December 2017 - Haskell-Cafe

Call for Papers: PACMPL issue ICFP 2018
by Lindsey Kuper 21 Dec '17

21 Dec '17

PACMPL issue ICFP 2018 Call for Papers accepted papers to be invited for presentation at The 23rd ACM SIGPLAN International Conference on Functional Programming St. Louis, Missouri, USA http://icfp18.sigplan.org/ ### Important dates Submissions due: 16 March 2018 (Friday) Anywhere on Earth https://icfp18.hotcrp.com Author response: 2 May (Wednesday) - 4 May (Friday) 14:00 UTC Notification: 18 May (Friday) Final copy due: 22 June (Friday) Conference: 24 September (Monday) - 26 September (Wednesday) ### 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 issue ICFP 2018 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 2018 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 16, 2018, 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 27 pages for a full paper or 14 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. See also PACMPL's Information and Guidelines for Authors at <https://pacmpl.acm.org/authors.cfm>. **Submission**: Submissions will be accepted at <https://icfp18.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 Wednesday, May 2, 2018, 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 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 2018. 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 2018 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 18, 2018. 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 22, 2018), 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. **PACMPL issue ICFP 2018 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 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 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. * 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. * 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 <https://icfp18.sigplan.org/track/icfp-2018-Artifacts>. ### 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 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 principal editor will be happy to advise on any concerns about which category to submit to. ### ICFP Organizers General Chair: Robby Findler (Northwestern University, USA) Artifact Evaluation Co-Chairs: Simon Marlow (Facebook, UK) Ryan R. Newton (Indiana University, USA) Industrial Relations Chair: Alan Jeffrey (Mozilla Research, USA) Programming Contest Organiser: Matthew Fluet (Rochester Institute of Technology, USA) Publicity and Web Chair: Lindsey Kuper (Intel Labs, USA) Student Research Competition Chair: Ilya Sergey (University College London, UK) Video Co-Chairs: Jose Calderon (Galois, Inc., USA) Nicolas Wu (University of Bristol, UK) Workshops Co-Chair: David Christiansen (Indiana University, USA) Christophe Scholliers (Universiteit Gent, Belgium) ### PACMPL issue ICFP 2018 Principal Editor: Matthew Flatt (Univesity of Utah, USA) Review Committee: Sandrine Blazy (IRISA, University of Rennes 1, France) David Christiansen (Indiana University, USA) Martin Elsman (University of Copenhagen, Denmark) Marco Gaboardi (University at Buffalo, CUNY, USA) Sam Lindley (University of Edinburgh, UK) Heather Miller (Northweastern University, USA / EPFL, Switzerland) J. Garrett Morris (University of Kansas, USA) Henrik Nilsson (University of Nottingham, UK) François Pottier (Inria, France) Alejandro Russo (Chalmers University of Technology, Sweden) Ilya Sergey (University College London, UK) Michael Sperber (Active Group GmbH, Germany) Wouter Swierstra (Utrecht University, UK) Éric Tanter (University of Chile, Chile) Katsuhiro Ueno (Tohoku University, Japan) Niki Vazou (University of Maryland, USA) Jeremy Yallop (University of Cambridge, UK) External Review Committee: Michael D. Adams (University of Utah, USA) Amal Ahmed (Northeastern University, USA) Nada Amin (University of Cambridge, USA) Zena Ariola (University of Oregon) Lars Bergstrom (Mozilla Research) Lars Birkedal (Aarhus University, Denmark) Edwin Brady ( University of St. Andrews, UK) William Byrd (University of Alabama at Birmingham, USA) Giuseppe Castagna (CRNS / University of Paris Diderot, France) Sheng Chen (University of Louisiana at Lafayette, USA) Koen Claessen (Chalmers University ot Technology, Sweden) Ugo Dal Lago (University of Bologna, Italy / Inria, France) David Darais (University of Vermont, USA) Joshua Dunfield (Queen’s University, Canada) Richard Eisenberg (Bryn Mawr College, USA) Matthew Fluet (Rochester Institute of Technology, USA) Nate Foster (Cornell University, USA) Jurriaan Hage (Utrecht University, Netherlands) David Van Horn (University of Maryland, USA) Zhenjiang Hu (National Institute of Informatics, Japan) Suresh Jagannathan (Purdue University, USA) Simon Peyton Jones (Microsoft Research, UK) Naoki Kobayashi (University of Tokyo, Japan) Neelakantan Krishnaswami (University of Cambridge, UK) Kazutaka Matsuda (Tohoku University, Japan) Trevor McDonell (University of New South Wales, Australia) Hernan Melgratti (University of Buenos Aires, Argentina) Akimasa Morihata (University of Tokyo, Japan) Aleksandar Nanevski (IMDEA Software Institute, Spain) Kim Nguyễn (University of Paris-Sud, France) Cosmin Oancea (DIKU, University of Copenhagen, Denmark) Bruno C. d. S. Oliveira (University of Hong Kong, China) Tomas Petricek (University of Cambridge, UK) Benjamin Pierce (University of Pennsylvania, USA) Christine Rizkallah (University of Pennsylvania, USA) Tom Schrijvers (KU Leuven, Belgium) Manuel Serrano (Inria, France) Jeremy Siek (Indiana University, USA) Josef Svenningsson (Chalmers University of Technology, Sweden) Nicolas Tabareau (Inria, France) Dimitrios Vytiniotis (Microsoft Research, UK) Philip Wadler (University of Edinburgh, UK) Meng Wang (University of Kent, UK)

