March 2021 - Haskell-Cafe

Property checks and PostgreSQL?
by Ignat Insarov 14 Mar '21

14 Mar '21

Note: I posted this [elsewhere] — I hope it is permissible to also post here. There seems to be no single right venue for Haskell questions at this time. [elsewhere]: https://discourse.haskell.org/t/property-checks-and-postgresql/2078 So, I have a three layer stack: PostgreSQL data base, Haskell back end, PureScript front end. And there is a fault somewhere in this stack: sometimes some items present in the data base do not make it to the front end. A small fortune in programmer hours has been spent searching for the fault but there is no progress. No one can figure out even roughly where the fault is. Our last hope is to property check the whole thing, from end to end, verifying that any things put into the data base can be retrieved by the front end. We have in place a fixture that creates a new PostgreSQL cluster, runs the back end on it and makes some scripted HTTP requests. We use it to run unit tests, such like _«if I make this `PUT` request and then that `GET` request to the same end point, I should get back the same thing as I put in»_. In principle it would not be a problem to make a property check out of this property. Practically, tearing down the cluster, building a new pristine one and initializing the data base anew takes seconds, so this way even a trivial property check would take minutes to run. Theoretically, we can carefully reset the data base back to the initial state after each run of the property check. Given that only a small amount of data is modified, it would take imperceptibly little time. But it is hard to know what exactly should be reset, and it is easy to get it wrong if done by hand! One trick we do use is SQL transactions. We take a single connexion, start a transaction, perform a check and then roll the transaction back. Unfortunately, even this is not completely safe: it looks as though sequences used to draw primary keys from are not reset to their previous values! _(Or maybe we are doing something wrong — I am not really a PostgreSQL guru so I am not too sure of myself.)_ But even ignoring this problem _(it is irrelevant to most checks)_, there is another, more severe problem: transactions guarantee that uncommitted data is only visible inside the same connexion. So, there is no way to request it from the HTTP API. This trick is suitable for fuzzing the data base – back end layer, but nothing beyond that. Another thing I heard being done is to selectively drop and re-create exactly the tables affected by a run of a property check. This seems challenging to automate though. How can I know in a general way if a table has been touched? And then, how can I re-create exactly the affected tables given that the data base initialization code is an opaque blob? I wonder if this problem is already solved. In any case, any advice is welcome!

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Package Takeover: `toml`
by Emily Pillmore 13 Mar '21

13 Mar '21

To whom it may concern, I'd like to take over maintenance of the package `toml`. I have a burning need. I tried to contact Spiros a year ago, but no response has come. The last release was 2017, and it's safe to say the package is totally abandoned. My hackage user is topos. Cheers, E

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data kinds and exhaustiveness of typeclass instances
by Paul Brauner 13 Mar '21

13 Mar '21

Hello, the following program doesn't typecheck in GHC 9: data Tag = A | B data Foo (a :: Tag) = Foo class C a where f :: a -> Int instance C (Foo A) where f x = 1 instance C (Foo B) where f x = 2 g :: Foo a -> Int g = f Yet one could argue that for all a :: Tag, C (Foo a) holds because a :: Tag can only take two values: A or B, and C (Foo A) and C (Foo B) hold. Is there a way to somehow convince GHC of that fact so that g typechecks? Cheers, Paul

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[ANN] singletons-3.0, singletons-th-3.0, and singletons-base-3.0
by Ryan Scott 12 Mar '21

