February 2017 - Haskell-Cafe

Getting added to the Haskell group on Github.
by aditya siram 04 Feb '17

04 Feb '17

Hi, Some time ago the maintainership of c2hs [1] was silently passed to me. However I am unable to request access to the repo because I am not on the Haskell team [2]. Apparently I have to be added by someone already on the team but I'm having trouble getting in touch with someone. If anyone reading this is on the team [2] please add me. I'm @deech on Github. Thanks! -deech [1] https://github.com/haskell/c2hs [2] https://github.com/orgs/haskell/people

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Purpose of Control.Monad.Base class
by Guru Devanla 04 Feb '17

04 Feb '17

Hello, I am trying to understand the purpose of the MonadBase class. https://hackage.haskell.org/package/transformers-base-0.4.4/docs/Control-Mo… Google search did not give me any helpful answers. Please could some one help me understand the purpose of this class and also point me to any good resources. Thanks Guru

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using ghci with compiled code
by Dennis Raddle 03 Feb '17

03 Feb '17

I want to use ghci to interpret parts of my program, and I would like it to pick up pre-compiled object files to speed things up in other parts. Right now whenever I launch ghci it seems to "compile" everything (I think to byte code).. what can I do? D

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Best way to provide monad transformer instances in library design?
by Matt 03 Feb '17

03 Feb '17

I'm writing a wrapper around ekg which provides an MTL-style class for recording metrics [1], and now I'm trying to figure out how to make the class usable for clients. I'm conflicted on the numerous ways to approach the problem. I would like to provide the best UX for the library, and want to consider viewpoints other than my own. Currently, the class is: class MonadMetrics m where getMetrics :: m Metrics I currently don't provide any concrete transformer, instead requiring that users define it for their own stacks. I'd like to make it easier to use. I can provide a concrete transformer newtype MetricT m a = MetricT (ReaderT Metrics m a) instance Monad m => MonadMetrics (MetricT m) where getMetrics = MetricT ask and then provide a wide variety of pass through instances. This approach requires a large amount of boilerplate, and for any instances I forget to provide, will require that clients make their own orphan instances. It also incurs additional dependencies just for writing instances. I can provide a more abstract transformer and a more general Metrics class: class HasMetrics r where metrics :: Lens' r Metrics newtype MetricT2 m a = MetricT2 (m a) instance (MonadReader r m, HasMetrics r) => MonadMetrics (MetricT2 m) where getMetrics = MetricT (view metrics) but this still requires those pass through instances. Finally, I can scrap `MonadMetrics` as anything other than a class alias: class (MonadReader r m, MonadIO m, HasMetrics r) => MonadMetrics r m instance (MonadReader r m, MonadIO m, HasMetrics r) => MonadMetrics r m but now I have UndecidableInstances and a multiparam type class, and I'm not sure how well this actually plays out in the wild. What are your thoughts on this issue? Matt Parsons [1]: https://github.com/sellerlabs/monad-metrics

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segfault with Hint (InterpreterT)
by Dennis Raddle 03 Feb '17

03 Feb '17

My program segfaults the second time it run runInterpreterT. Just wondering if that's not supposed to be allowed. I'm actually not sure what triggers the segfault -- it's also doing some MIDI I/O. D

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is there less available support for beginners these days?
by Dennis Raddle 03 Feb '17

03 Feb '17

I'm a Haskell hobbyist enthusiast -- first learned it in 2008, struggling having come from an imperative language background, and I only get to use it in hobby projects, but I love it more each year. I still consider myself a beginner in many areas. Unfortunately it used to be much easier to get responses on the beginner's list or the #haskell channel. I've now had several posts go completely unanswered on the beginner's list, and can't get help from #haskell most of the time (by "help" I mean someone who takes just a few minutes with me). I've tried posting on Haskell cafe in the hopes I would get more responses and I'm not sure if that's going to work (or is appropriate). D

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Project hosting platform - looking for new developer
by fr33domlover 03 Feb '17

