- Haskell-Cafe - Haskell.org

Honest constant-space mergesort
by Ryan Reich 13 Jan '19

13 Jan '19

I am, for entertainment, trying to figure out how to implement a constant-space bottom-up mergesort in Haskell. This is quite easy to at least describe imperatively: break the list into ordered blocks, merge adjacent ones, and repeat with double the length until there's only one block and you're done. For a linked list like [a] in Haskell, this should be constant-space. Unfortunately, it is not. You can definitely express the above algorithm nicely if you organize those blocks into an [[a]], but then you are creating O(n) new lists. Even if you go to extremes to keep the blocks concatenated, though, you still have the issue that every time you merge two lists, you are creating a new list and that involves the allocator. (Note that the implementation of Data.List.sort does this.) The C equivalent would proceed by just overwriting the linking pointers in each node, without allocating new nodes to do so, but of course, that's not going to work in Haskell. I considered using something like STRef to represent the links (so, departing from the plain [a] type) but even then, if you overwrite the contents of one of those, the garbage collector still needs to know. This is better than allocating more space, but it is not necessary because the algorithm never actually makes any of the nodes unreachable. Also, I understand that using a lot of STRefs is bad for efficiency. Is there any way to do this in (GHC) Haskell and get the algorithm down to the bare minimum? If there is, is it any different than just "writing C in Haskell"? That is, can one coerce GHC into emitting the mutable-link operations and not doing collections, from high-level code? Although I've definitely seen examples of algorithms that compile down to such unboxed depths, this one has been really resistant to improvement. Thanks, Ryan Reich

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Re: [Haskell-cafe] About 'Overloaded functions are not your friend' line in GHC manual
by Johannes Waldmann 12 Jan '19

12 Jan '19

At run-time, only the branch (Left/Right) is "picked" at run-time. For each branch, the "show" instance is determined at compile-time. The problem is that such instances are represented by dictionaries, and this adds extra cost (extra arguments) to function calls - but only in those cases where these dictionaries are not inlined (that is, removed) statically. But I am thinking that GHC has become very good at inlining. - J.W.

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LambdaConf 2019 Call for Proposals
by John A. De Goes 12 Jan '19