12 Mar '21

I'm happy to announce the 3.0 release of the singletons library, as well as the debut of its companion libraries, singletons-th and singletons-base. There's been a fair bit of improvements, cleanup, and reorganization since the last release, so I thought it appropriate to give this release a super-major version bump. This announcement will go over some highlights of the new releases, but if you want the full details, you can consult the changelog entries for each library below: * singletons-3.0: http://hackage.haskell.org/package/singletons-3.0/changelog * singletons-th-3.0: http://hackage.haskell.org/package/singletons-th-3.0/changelog * singletons-base-3.0: http://hackage.haskell.org/package/singletons-base-3.0/changelog ########################## ## Why three libraries? ## ########################## Instead of there just being one singletons library, there are now _three_ libraries: * singletons: A small, foundational library that defines basic singleton-related types and definitions. The new singletons library supports GHC 8.0 or later, does not depend on any external libraries, and does not make use of Template Haskell. * singletons-th: A library that defines Template Haskell functionality for promoting term-level functions to type-level equivalents and singling functions to dependently typed equivalents. This library requires bleeding-edge GHC features, and as a result, it only supports GHC 9.0 or later. * singletons-base: A library that uses singletons-th to define promoted and singled functions from the base library, including the Prelude. Similarly to singletons-th, this library only supports GHC 9.0 or later. Previous singletons releases combined all of this functionality into a single library. However, discussions with singletons users revealed that this was a frequent source of pain: * The total time it took to compile the old singletons library dissuaded some from using it, especially for those who only wanted to use a small part of the library. * Some users wished to use singletons in environments where Template Haskell is not an option. * Some users wanted to use core definitions from Data.Singletons, but because the old singletons library only ever supported one GHC at a time, this was impractical. The new package structure in the 3.0 release is an attempt to address some of these pain points. In particular, most of the code which results in extensive compile times is now confined to the singletons-base library. For the full discussion that led up to this, see [1]. ########################### ## Module reorganization ## ########################### Splitting up singletons into smaller packages provided a rare opportunity to clean up the module naming conventions, which were confusing and inconsistent in many places. The 3.0 releases now use the following conventions: * All modules in singletons now begin with Data.Singletons.*. * All modules in singletons-th now begin with Data.Singletons.TH.*. * Most modules in singletons-base now reflect the modules from base from which they take inspiration. For example, the Prelude module now has a Prelude.Singletons counterpart in singletons. Similarly, there are also modules like Control.Monad.Singletons, GHC.TypeLits.Singletons, etc. All other modules that do not correspond to something in base now begin with Data.Singletons.Base.*. The convention now used in singletons-base was inspired by the conventions used in the lens library. The end result is that is now somewhat easier to figure out what parts of singletons-base to import. Because this is a significant departure from previous singletons releases, if you want to continue supporting pre-3.0 releases, you will likely need to put something like this in your .cabal file: flag singletons-3-0 description: Use @singletons-3.0@ or later. default: True library ... if flag(singletons-3-0) build-depends: singletons-base >= 3.0 -- You may also need to depend on singletons >= 3.0 or singletons-th >= 3.0 else build-depends: singletons < 3.0 ################### ## Other changes ## ################### Besides the package and module reorganization mentioned above, singletons-{,th,base}-3.0 feature a variety of new quality-of-life improvements. Here are a handful of the more notable improvements: * The OptionsM type, used for configuring how the Template Haskell machinery in singletons-th works, is now an instance of Quote. This avoids the need to use the Control.Monad.Trans.Class.lift function to lift quoted declarations into OptionsM. [2] * Data.Singletons.TH.Options now defines a promotedDataTypeOrConName option. Overriding this option can be useful in situations where one wishes to promote types such as `Nat`, `Symbol`, or data types built on top of them. [3] * The internals of the ShowSing class have been refactored to allow deriving Show instances for Sing types (e.g., deriving instance ShowSing a => Show (SList (z :: [a]))). [4] * Improve the quality of GHCi's output when using :kind on types defined by singletons-th. [5] If you encounter any issues, feel free to leave a bug report at https://github.com/goldfirere/singletons/issues. Happy (type-level) hacking, Ryan S. ----- [1] https://github.com/goldfirere/singletons/issues/420 [2] https://github.com/goldfirere/singletons/pull/484 [3] https://github.com/goldfirere/singletons/pull/462 [4] https://github.com/goldfirere/singletons/pull/486 [5] https://github.com/goldfirere/singletons/pull/446