03 Feb '17

(tl;dr Git and Darcs hosting platform with planned federation written in Yesod, I'm looking for someone excited about the idea to take over development) Hello Haskellers, Several months ago I started developing a project hosting platform called Vervis. It's written in Haskell using the Yesod web framework. The intention was to make it federated and allow people for the first time in git (and darcs) history to enjoy both the full access to the community and avoid the proprietaryness and centralization of githu8. I'm probably still really a beginner and I never made any web application before Vervis, or even an actual mature stable software project. But I made huge progress with Vervis very fast, and I created something you can actually see with your eyes :) However due to certain life preferences I stopped working on it and I may never go back to it. But I was hoping someone else would want to continue its development. The code is hosted both at Darcs Hub and in a Vervis instance, so the project is self-hosting: https://dev.seek-together.space/s/fr33domlover/r/vervis I didn't work on design much, just focused on functionality. Among the unique features already present one one way or another: - Supports both Git and Darcs - Rendering of documents using Pandoc, supporting more formats and variants than any other platform could - Support of Libravatar for user avatar images - Flexible integrated system for project management and issues, on the level you can find in project management platforms, but here it's integrated into the code hosting platform itself - Hierarchical discussion threads - A single project can contain more than one repo, i.e. repos can share the same project plan and the same issue database - Merely being written in Haskell and Yesod makes it very stable once it builds successfully There are still many quirks, including the UI of the registration system and UI in general, so I'm sorry in advance if you try to create an account there and get errors or confusing UIs. It's not polished or anything yet. I also made several support projects and implemented some helper tools, such as some support for parsing darcs repos (attoparsec stuff that can and maybe should be merged into Darcs itself), SSH server component that uses an improved version of the ssh package, hierarchical SQL query API with an interface similar to the FGL library, code for working with git repos... and probably more things I'm forgetting. If you have any questions, I'm here to answer :) --fr33

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Haskell jobs in Brisbane, Australia
by Tony Morris 31 Jan '17

31 Jan '17

Hi all, Data61, CSIRO has recently announced three new positions in Brisbane, Australia for work in FP. Haskell will be the primary language used. For a few reasons remote work is not available for these positions, but if you’re willing to relocate, these would be a fantastic opportunity. https://jobs.csiro.au/job/Brisbane%2C-QLD-Senior-Functional-Programming-Eng… https://jobs.csiro.au/job/Brisbane%2C-QLD-Functional-Programming-Engineers/… https://jobs.csiro.au/job/Brisbane%2C-QLD-Functional-Programming-Engineer/3…

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Second Call for Papers: ICFP 2017
by Lindsey Kuper 31 Jan '17