12 Jan '19

Dear Haskell Enthusiasts: The LambdaConf 2019 Call for Proposals is open, and we warmly welcome Haskell proposals on topics of interest to aspiring and practicing functional programmers. Historically, Haskell content accounts for more than 50% of content across all 5-8 tracks of the event. Last year’s speakers included Michael Snoyberg, Dana Scott, Jeremy Siek, and many others from both industry and academia. To submit a proposal for LambdaConf 2019, please visit the following website: https://www.papercall.io/lambdaconf-2019 Travel assistance is available, including lodging. ## INTRODUCTION LambdaConf is the largest interdisciplinary functional programming conference in the Mountain West, and one of the largest and most respected functional programming conferences in the world. The conference takes place June 5rd - 7th, in Boulder, Colorado, at the University of Colorado Boulder, and is surrounded by commercial training opportunities. If you are an educator, a researcher, a speaker, a speaker coach, or someone aspiring to one of the preceding, then we warmly welcome you to submit a proposal for LambdaConf 2019. No prior experience is necessary for most proposals, and we welcome beginner-level content. The Call for Proposals closes at the end of January 2019. We recommend submitting as early as you can to ensure sufficient time for editing. LambdaConf attracts everyone from the FP-curious to researchers advancing state-of-the-art; hobbyists, professionals, academics and students. Material at all levels, including beginner content and very advanced content, will find an audience at LambdaConf. Historically, LambdaConf has enjoyed a large selection of sessions on statically-typed functional programming, and a smaller selection of sessions on dynamically-typed functional programming. Some sessions are not tied to specific programming languages, but rather cover topics in abstract algebra, category theory, type theory, programming language theory, functional architecture, and so on, either generically or in a way that applies across many programming languages. ## TOPICS LambdaConf looks for sessions in the following areas: - LANGUAGES. Proposals that overview or dive into specific features of functional, math, or logic programming languages (both new and existing), with the goal of exposing developers to new ideas or helping them master features of languages they already know. LIBRARIES. Proposals that discuss libraries that leverage functional or logic programming to help programmers solve real-world problems. - CONCEPTS. Proposals that discuss functional programming idioms, patterns, or abstractions; or concepts from mathematics, logic, and computer science, all directed at helping developers write software that’s easier to test, easier to reason about, and easier to change safely. - APPLICATIONS. Proposals that discuss how functional programming can help with specific aspects of modern software development, including scalability, distributed systems, concurrency, data processing, security, performance, correctness, user-interfaces, machine learning, and big data. - USE CASES. Proposals that discuss how functional programming enabled a project or team to thrive, or deliver more business value than possible with other approaches. - CHERRY PICKING. Proposals that show how techniques and approaches from functional programming can be adapted and incorporated into mainstream development languages and practices, to the benefit of developers using them. - CAUTIONARY TALES. Proposals that call attention to difficulties of functional programming (both as a cautionary tale but also to raise awareness), especially such proposals that suggest alternatives or a path forward. - EFFICACY. Proposals that present data, measurements, or analysis that suggests different techniques, paradigms, languages, libraries, concepts, or approaches have different efficacies for given specified metrics, which provide actionable takeaways to practicing functional and logic programmers. - OFF-TOPIC. Proposals that have appeal to a mainstream developer audience (the number of off-topic proposals we accept is small, but we do accept some, especially for keynotes). ## SESSION TYPES LambdaConf accepts proposals for the following types of sessions: - LEAP WORKSHOPS (6h). Leap Workshops are approximately 6 hours in length. They are in-depth, hands-on workshops designed to teach mainstream functional programming topics in enough detail, attendees can immediately apply what they learn in their jobs. We require that speakers follow our recommended format for Leap Workshops, although we allow exceptions for experienced teachers. - HOP WORKSHOPS (2h). Hop Workshops are 2 hours in length. Like Leap Workshops, these workshops are in-depth and hands-on, but they cover reduced content and may be specialized to topics that may not have mainstream appeal. We require that speakers follow our recommended format for Hop Workshops, although we allow exceptions for experienced teachers. - DE NOVO SESSIONS (50m). De Novo Sessions are 50 minutes in length. These sessions are designed to present original work from industry and academia. While the requirements for proposals are more rigorous, there is less competition for De Novo slots. - EDUCATIONAL SESSIONS (50m). Educational Sessions are 50 minutes in length. These sessions are designed to clearly and concisely teach one useful concept, skill, aspect, library, or language to attendees. - KEYNOTES (40m). Keynotes are 40 minutes in length, and are presented before all attendees (there are no other sessions concurrent with keynotes). Keynotes are designed to offer thought-provoking, opinionated, and insightful commentary on topics of interest to the community. If you are accepted for a specific type of proposal (e.g. Educational), we cannot guarantee that you will get a slot of this type. Based on scheduling requirements, feedback from the committee, or feedback from your speaker coach, we may require you to change the format of your session. Regards, -- John A. De Goes john(a)degoes.net Follow me on Twitter @jdegoes

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Second Call for Papers: PACMPL issue ICFP 2019
by William Fearon 11 Jan '19

11 Jan '19

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About 'Overloaded functions are not your friend' line in GHC manual
by Michail Pevnev 11 Jan '19

11 Jan '19

Hello everyone. I've been browsing through GHC manual some time ago, mostly to see what extensions there are, and stumbled upon the following paragraph in the 'Advice on: sooner, faster, smaller, thriftier' section, 'faster' subsection: Overloaded functions are not your friend: Haskell’s overloading (using type classes) is elegant, neat, etc., etc., but it is death to performance if left to linger in an inner loop. I don't quite understand the reason behind this. I was under impression that function overloading - existential shenanigans aside - is resolved mostly at compile time. Mostly, because obviously, say, 'show' for an Either still has to examine its argument at runtime and pick appropriate 'show' for one of the inner types. But the same happens for any examination of an Either, really. Does overloading make it worse, and if so, how and why? Or am I completely off track here, and the actual reason is something entirely different? -- Michail.