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ANN: hledger-1.21
by Simon Michael 10 Mar '21

10 Mar '21

I'm very pleased to announce hledger 1.21 ! http://hledger.org/release-notes.html#hledger-1-21 describes the user-visible changes. Highlights include: - About 10% faster/more space-efficient, and much more in some cases - Jump to any topic in the manuals from the command line - More polish for valuation and balance reports, and a new value change report - Improvements to check, print and other commands. Thank you to the following release contributors this time around: Dmitry Astapov, Arnout Engelen, Damien Cassou, aragaer, Doug Goldstein, Caleb Maclennan, Felix Van der Jeugt, heavy first time contributor Vladimir Zhelezov, and especially Stephen Morgan. hledger (http://hledger.org) is a robust, cross-platform, plain text accounting tool, with command-line, terminal and web UIs. It is an actively maintained, largely compatible reimplementation of Ledger CLI with many improvements. You can use it to track money, time, investments, cryptocurrencies, inventory and more. See also the Plain Text Accounting site (http://plaintextaccounting.org) http://hledger.org/download shows all the ways to install hledger on mac, windows or unix (stack, cabal, brew, nix, CI binaries, your package manager..). Or, run this bash script to install or upgrade to the latest release: $ curl -sO https://raw.githubusercontent.com/simonmichael/hledger/master/hledger-insta… $ less hledger-install.sh # security review $ bash hledger-install.sh New users, check out https://hledger.org/quickstart. To get help, see http://hledger.org#help-feedback, and join our chat channel via Freenode (#hledger, http://irc.hledger.org) or Matrix (#freenode_#hledger:matrix.org, http://matrix.hledger.org) Beginners and experts, contributors, sponsors, and all feedback are most welcome! Wishing you health and prosperity, -Simon

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Multitier Haskell?
by Adriaan Leijnse 08 Mar '21

08 Mar '21

Dear Café, Are there any techniques for compiling a single client-server program such that clients are not distributed any server code? I'm currently looking at implementing an FRP library with a type system like Gavial's [0]. The types will make it clear on which side code belongs. My current thinking is to have Client and Server instances of a ClientServer type class which can safely implement the functions of the other side as no-ops/bottom, but I imagine that would at leastrequire a dead code elimination step after compilation to get the desired effect? Adriaan [0] https://arxiv.org/pdf/2002.06188.pdf

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Re: [Haskell-cafe] FMBC 2021 - 1st CFP
by Bruno Bernardo 08 Mar '21