31 Jan '17

ICFP 2017 The 22nd ACM SIGPLAN International Conference on Functional Programming Oxford, United Kingdom http://icfp17.sigplan.org/ Second Call for Papers ### Important dates Submissions due: Monday, February 27, Anywhere on Earth https://icfp17.hotcrp.com Author response: Monday, April 17, 2017, 15:00 (UTC) - Thursday, April 20, 2017, 15:00 (UTC) Notification: Monday, 1 May, 2017 Final copy due: Monday, 5 June 2017 Early registration: TBA Conference: Monday, 4 September - Wednesday, 6 September, 2017 ### Updates for Second Call for Papers This revision, published January 31, 2017, includes the following updates: * A link to the list of frequently asked questions about the double-blind review process. * A new requirement for citations in author-year format. This follows a decision for PACMPL that was made after the original version of this call was distributed. As such, author-year format will be required for final (accepted) versions of papers in ICFP 2017, but not for submissions. * Additional guidance for authors using LaTeX. * A clarification about the expectations for author attendance in the event of visa-related problems. ### New this year Those familiar with previous ICFP conferences should be aware of two significant changes that are being introduced in 2017: 1. Papers selected for ICFP 2017 will be published as the ICFP 2017 issue of a new journal, Proceedings of the ACM on Programming Languages (PACMPL), which replaces the previous ICFP conference proceedings. The move to PACMPL will have two noticeable impacts on authors: * A new, two-phase selection and reviewing process that conforms to ACM’s journal reviewing guidelines. * A new, single-column format for submissions. 2. Authors of papers that are conditionally accepted in the first phase of the reviewing process will have the option to submit materials for Artifact Evaluation. Further details on each of these changes are included in the following text. ### Scope ICFP 2017 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. ICFP 2017 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 program chair if you have questions or are concerned about the appropriateness of a topic. ### Preparation of submissions ICFP 2017 will employ a lightweight double-blind reviewing process, as described below. **Deadline**: The deadline for submissions is Monday, February 27, 2017, Anywhere on Earth (<https://en.wikipedia.org/wiki/Anywhere_on_Earth>). This deadline will be strictly enforced. **Formatting**: (NOTE: NEW FORMAT REQUIREMENTS FOR ICFP 2017) 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 Large" template that is available (in both LaTeX and Word formats) from <http://www.acm.org/publications/proceedings-template>. For authors using LaTeX, a lighter-weight package, including only the essential files, is available from <http://sigplan.org/Resources/Author/#acmart-format>; the appropriate template for ICFP 2017 authors is in the file `acmart-pacmpl-template.tex`. As documented in the template, submissions should be prepared using the `acmlarge` and `anonymous` options. The use of the `review` option is also strongly encouraged but not required. (The `review` option will add line numbers, which will make it easier for reviewers to reference specific parts of your paper in their comments, but should have absolutely no other effect on the typesetting.) Details of available technical support for LaTeX-specific questions is available at <http://www.acm.org/publications/proceedings-template#h-technical-support>. There is a limit of 24 pages for a full paper or 12 pages for an Experience Report; in either case, the bibliography will not be counted against these limits. These page limits have been chosen to allow essentially the same amount of content with the new single-column format as was possible with the two-column format used in past ICFP conferences. Submissions that exceed the page limits or, for other reasons, do not meet the requirements for formatting, will be summarily rejected. **Citations**: As part of PACMPL, ICFP 2017 papers are expected to use author-year citations for references to other work. Author-year citations may be used as either a noun phrase, such as "The lambda calculus was originally conceived by Church (1932)", or a parenthetic phase, such as "The lambda calculus (Church 1932) was intended as a foundation for mathematics". A useful test for correct usage it to make sure that the text still reads correctly when the parenthesized portions of any references are omitted. Take care with prepositions; in the first example above, "by" is more appropriate than "in" because it allows the text to be read correctly as a reference to the author. Sometimes, readability may be improved by putting parenthetic citations at the end of a clause or a sentence, such as "A foundation for mathematics was provided by the lambda calculus (Church 1932)". In LaTeX, use `\citet{Church-1932}` for citations as a noun phrase, "Church (1932)", and `\citep{Church-1932}` for citations as a parenthetic phrase, "(Church 1932)"; for details, see Sections 2.3--2.5 of the natbib documentation (<http://ctan.org/pkg/natbib>). **Submission**: Submissions will be accepted at <https://icfp17.hotcrp.com/> (in preparation at the time of writing). 