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overloading functions
by Damien Mattei 11 Jan '19

11 Jan '19

Hi, i have this definition: {-# LANGUAGE FlexibleInstances #-} class ConcatenateMaybeString a where cms :: Maybe String -> a -> Maybe String instance ConcatenateMaybeString (Maybe String) where cms mf ms = mf >>= (\f -> ms >>= (\s -> return (f ++ s))) instance ConcatenateMaybeString String where cms mf s = mf >>= (\f -> return (f ++ s)) when i use it on : f `cms` ("." ::String) `cms` s it works but not on this: f `cms` "." `cms` s "." is too ambigious to compile: *Main> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:373:43: error: • Ambiguous type variable ‘a0’ arising from a use of ‘cms’ prevents the constraint ‘(ConcatenateMaybeString a0)’ from being solved. Probable fix: use a type annotation to specify what ‘a0’ should be. These potential instances exist: instance ConcatenateMaybeString (Maybe String) -- Defined at UpdateSidonie.hs:169:11 instance ConcatenateMaybeString String -- Defined at UpdateSidonie.hs:177:11 • In the first argument of ‘cms’, namely ‘f `cms` "."’ In the expression: f `cms` "." `cms` s In the expression: let f = fmap head resBDwords s = fmap (head . tail) resBDwords mp = Just "." :: Maybe String .... in f `cms` "." `cms` s | 373 | in f `cms` "." `cms` s) :: Maybe String | ^^^^^^^^^^^ UpdateSidonie.hs:373:51: error: • Ambiguous type variable ‘a0’ arising from the literal ‘"."’ prevents the constraint ‘(Data.String.IsString a0)’ from being solved. Probable fix: use a type annotation to specify what ‘a0’ should be. These potential instances exist: instance Data.String.IsString Query -- Defined in ‘Database.MySQL.Simple.Types’ instance Data.String.IsString Tx.Text -- Defined in ‘Data.Text’ instance (a ~ Char) => Data.String.IsString [a] -- Defined in ‘Data.String’ ...plus six instances involving out-of-scope types (use -fprint-potential-instances to see them all) • In the second argument of ‘cms’, namely ‘"."’ In the first argument of ‘cms’, namely ‘f `cms` "."’ In the expression: f `cms` "." `cms` s | 373 | in f `cms` "." `cms` s) :: Maybe String | ^^^ Failed, no modules loaded. Prelude> any idea? Damien

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Second Call for Papers: PACMPL issue ICFP 2019
by Sam Tobin-Hochstadt 11 Jan '19

11 Jan '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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Call for papers, MPC 2019, Portugal
by Graham Hutton 09 Jan '19