08 Mar '21

Apologies for the poor formatting of my previous message. You'll find below a hopefully more readable version.Sorry again for the noise. ==================================== [ Please distribute, apologies for multiple postings. ] ======================================================================== 3rd International Workshop on Formal Methods for Blockchains (FMBC) 2021 - First Call https://fmbc.gitlab.io/2021 July 18 or 19 (TBA), 2021 Co-located with the 33nd International Conference on Computer-Aided Verification (CAV 2021) http://i-cav.org/2021/ ------------------------------------------------------------- IMPORTANT DATES -------------------------------- Abstract submission: April 22, 2021 Full paper submission: April 29, 2021 Notification: June 10, 2021 Camera-ready: July 8, 2021 Workshop: July 18 or 19 (TBA), 2021 Deadlines are Anywhere on Earth: https://en.wikipedia.org/wiki/Anywhere_on_Earth -------------------------------- -------------------------------- TOPICS OF INTEREST -------------------------------- Blockchains are decentralized transactional ledgers that rely on cryptographic hash functions for guaranteeing the integrity of the stored data. Participants on the network reach agreement on what valid transactions are through consensus algorithms. Blockchains may also provide support for Smart Contracts. Smart Contracts are scripts of an ad-hoc programming language that are stored in the Blockchain and that run on the network. They can interact with the ledger’s data and update its state. These scripts can express the logic of possibly complex contracts between users of the Blockchain. Thus, Smart Contracts can facilitate the economic activity of Blockchain participants. With the emergence and increasing popularity of cryptocurrencies such as Bitcoin and Ethereum, it is now of utmost importance to have strong guarantees of the behavior of Blockchain software. These guarantees can be brought by using Formal Methods. Indeed, Blockchain software encompasses many topics of computer science where using Formal Methods techniques and tools are relevant: consensus algorithms to ensure the liveness and the security of the data on the chain, programming languages specifically designed to write Smart Contracts, cryptographic protocols, such as zero-knowledge proofs, used to ensure privacy, etc. This workshop is a forum to identify theoretical and practical approaches of formal methods for Blockchain technology. Topics include, but are not limited to: * Formal models of Blockchain applications or concepts * Formal methods for consensus protocols * Formal methods for Blockchain-specific cryptographic primitives or protocols * Design and implementation of Smart Contract languages * Verification of Smart Contracts -------------------------------- -------------------------------- SUBMISSION -------------------------------- Submit original manuscripts (not published or considered elsewhere) with a page limit of 12 pages for full papers and 6 pages for short papers (excluding bibliography and short appendix of up to 5 additional pages). Alternatively you may also submit an extended abstract of up to 3 pages (including bibliography) summarizing your ongoing work in the area of formal methods and blockchain. Authors of selected extended-abstracts are invited to give a short lightning talk. Submission link: https://easychair.org/conferences/?conf=fmbc2021 Authors are encouraged to use LaTeX and prepare their submissions according to the instructions and styling guides for OASIcs provided by Dagstuhl. Instructions for authors: https://submission.dagstuhl.de/documentation/authors#oasics At least one author of an accepted paper is expected to present the paper at the workshop as a registered participant. -------------------------------- -------------------------------- PROCEEDINGS -------------------------------- All submissions will be peer-reviewed by at least three members of the program committee for quality and relevance. Accepted regular papers (full and short papers) will be included in the workshop proceedings, published as a volume of the OpenAccess Series in Informatics (OASIcs) by Dagstuhl. -------------------------------- -------------------------------- INVITED SPEAKER -------------------------------- To be confirmed -------------------------------- -------------------------------- PROGRAM COMMITTEE -------------------------------- PC CO-CHAIRS * Bruno Bernardo (Nomadic Labs, France) (bruno(a)nomadic-labs.com) * Diego Marmsoler (University of Exeter, UK) (D.Marmsoler(a)exeter.ac.uk) PC MEMBERS To be confirmed

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FMBC 2021 - 1st CFP
by Bruno Bernardo 08 Mar '21