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Re: [Haskell-cafe] Are bottoms ever natural?
by Alexey Muranov 20 Dec '17

20 Dec '17

Siddharth, how would you deal with functions that terminate for some arguments/inputs but do not terminate for the others ? Alexey. On Tue, 2017-12-19 at 07:20 +0000, (IIIT) Siddharth Bhat wrote: > > Is that really true, though? > > Usually when you have an infinite loop, you have progress of some > > sort. Infinite loops with no side effects can be removed from the > > program according to the C standard, for example. So, in general, we > > should allow programmers to express termination / progress, right? > At > > that point, no computation ever "bottoms out"? > > Shouldn't a hypothetical purely functional programming language > > better control this (by eg. Forcing totality?) It seems like we lose > > much of the benefits of purity by muddying the waters with > > divergence. > > From an optimising compiler perspective, Haskell is on some weird > > lose-lose space, where you lose out on traditional compiler > > techniques that work on strict code, but it also does not allow the > > awesome stuff you could do with "pure" computation because bottom > > lurks everywhere. > > What neat optimisations can be done on Haskell that can't be done in > > a traditional imperative language? I genuinely want to know. > > What are your thoughts on this? > > Cheers > > Siddharth > > > > On Tue 19 Dec, 2017, 08:09 Brandon Allbery, <allbery.b(a)gmail.com> > > wrote: >> > > Define "natural". >> > > >> > > You might want to look into the concept of Turing completeness. >> One >> > > could define a subset of Haskell in which bottoms cannot occur... >> > > but it turns out there's a lot of useful things you can't do in >> > > such a language. (In strict languages, these often are expressed >> as >> > > infinite loops of one kind or another. Note also that any >> > > dependency on external input is an infinite loop from the >> > > perspective of the language, since it can only be broken by the >> > > external entity providing the input.) >> > > >> > > On Tue, Dec 19, 2017 at 1:47 AM, (IIIT) Siddharth Bhat >> <siddharth.b >> > > hat(a)research.iiit.ac.in> wrote: >>> > > > I've been thinking about the issue of purity and speculation >>> > > > lately, and from what little I have read, it looks like the >>> > > > presence of bottom hiding inside a lazy value is a huge issue. >>> > > > >>> > > > How "natural" is it for bottoms to exist? If one were to >>> change >>> > > > Haskell and declare that any haskell value can be speculated >>> > > > upon, what ramifications does this have? >>> > > > >>> > > > Is it totally broken? Is it "correct" but makes programming >>> > > > unpleasant? What's the catch? >>> > > > >>> > > > Thanks, >>> > > > Siddharth

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Are bottoms ever natural?
by (IIIT) Siddharth Bhat 20 Dec '17

20 Dec '17

I've been thinking about the issue of purity and speculation lately, and from what little I have read, it looks like the presence of bottom hiding inside a lazy value is a huge issue. How "natural" is it for bottoms to exist? If one were to change Haskell and declare that any haskell value can be speculated upon, what ramifications does this have? Is it totally broken? Is it "correct" but makes programming unpleasant? What's the catch? Thanks, Siddharth -- Sending this from my phone, please excuse any typos!

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Re: [Haskell-cafe] Are bottoms ever natural?
by Alexey Muranov 19 Dec '17

19 Dec '17

Siddharth, are you aware that the question whether a given element belongs to the domain of a given computable function is algorithmically undecidable? Alexey. On Tue, 2017-12-19 at 16:51 +0000, Siddharth Bhat wrote: > > So, I have two opinions to share on this: > > Regarding the plane example, it is wrong to attempt to graph it on a > > plane, precisely because the domain is not the plane. > > I am confused by your assertion that it is impossible to avoid > > divergence in mathematics: we do not define division as a *partial* > > function. Rather we define it as a *total* function on a *restricted > > domain*. > > So, what one would need is the ability to create these restricted > > domains, which certain languages have as far as I am aware. > > At that point, now that we track our domains and codomains > correctly, > > all should work out? > > I would still like an answer to interesting transformations that are > > enabled by having purity and laziness and are not encumbered by the > > presence of bottoms. > > Cheers, > > Siddharth > >

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Writing an interpreter for a language with mutability
by Jeremy Mikkola 19 Dec '17