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 15:00 UTC on Monday, April 17, 2017, to read reviews and respond to them. **Supplementary Materials**: Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. The material should be uploaded at submission time, as a single pdf or a tarball, not via a URL. This supplementary material may or may not be anonymized; if not anonymized, it will only be revealed to reviewers 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 program chair 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 presentation at ICFP 2017. A [list of frequently asked questions and answers](http://icfp17.sigplan.org/track/icfp-2017-papers#Submission-and-Re… that address common concerns is available on the conference website and will be updated as necessary to clarify and expand on this process. **ICFP 2017 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 PC 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 1, 2017. Authors of conditionally accepted papers will be provided with committee reviews (just as in previous conferences) along with a set of mandatory revisions. After five weeks (June 5, 2017), 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 five 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. This process is intended as a refinement of the review process that has been used in previous ICFP conferences. By incorporating a second stage, the process will conform to ACM’s journal reviewing guidelines for PACMPL. **ICFP 2017 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 PC and external 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 Large format. The page limits for final versions of papers will be increased to ensure that authors have space to respond to reviewer comments and mandatory revisions. * Authors of accepted submissions will be required to agree to one of the three ACM licensing options: copyright transfer to ACM; retaining copyright but granting ACM exclusive publication rights; or open access on payment of a fee. Further information about ACM author rights is available from <http://authors.acm.org>. * At least one author of each accepted submission will be expected to attend and present their paper at the conference. (ICFP welcomes all authors, regardless of nationality. If any author of an accepted submission has visa-related difficulties in travelling to the conference, we will make arrangements to enable remote participation, and not require them to attend the conference in order to present their talk.) 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. * We intend that the proceedings 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. * **AUTHORS TAKE NOTE: The official publication date is the date the proceedings 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 a committee, separate from the program 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 proceedings, 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. Further information about the motivations and expectations for Artifact Evaluation can be found at <http://icfp17.sigplan.org/track/icfp-2017-Artifacts>. ### Special categories of papers In addition to research papers, 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 ICFP paper by its title, by its length, and by the criteria used to evaluate it. * Both in the proceedings and in any citations, the title of each accepted Experience Report must begin with the words "Experience Report" followed by a colon. 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 program 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 off submitted it as a full paper, which will be judged by the usual criteria of novelty, originality, and relevance. The program chair will be happy to advise on any concerns about which category to submit to. ### Organizers General Chair: Jeremy Gibbons (University of Oxford, UK) Program Chair: Mark Jones (Portland State University, USA) Artifact Evaluation Chair: Ryan R. Newton (Indiana University, USA) Industrial Relations Chair: Ryan Trinkle (Obsidian Systems LLC, USA) Programming Contest Organiser: Sam Lindley (University of Edinburgh, UK) Publicity and Web Chair: Lindsey Kuper (Intel Labs, USA) Student Research Competition Chair: Ilya Sergey (University College London, UK) Video Chair: Jose Calderon (Galois, Inc., USA) Workshops Co-Chair: Andres Löh (Well-Typed LLP) Workshops Co-Chair: David Christiansen (Indiana University, USA) Program Committee: Bob Atkey (University of Strathclyde, Scotland) Adam Chlipala (MIT, USA) Dominique Devriese (KU Leuven, Belgium) Martin Erwig (Oregon State, USA) Matthew Flatt (University of Utah, USA) Ronald Garcia (University of British Columbia, Canada) Kathryn Gray (University of Cambridge, England) John Hughes (Chalmers University and Quvik, Sweden) Chung-Kil Hur (Seoul National University, Korea) Graham Hutton (University of Nottingham, England) Alan Jeffrey (Mozilla Research, USA) Ranjit Jhala (University of California, San Diego, USA) Shin-ya Katsumata (Kyoto University, Japan) Lindsey Kuper (Intel Labs, USA) Dan Licata (Wesleyan University, USA) Ben Lippmeier (Digital Asset, Australia) Gabriel Scherer (Northeastern University, USA) Alexandra Silva (University College London, England) Nikhil Swamy (Microsoft Research, USA) Sam Tobin-Hochstadt (Indiana University, USA) Nicolas Wu (University of Bristol, England) Beta Ziliani (CONICET and FAMAF, Universidad Nacional de Córdoba, Argentina)

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Ensuring all values of an ADT are explicitly handled OR finding all occurrences of type X in my app
by Saurabh Nanda 31 Jan '17

31 Jan '17

Hi, If I have the following ADT data BookingState = Confirmed | Cancelled which had a very high chance of being expanded in the future to have more values. How do I ensure that every pattern match on BookingState matches each value explicitly. Basically prevent the '_' matcher ? If that is not possible, is the following possible instead: evolve BookingState and then find all possible occurrences of values of BookingState throughout my app? I'm basically trying to make sure that any a newly added state, which may require special-case handling is not automatically handled by the '_' branch. -- Saurabh.

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