08 Mar '21

[ Please distribute, apologies for multiple postings. ]========================================================================3rd International Workshop on Formal Methods for Blockchains (FMBC) 2021 - First Callhttps://fmbc.gitlab.io/2021July 18 or 19 (TBA), 2021Co-located with the 33nd International Conference on Computer-Aided Verification (CAV 2021)http://i-cav.org/2021/------------------------------------------------… DATES--------------------------------Abstract submission: April 22, 2021Full paper submission: April 29, 2021Notification: June 10, 2021Camera-ready: July 8, 2021Workshop: July 18 or 19 (TBA), 2021Deadlines are Anywhere on Earth:https://en.wikipedia.org/wiki/Anywhere_on_Earth----------------------… OF INTEREST--------------------------------Blockchains are decentralized transactional ledgers that rely oncryptographic hash functions for guaranteeing the integrity of thestored data. Participants on the network reach agreement on what validtransactions are through consensus algorithms.Blockchains may also provide support for Smart Contracts. SmartContracts are scripts of an ad-hoc programming language that arestored in the Blockchain and that run on the network. They caninteract with the ledger’s data and update its state. These scriptscan express the logic of possibly complex contracts between users ofthe Blockchain. Thus, Smart Contracts can facilitate the economicactivity of Blockchain participants.With the emergence and increasing popularity of cryptocurrencies suchas Bitcoin and Ethereum, it is now of utmost importance to have strongguarantees of the behavior of Blockchain software.These guarantees can be brought by using Formal Methods. Indeed,Blockchain software encompasses many topics of computer science whereusing Formal Methods techniques and tools are relevant: consensusalgorithms to ensure the liveness and the security of the data on thechain, programming languages specifically designed to write SmartContracts, cryptographic protocols, such as zero-knowledge proofs,used to ensure privacy, etc.This workshop is a forum to identify theoretical and practicalapproaches of formal methods for Blockchain technology. Topicsinclude, but are not limited to:* Formal models of Blockchain applications or concepts* Formal methods for consensus protocols* Formal methods for Blockchain-specific cryptographic primitives or protocols* Design and implementation of Smart Contract languages* Verification of Smart Contracts----------------------------------------------------------------SUBMISSION--------------------------------Submit original manuscripts (not published or considered elsewhere)with a page limit of 12 pages for full papers and 6 pages for shortpapers (excluding bibliography and short appendix of up to 5additional pages).Alternatively you may also submit an extended abstract of up to 3pages (including bibliography) summarizing your ongoing work in thearea of formal methods and blockchain. Authors of selectedextended-abstracts are invited to give a short lightning talk.Submission link: https://easychair.org/conferences/?conf=fmbc2021Authors are encouraged to use LaTeX and prepare their submissionsaccording to the instructions and styling guides for OASIcs providedby Dagstuhl.Instructions for authors: https://submission.dagstuhl.de/documentation/authors#oasicsAt least one author of an accepted paper is expected to present thepaper at the workshop as a registered participant.----------------------------------------------------------------PROCEEDINGS--------------------------------All submissions will be peer-reviewed by at least three members of theprogram committee for quality and relevance. Accepted regular papers(full and short papers) will be included in the workshop proceedings,published as a volume of the OpenAccess Series in Informatics (OASIcs)by Dagstuhl.----------------------------------------------------------------INVITED SPEAKER--------------------------------To be confirmed----------------------------------------------------------------PROGRAM COMMITTEE--------------------------------PC CO-CHAIRS* Bruno Bernardo (Nomadic Labs, France) (bruno(a)nomadic-labs.com)* Diego Marmsoler (University of Exeter, UK) (D.Marmsoler(a)exeter.ac.uk)PC MEMBERSTo be confirmed

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[ANN] Copilot 3.2.1 - hard realtime C runtime verification
by Frank Dedden 08 Mar '21

08 Mar '21

Dear all, We are very pleased to announce the release of Copilot 3.2.1, a stream-based DSL for writing and monitoring embedded C programs, with an emphasis on correctness and hard realtime requirements. Copilot is typically used as a high-level runtime verification framework, and supports temporal logic (LTL, PTLTL and MTL), clocks and voting algorithms. Among others, Copilot has been used at the Safety Critical Avionics Systems Branch of NASA Langley Research Center for monitoring test flights of drones. In collaboration with the development team at Galois, Inc., the new release introduces a backend to their What4 solver frontend. With this addition, it is now possible to prove properties of Copilot specifications using What4 [4]. We thank Galois for their help and their contribution. The second big focus of this release was the documentation: the public API documentation has been improved and completed. Additionally, several small improvements and fixes are made in the C99 code generator. The newest release is available on hackage [1]. For more information, including documentation, examples and links to the source code, please visit the webpage [2]. Current emphasis is on facilitating the use with other systems, and improving the codebase in terms of stability and test coverage. Users are encouraged to participate by opening issues and asking questions via our github repo [3]. Kind regards, The Copilot developers: - Frank Dedden - Alwyn Goodloe - Ivan Perez [1] http://hackage.haskell.org/package/copilot [2] https://copilot-language.github.io [3] https://github.com/Copilot-Language/copilot [4] https://github.com/GaloisInc/what4