19 Dec '17

I am quite certain I am not the first to try to do this, but my google-fu is failing me today. How does one go about interpreting a language with mutable objects in Haskell? The best approach I can think of is to represent the language's memory as a `Data.Map.Map RefID LanguageObject` where RefID is some type (probably Int) used as a reference. The LanguageObject structure might contain some values of type RefID to refer to other objects. Mutating an object involves simply replacing the value in the map at a given RefID. I don't like this approach for two reasons: 1. Map lookups aren't very efficient compared with actual references to the value. 2. I have to re-invent garbage collection, removing objects from the map when they no longer have incoming references. (Unlike simple interpreters for languages with immutable values, I can't rely on Haskell's garbage collector to do the work for me.) Is there a better approach? - Jeremy

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Cybersecurity Posts (2) at the University of Sheffield
by Achim D. Brucker 19 Dec '17

19 Dec '17

(Apologies for duplicates - resending after I forgot to attach the body of the mail ...) Dear all, As part of an ambitious strategy for growth, the Department of Computer Science is seeking to appoint two Lecturers/Senior Lecturers/Readers with outstanding expertise in the field of cybersecurity. Computer and information security is a rapidly growing interest at Sheffield. In the Department of Computer Science we have recently established a Security of Advanced Systems group, led by Professor John Clark. There is related expertise in other departments such as Law, Mathematics and Statistics, and Politics. Furthermore, Sheffiield’s strength in engineering brings many opportunities for collaborative research in cybersecurity, in areas such as robotics and advanced manufacturing. To drive our research agenda in cybersecurity forward, we are now seeking to make two appointments at Lecturer, Senior Lecturer or Reader Level. Candidates will demonstrate an excellent research record in cybersecurity. Suitable areas of expertise include (but are not limited to): security policies, threat modelling, authentication, access control, malware and malware detection, network security, secure protocols, intrusion detection, and human factors and security. Candidates who work at the interface of artificial intelligence and cybersecurity are particularly encouraged to apply. In 2016 we were recognised for our work on diversity and equality with a Silver Athena Swan award. Interested candidates may find details of these (and other) posts at: https://www.sheffield.ac.uk/dcs/jobs/index Best, Achim -- Dr. Achim D. Brucker | Software Assurance & Security | University of Sheffield https://www.brucker.ch | https://logicalhacking.com/blog @adbrucker | @logicalhacking

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Cybersecurity Posts (2) at the University of Sheffield
by Achim D. Brucker 19 Dec '17

19 Dec '17

(Apologies for duplicates) -- Dr. Achim D. Brucker | Software Assurance & Security | University of Sheffield https://www.brucker.ch | https://logicalhacking.com/blog @adbrucker | @logicalhacking

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coercing D a to D b
by Evan Laforge 19 Dec '17

19 Dec '17

Given: data D a = A a | B Int Char dmapNoCoerce :: (a -> b) -> D a -> D b dmapNoCoerce f (A a) = A (f a) dmapNoCoerce _ (B i c) = B i c I have to reconstruct a B change it from D a to D b. But at the lower level, couldn't this be implemented as a type cast? What prevents such an optimization? I can write this as dmapCoerce :: (a -> b) -> D a -> D b dmapCoerce f (A a) = A (f a) dmapCoerce _ b@(B {}) = Unsafe.Coerce.unsafeCoerce b >From the core, it looks like dmapCoerce indeed has a cast with no allocation, while dmapNoCoerce allocates a new B. It seems to work, but is it safe? Is there a more principled way to do it? I can't convince Data.Coerce to cooperate, presumably because 'a' and 'b' are not coercible themselves, and it doesn't believe me if I try to tell it the type role is phantom.

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error terminated list
by Evan Laforge 18 Dec '17

18 Dec '17

I've been using this lately to represent a process which can fail but still produce data: data UntilFail err a = a :+ UntilFail err a | Done | Fail err I know there's been discussion of this type before, but I don't remember what it was called and I can't find the right search terms to turn it up. Ring any bells? I have some awkward names for it, but maybe someone else thought of something more elegant. I have some ad-hoc functions for it: mapUntilFail :: (a -> Either err b) -> UntilFail err a -> UntilFail err b concatMapUntilFail :: (a -> UntilFail err b) -> UntilFail err a -> UntilFail err b -- | Like 'concatMapUntilFail', but consume a variable number of results. -- -- A more precise type would end with @Done [a]@. processUntilFail :: (a -> [a] -> (UntilFail err b, [a])) -> [a] -> UntilFail err b Maybe someone who has thought more deeply about it has come up with more elegant abstractions.

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Haskell IDE Engine logo poll
by Damien Flament 18 Dec '17

18 Dec '17

Hi ! The final poll is available featuring all logos submitted until last Friday. Please vote for your favourite*s* here: http://freeonlinesurveys.com/p/Cw1FRJ2G?qid=1017541 Thanks. Damien.

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