09 Jan '19

Dear all, The next Mathematics of Program Construction (MPC) conference will be held in Portugal in October 2019, co-located with the Symposium on Formal Methods (FM). Paper submission is 3rd May 2019. Please share, and submit your best papers! Best wishes, Graham Hutton Program Chair, MPC 2019 ====================================================================== *** CALL FOR PAPERS -- MPC 2019 *** 13th International Conference on Mathematics of Program Construction 7-9 October 2019, Porto, Portugal Co-located with Formal Methods 2019 https://tinyurl.com/MPC-Porto ====================================================================== TIMELINE: Abstract submission 26th April 2019 Paper submission 3rd May 2019 Author notification 14th June 2019 Camera ready copy 12th July 2019 Conference 7-9 October 2019 BACKGROUND: The International Conference on Mathematics of Program Construction (MPC) aims to promote the development of mathematical principles and techniques that are demonstrably practical and effective in the process of constructing computer programs. MPC 2019 will be held in Porto, Portugal from 7-9 October 2019, and is co-located with the International Symposium on Formal Methods, FM 2019. Previous conferences were held in Königswinter, Germany (2015); Madrid, Spain (2012); Québec City, Canada (2010); Marseille, France (2008); Kuressaare, Estonia (2006); Stirling, UK (2004); Dagstuhl, Germany (2002); Ponte de Lima, Portugal (2000); Marstrand, Sweden (1998); Kloster Irsee, Germany (1995); Oxford, UK (1992); Twente, The Netherlands (1989). SCOPE: MPC seeks original papers on mathematical methods and tools put to use in program construction. Topics of interest range from algorithmics to support for program construction in programming languages and systems. Typical areas include type systems, program analysis and transformation, programming language semantics, security, and program logics. The notion of a 'program' is interpreted broadly, ranging from algorithms to hardware. Theoretical contributions are welcome, provided that their relevance to program construction is clear. Reports on applications are welcome, provided that their mathematical basis is evident. We also encourage the submission of 'programming pearls' that present elegant and instructive examples of the mathematics of program construction. SUBMISSION: Submission is in two stages. Abstracts (plain text, maximum 250 words) must be submitted by 26th April 2019. Full papers (pdf, formatted using the llncs.sty style file for LaTex) must be submitted by 3rd May 2019. There is no prescribed page limit, but authors should strive for brevity. Both abstracts and papers will be submitted using EasyChair. Papers must present previously unpublished work, and not be submitted concurrently to any other publication venue. Submissions will be evaluated by the program committee 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. Accepted papers must be presented in person at the conference by one of the authors. The proceedings of MPC 2019 will be published in the Lecture Notes in Computer Science (LNCS) series, as with all previous instances of the conference. Authors of accepted papers will be expected to transfer copyright to Springer for this purpose. After the conference, authors of the best papers from MPC 2019 and MPC 2015 will be invited to submit revised versions to a special issue of Science of Computer Programming (SCP). For any queries about submission please contact the program chair, Graham Hutton <graham.hutton(a)nottingham.ac.uk>. KEYNOTE SPEAKERS: Assia Mahboubi INRIA, France Annabelle McIver Macquarie University, Australia PROGRAM COMMITTEE: Patrick Bahr IT University of Copenhagen, Denmark Richard Bird University of Oxford, UK Corina Cîrstea University of Southampton, UK Brijesh Dongol University of Surrey, UK João F. Ferreira University of Lisbon, Portugal Jennifer Hackett University of Nottingham, UK William Harrison University of Missouri, USA Ralf Hinze University of Kaiserslautern, Germany Zhenjiang Hu National Institute of Informatics, Japan Graham Hutton (chair) University of Nottingham, UK Cezar Ionescu University of Oxford, UK Mauro Jaskelioff National University of Rosario, Argentina Ranjit Jhala University of California, USA Gabriele Keller Utrecht University, The Netherlands Ekaterina Komendantskaya Heriot-Watt University, UK Chris Martens North Carolina State University, USA Bernhard Möller University of Augsburg, Germany Shin-Cheng Mu Academia Sinica, Taiwan Mary Sheeran Chalmers University of Technology, Sweden Alexandra Silva University College London, UK Georg Struth University of Sheffield, UK CONFERENE VENUE: The conference will be held at the Alfândega Porto Congress Centre, a 150 year old former custom's house located in the historic centre of Porto on the bank of the river Douro. The venue was renovated by a Pritzer prize winning architect and has received many awards. LOCAL ORGANISERS: José Nuno Oliveira University of Minho, Portugal For any queries about local issues please contact the local organiser, José Nuno Oliveira <jno(a)di.uminho.pt>. ====================================================================== This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please contact the sender and delete the email and attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. Email communications with the University of Nottingham may be monitored where permitted by law.

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concatenate two Maybe String...
by Damien Mattei 08 Jan '19