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Second Call for Papers for the Haskell Symposium 2021
by Hage, J. (Jurriaan) 08 Mar '21

08 Mar '21

Dear all, This is the second call for papers for the upcoming Haskell Symposium. The deadline of the first round is approaching fast. Please forward to anyone that you believe might be interested. Apologies for receiving multiple copies of this announcement. Best regards, Jurriaan Hage Chair ================================================================================ ACM SIGPLAN CALL FOR SUBMISSIONS Haskell Symposium 2021 ** virtual ** Thu 26 -- Fri 27 August, 2021 http://www.haskell.org/haskell-symposium/2021/ ================================================================================ The ACM SIGPLAN Haskell Symposium 2021 will be co-located with the 2021 International Conference on Functional Programming (ICFP). Due to COVID-19 it will take place **virtually** this year. Like last year, we will be using a lightweight double-blind reviewing process. See further information below. Different from last year is that we offer a new submission category: the tutorial. Details can be found below. The Haskell Symposium presents original research on Haskell, discusses practical experience and future development of the language, and promotes other forms of declarative programming. Topics of interest include: * Language design, with a focus on possible extensions and modifications of Haskell as well as critical discussions of the status quo; * Theory, such as formal semantics of the present language or future extensions, type systems, effects, metatheory, and foundations for program analysis and transformation; * Implementations, including program analysis and transformation, static and dynamic compilation for sequential, parallel, and distributed architectures, memory management, as well as foreign function and component interfaces; * Libraries, that demonstrate new ideas or techniques for functional programming in Haskell; * Tools, such as profilers, tracers, debuggers, preprocessors, and testing tools; * Applications, to scientific and symbolic computing, databases, multimedia, telecommunication, the web, and so forth; * Functional Pearls, being elegant and instructive programming examples; * Experience Reports, to document general practice and experience in education, industry, or other contexts; * Tutorials, to document how to use a particular language feature, programming technique, tool or library within the Haskell ecosystem; * System Demonstrations, based on running software rather than novel research results. Regular papers should explain their research contributions in both general and technical terms, identifying what has been accomplished, explaining why it is significant, and relating it to previous work, and to other languages where appropriate. Experience reports and functional pearls need not necessarily report original academic research results. For example, they may instead report reusable programming idioms, elegant ways to approach a problem, or practical experience that will be useful to other users, implementers, or researchers. The key criterion for such a paper is that it makes a contribution from which other Haskellers can benefit. It is not enough simply to describe a standard solution to a standard programming problem, or report on experience where you used Haskell in the standard way and achieved the result you were expecting. A new submission category for this year's Haskell Symposium is the tutorial. Like with the experience report and the functional pearl, the key criterion for such a paper is that it makes a contribution from which other Haskellers can benefit. What distinguishes a tutorial is that its focus is on explaining an aspect of the Haskell language and/or ecosystem in a way that is generally useful to a Haskell audience. Tutorials for many such topics can be found online; the distinction here is that by writing it up for formal review it will be vetted by experts and formally published. System demonstrations should summarize the system capabilities that would be demonstrated. The proposals will be judged on whether the ensuing session is likely to be important and interesting to the Haskell community at large, whether on grounds academic or industrial, theoretical or practical, technical, social or artistic. Please contact the program chair with any questions about the relevance of a proposal. If your contribution is not a research paper, please mark the title of your experience report, functional pearl, tutorial or system demonstration as such, by supplying a subtitle (Experience Report, Functional Pearl, Tutorial Paper, System Demonstration). Submission Details ================== Early and Regular Track ----------------------- The Haskell Symposium uses a two-track submission process so that some papers can gain early feedback. Strong papers submitted to the early track are accepted outright, and the others will be given their reviews and invited to resubmit to the regular track. Papers accepted via the early and regular tracks are considered of equal value and will not be distinguished in the proceedings. Although