheck $ let setup = do var <- newEmptyMVar registerInvariant $ do value <- inspectMVar var when (value == Just 1) $ throwM Overflow pure var in withSetup setup $ \var -> do fork $ putMVar var 0 fork $ putMVar var 1 tryReadMVar var :} [fail] Successful [invariant failure] S0--P2- [fail] Deterministic [invariant failure] S0--P2- Nothing S0---- Just 0 S0--P1--S0-- ``` In the `[invariant failure]` case, thread 2 is scheduled, writing the forbidden value "1" to the MVar, which terminates the test. Here is an example of a setup action with a teardown action: ``` > :{ autocheck $ let setup = newMVar () teardown var (Right _) = show <$> tryReadMVar var teardown _ (Left e) = pure (show e) in withSetupAndTeardown setup teardown $ \var -> do fork $ takeMVar var takeMVar var :} [pass] Successful [fail] Deterministic "Nothing" S0--- "Deadlock" S0-P1--S0- ``` The teardown action can perform arbitrary concurrency effects, including inspecting any mutable state returned by the setup action. Setup and teardown actions were previously available in a slightly different form as the `dontCheck` and `subconcurrency` functions, which have been removed (see the migration guide if you used these). [1]: http://hackage.haskell.org/package/dejafu [2]: https://dejafu.readthedocs.io/en/latest/migration_1x_2x.html [3]: http://hackage.haskell.org/package/hunit-dejafu [4]: http://hackage.haskell.org/package/tasty-dejafu [5]: http://hackage.haskell.org/package/concurrency [6]: https://dejafu.readthedocs.io/en/latest/typeclass.html [7]: http://hackage.haskell.org/package/auto-update [8]: https://github.com/barrucadu/dejafu/blob/master/dejafu-tests/lib/Examples/A… -- Michael Walker (http://www.barrucadu.co.uk)

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Weird defaulting on newEmptyTMVar
by Ruben Astudillo 12 Feb '19

12 Feb '19

Dear list Playing on ghci I encountered the following type GHCi, version 8.6.3: http://www.haskell.org/ghc/ :? for help Prelude> :m +Control.Concurrent.STM Prelude Control.Concurrent.STM> var1 <- atomically $ newEmptyTMVar Prelude Control.Concurrent.STM> :t var1 var1 :: TMVar GHC.Types.Any Prelude Control.Concurrent.STM> I would think `var1 :: TMVar a` as the documentation says it should. Reading on `GHC.Types` `Any` seems to be a type family related to some laws on `unsafeCoerce`. Can anybody explain me why is this sensible defaulting and how to obtain the expected result? -- -- Ruben -- pgp: 4EE9 28F7 932E F4AD

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Taking over haskell-src-meta
by Dan Burton 11 Feb '19

11 Feb '19

I, Dan Burton, intend to take over maintenance of haskell-src-meta. -- Dan Burton

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Re: [Haskell-cafe] ANN:DBFunctor-0.1.1.0 - Functional Data Management / ETL Data Processing in Haskell
by Olaf Klinke 11 Feb '19

11 Feb '19

Dear Nikos, (not CC'ing the list yet because I don't know whether my remarks miss the design goals of DBFunctor) As a data scientist I have some questions regarding the design of DBFunctor. 1. Why is RTable, or rather RDataType a closed type? Was this forced by compatibility to other databases? There might be RTabular Types whose elements are hard to represent as RDataType, or the conversion deletes some of the data's semantics. What is the essence that makes the operations in the Julius language work? For example, the inner join could have the more general type MonadPlus m => (a -> b -> Bool) -> m a -> m b -> m (a,b) 2. Is there a way of saying that a column may not contain NULL values? 3. What about the efficiency of the operations? I see no complexities stated in the documentation of Etl.Julius. 4. I like the feature of named operations (:=>). But can we bind a sub-operation to a Haskell variable without leaving Julius? After all, takeNamedResult can lead to exceptions because the user may reference a name that has not been defined. With haskell variables, the compiler prevents this. Why not make the Julius language a monad, so that one can use do-notation for building the result? E.g. type JuliusExpr = JuliusLang RTable instance Monad JuliusLang where ... Kind regards, Olaf

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