08 Jan '19

hello, i have a variable resBDwords of type ( i expect) Maybe [String], for info it is integer and fractional part of a number example looks like this : resBDwords =Just ["-04","3982"] i want to concatanate "-04" and "3982" in the example, i begin to understand fmap to use the functor hidden in the Maybe ,it worked previously: let resBDstr = fmap Tx.unpack resBDtxt putStr "resBDstr =" putStrLn (show resBDtxt) let resBDwords = fmap words resBDstr putStr "resBDwords =" putStrLn (show resBDwords) which gives: resBDtxt ="-04 3982" resBDstr =Just "-04 3982" just after in my code i have this to concatanate the two strings f and s that are the first and second element of the array: putStr "resBDwords =" putStrLn (show resBDwords) let lgBDwords = length resBDwords let resBDstrFloat = if lgBDwords == 0 then trace "WARNING: BD contains no words" Nothing else if lgBDwords == 1 then trace "WARNING: BD contains only one word" fmap head resBDwords else let f = fmap head resBDwords s = fmap (head . tail) resBDwords in f ++ "." ++ S but i do not know how to concatanate the Maybe String in an elegant way, using somethin like fmap variable which have handled Nothing (from Maybe) automatically i need the counter part for multipe variable i do not want to do it using the hard way with case... of Just x -> nothing ......... i got this error : *Main> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:339:43: error: • Couldn't match expected type ‘[Char]’ with actual type ‘Maybe String’ • In the first argument of ‘(++)’, namely ‘f’ In the expression: f ++ "." ++ s In the expression: let f = fmap head resBDwords s = fmap (head . tail) resBDwords in f ++ "." ++ s | 339 | in f ++ "." ++ s | ^ UpdateSidonie.hs:339:43: error: • Couldn't match expected type ‘Maybe String’ with actual type ‘[Char]’ • In the expression: f ++ "." ++ s In the expression: let f = fmap head resBDwords s = fmap (head . tail) resBDwords in f ++ "." ++ s In the expression: if lgBDwords == 1 then trace "WARNING: BD contains only one word" fmap head resBDwords else let f = fmap head resBDwords s = fmap (head . tail) resBDwords in f ++ "." ++ s | 339 | in f ++ "." ++ s | ^^^^^^^^^^^^^ UpdateSidonie.hs:339:55: error: • Couldn't match expected type ‘[Char]’ with actual type ‘Maybe String’ • In the second argument of ‘(++)’, namely ‘s’ In the second argument of ‘(++)’, namely ‘"." ++ s’ In the expression: f ++ "." ++ s | 339 | in f ++ "." ++ s | ^ Failed, no modules loaded. for now this page has been of valuable help: https://pbrisbin.com/posts/maybe_is_just_awesome/ i'm sure it's an obvious question but.... :-)

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concatenate two Maybe String...
by Jake 08 Jan '19