all papers may be submitted to the early track, authors of functional pearls and experience reports are particularly encouraged to use this mechanism. The success of these papers depends heavily on the way they are presented, and submitting early will give the program committee a chance to provide feedback and help draw out the key ideas. Formatting ---------- Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines. Authors should use the `acmart` format, with the `sigplan` sub-format for ACM proceedings. For details, see: http://www.sigplan.org/Resources/Author/#acmart-format It is recommended to use the `review` option when submitting a paper; this option enables line numbers for easy reference in reviews. Functional pearls, experience reports, tutorials and demo proposals should be labelled clearly as such. Lightweight Double-blind Reviewing ---------------------------------- Haskell Symposium 2021 will use 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 judgment 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. A reviewer will learn the identity of the author(s) of a paper after a review is submitted. Page Limits ----------- The length of submissions should not exceed the following limits: Regular paper: 12 pages Functional pearl: 12 pages Tutorial: 12 pages Experience report: 6 pages Demo proposal: 2 pages There is no requirement that all pages are used. For example, a functional pearl may be much shorter than 12 pages. In all cases, the list of references is not counted against these page limits. Deadlines --------- Early track: Submission deadline: 19 March 2021 (Fri) Notification: 23 April 2021 (Fri) Regular track and demos: Submission deadline: 21 May 2021 (Fri) Notification: 23 June 2021 (Wed) Deadlines are valid anywhere on Earth. Submission ---------- Submissions must adhere to SIGPLAN's republication policy (http://sigplan.org/Resources/Policies/Republication/) and authors should be aware of ACM's policies on plagiarism (https://www.acm.org/publications/policies/plagiarism) Program Committee members are allowed to submit papers, but their papers will be held to a higher standard. The paper submission deadline and length limitations are firm. There will be no extensions, and papers violating the length limitations will be summarily rejected. Papers should be submitted through HotCRP at: https://haskell21.hotcrp.com/ Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. 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 can distinguish between 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). 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 conference 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. Proceedings =========== Accepted papers will be included in the ACM Digital Library. Their authors will be required to choose one of the following options: - Author retains copyright of the work and grants ACM a non-exclusive permission-to-publish license (and, optionally, licenses the work with a Creative Commons license); - Author retains copyright of the work and grants ACM an exclusive permission-to-publish license; - Author transfers copyright of the work to ACM. For more information, please see ACM Copyright Policy (http://www.acm.org/publications/policies/copyright-policy) and ACM Author Rights (http://authors.acm.org/main.html) Accepted proposals for system demonstrations will be posted on the symposium website but not formally published in the proceedings. Publication date: The official publication date of accepted papers 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. Artifacts ========= Authors of accepted papers are encouraged to make auxiliary material (artifacts like source code, test data, etc.) available with their paper. They can opt to have these artifacts published alongside their paper in the ACM Digital Library (copyright of artifacts remains with the authors). If an accepted paper's artifacts are made permanently available for retrieval in a publicly accessible archival repository like the ACM Digital Library, that paper qualifies for an Artifacts Available badge (https://www.acm.org/publications/policies/artifact-review-badging#available) Applications for such a badge can be made after paper acceptance and will be reviewed by the PC chair. Program Committee ================= Edwin Brady University of St Andrews Koen Claessen Chalmers University of Technology Dominique Devriese Vrije Universiteit Brussel Andy Gill University of Kansas Jurriaan Hage (chair) Universiteit Utrecht Zhenjiang Hu Peking University Ranjit Jhala University of California Patricia Johann Appalachian State University Yukiyoshi Kameyama University of Tsukuba George Karachalias Tweag Ralf Laemmel University of Koblenz-Landau Daan Leijen Microsoft Research Ben Lippmeier Ghost Locomotion Neil Mitchell Facebook Alberto Pardo Universidad de la Republica, Uruguay Matt Roberts Macquarie University Janis Voigtlaender University of Duisburg-Essen Nicolas Wu Imperial College London If you have questions, please contact the chair at: j.hage(a)uu.nl ================================================================================

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