08 Jan '19

On Mon, Jan 7, 2019 at 5:55 PM Jake <jake.waksbaum(a)gmail.com> wrote: > Like Josh mentioned, Applicative Functors > <http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.h…> > are what you want. There are two idiomatic ways to do it: > > - You can just use liftA2, which has type (a -> b -> c) -> f a -> f b > -> f c. That means it lifts a binary function to some applicative functor > like maybe, so liftA2 (++) :: Maybe String -> Maybe String -> Maybe String > - In general, for any arity f that you want to lift to an applicative > functor. So you have a function g that takes a bunch of arguments of types > a, b, c, ... and gives you back an r and you want to get a function that > takes an f a, f b, f c, ... and gives you back an f r, you can write f <$> > a <*> b <*> c ... This works because <$> let's you apply g to type f a and > gives you an f (b -> c ..) -> f r, and <*> basically let's you take the > function back out of the f and apply it to the b to get a f (c ..) -> f r > and so on. *tl;dr* you can write (++) <$> s1 <*> s2. In fact, liftA2 > must satisfy the equation liftA2 > <http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.h…> > f x y = f <$> x <*> > <http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.h…> > y so these are the same thing. > > בתאריך יום ב׳, 7 בינו׳ 2019, 17:41, מאת ☂Josh Chia (謝任中) < > joshchia(a)gmail.com>: > >> Firstly, because "resBDwords :: Maybe String", not "resBDwords :: >> String", "lgBDwords = length resBDwords" probably is not what you want -- >> it does not give you the number of words in the String that may be in there. >> >> Second, for the problem you asked about, you could just use a function >> that takes a String and do it "the hard way" like you said, using case >> outside before calling the function. Another way is to use an applicative >> functor to allow you to have a "Maybe String -> Maybe String -> Maybe >> String". This is used once for each "++" that you want to do. >> >> I don't know exactly what you need to accomplish but I would just write a >> function "f :: String -> Maybe String" implementing the logic you listed in >> the second code snippet but operating on String instead of "Maybe String" >> and do "join . fmap f $ resBDwords". >> >> On Tue, Jan 8, 2019 at 12:13 AM Damien Mattei <mattei(a)oca.eu> wrote: >> >>> hello, >>> >>> i have a variable resBDwords of type ( i expect) Maybe [String], for >>> info it is integer and fractional part of a number >>> >>> example looks like this : >>> resBDwords =Just ["-04","3982"] >>> >>> i want to concatanate "-04" and "3982" in the example, i begin to >>> understand fmap to use the functor hidden in the Maybe ,it worked >>> previously: >>> >>> let resBDstr = fmap Tx.unpack resBDtxt >>> putStr "resBDstr =" >>> putStrLn (show resBDtxt) >>> >>> let resBDwords = fmap words resBDstr >>> putStr "resBDwords =" >>> putStrLn (show resBDwords) >>> >>> which gives: >>> >>> resBDtxt ="-04 3982" >>> resBDstr =Just "-04 3982" >>> >>> >>> just after in my code i have this to concatanate the two strings f and s >>> that are the first and second element of the array: >>> >>> >>> putStr "resBDwords =" >>> putStrLn (show resBDwords) >>> >>> let lgBDwords = length resBDwords >>> >>> let resBDstrFloat = if lgBDwords == 0 >>> then trace "WARNING: BD contains no words" >>> Nothing >>> else >>> if lgBDwords == 1 >>> then trace "WARNING: BD contains only >>> one word" fmap head resBDwords >>> else let f = fmap head resBDwords >>> s = fmap (head . tail) >>> resBDwords >>> in f ++ "." ++ S >>> >>> but i do not know how to concatanate the Maybe String in an elegant way, >>> using somethin like fmap variable which have handled Nothing (from >>> Maybe) automatically i need the counter part for multipe variable >>> >>> i do not want to do it using the hard way with case... of Just x -> >>> nothing ......... >>> >>> i got this error : >>> *Main> :load UpdateSidonie >>> [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) >>> >>> UpdateSidonie.hs:339:43: error: >>> • Couldn't match expected type ‘[Char]’ >>> with actual type ‘Maybe String’ >>> • In the first argument of ‘(++)’, namely ‘f’ >>> In the expression: f ++ "." ++ s >>> In the expression: >>> let >>> f = fmap head resBDwords >>> s = fmap (head . tail) resBDwords >>> in f ++ "." ++ s >>> | >>> 339 | in f ++ "." ++ s >>> | ^ >>> >>> UpdateSidonie.hs:339:43: error: >>> • Couldn't match expected type ‘Maybe String’ >>> with actual type ‘[Char]’ >>> • In the expression: f ++ "." ++ s >>> In the expression: >>> let >>> f = fmap head resBDwords >>> s = fmap (head . tail) resBDwords >>> in f ++ "." ++ s >>> In the expression: >>> if lgBDwords == 1 then >>> trace "WARNING: BD contains only one word" fmap head >>> resBDwords >>> else >>> let >>> f = fmap head resBDwords >>> s = fmap (head . tail) resBDwords >>> in f ++ "." ++ s >>> | >>> 339 | in f ++ "." ++ s >>> | ^^^^^^^^^^^^^ >>> >>> UpdateSidonie.hs:339:55: error: >>> • Couldn't match expected type ‘[Char]’ >>> with actual type ‘Maybe String’ >>> • In the second argument of ‘(++)’, namely ‘s’ >>> In the second argument of ‘(++)’, namely ‘"." ++ s’ >>> In the expression: f ++ "." ++ s >>> | >>> 339 | in f ++ "." ++ s >>> | ^ >>> Failed, no modules loaded. >>> >>> for now this page has been of valuable help: >>> >>> https://pbrisbin.com/posts/maybe_is_just_awesome/ >>> >>> i'm sure it's an obvious question but.... :-) >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> To (un)subscribe, modify options or view archives go to: >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>> Only members subscribed via the mailman list are allowed to post. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. > >

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