- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] FMBC 2021 - 1st CFP
by Bruno Bernardo 09 Mar '21

09 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 09 Mar '21

09 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@nomadic-labs.com)* Diego Marmsoler (University of Exeter, UK) (D.Marmsoler@exeter.ac.uk)PC MEMBERSTo be confirmed

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[ANN] Copilot 3.2.1 - hard realtime C runtime verification
by Frank Dedden 09 Mar '21

09 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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Announcing `passveil` 0.0.1.0
by raichoo 08 Mar '21

08 Mar '21

Hi everyone, My company antei (https://antei.de) has developed a password manager for our internal usage and we have decided to open source it. We drew a lot of inspiration from the well established `password-store`. It's build in Haskell and uses `darcs` for versioning the password store. We have chosen `darcs` because of its powerful merging capabilities which turned out to be useful for our use case. Apart from that, we have implemented a trust system, that can be used to transfer knowledge of a password to other people having access to the store, so we do not have to rely on transmitting sensitive data via insecure channels (https://sysadminotaur.devolutions.net/30-vault) Please note that `passveil` is in an early state and has only been tested and used internally. We strongly believe in open source and making our tools available to others and are constantly working on improving our code and want share our progress with everyone. Check out: https://passveil.antei.de for more information. Kind regards, raichoo

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Profiling analyzing ghc compilation
by Martin Heuschober 06 Mar '21

06 Mar '21

Hi, Is there a way to profile (time/space) GHC compilation itself, I'd be interested in what compilation steps in compiling my project take long and how much RAM it uses/why it uses so much RAM, so I can identify what to rewrite, remove, and split to get a leaner build. Thanks, Martin PS.: I hope this is the right place to ask this.

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Haskell.org Call For Nominations
by Rebecca Skinner 06 Mar '21

06 Mar '21

Dear Haskellers, As you may know, Emily Pillmore has recently stepped down from her role on the Haskell.org committee as she focuses full time on her new role as the CTO of the Haskell Foundation. We are currently seeking nominations for anyone interested in filling her vacated seat on the committee. The term for this seat will end in October of 2021, so it will be a good opportunity for anyone who is interested in serving for a short time, perhaps to get a better idea of whether they would like to be re-nominated for a full term at the end of the year. The role is about setting policy, providing direction/guidance for Haskell.org infrastructure, planning for the long term, and being fiscally responsible with the Haskell.org funds and donations. As overseers for policy regarding the open source side of Haskell, committee members must also be able to set aside personal or business related bias and make decisions with the good of the open source Haskell community in mind. For more information about the haskell committee please visit https://www.haskell.org/haskell-org-committee/ To nominate yourself, please send an email to committee(a)haskell.org no later than 12 March 2021. Q: If selected for this role, will I serve a normal 3-year term? A: No, you will serve the remainder of the term for the seat, which will end in October of 2021. Q: If I was nominated for a seat during the 2021 election and was not selected, am I eligible for re-nomination? A: We had a number of great candidates for the recent election and we would welcome anyone who was not selected to re-nominate themselves for this seat. Q: If selected, am I eligible for re-nomination at the end of the year? A: Yes, if you are selected and serve you may be re-nominated for a full 2022-2025 term. Q: Who can be nominated to serve on the committee? A: Anyone who has something to contribute. Committee members do not need to be deep technical Haskell experts. Instead we look for enthusiasm towards improving the Haskell community. We aim to represent the different facets of the community. We aim to be diverse in terms of industry or research, and in terms of gender, race and location. Q: What should I include in my nomination? A: Tell us a bit about who you are, and how you think you can help. Feel free to include anything that you think will help us make a better decision. Q: What is involved in membership? A: Being a member of the committee does not necessarily require a significant amount of time, but committee members should aim to be responsive during discussions when the committee is called upon to make a decision. For more information, or any other questions, please reach out to the committee by email. - Rebecca Skinner on behalf of the Haskell.org Committee

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Re: [Haskell-cafe] naturally, length :: a -> Int
by Olaf Klinke 05 Mar '21

05 Mar '21

> Date: Thu, 4 Mar 2021 11:36:14 +0100 > From: Ben Franksen <ben.franksen(a)online.de> > To: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] naturally, length :: a -> Int > Message-ID: <s1qd6u%24l43%241(a)ciao.gmane.io> > Content-Type: text/plain; charset=utf-8 > > Am 03.03.21 um 16:17 schrieb Olaf Klinke: > > Which bugs can be caught at compile-time by having length return > > natural numbers? Regarding space, Int instead of Word only wastes the > > sign bit, doesn't it? > > > > I like the idea that Johannes Waldmann and Jaro Reinders brought up: > > Why is length :: Foldable a => a -> Int so convenient? Short answer: > > Because of "affine" things like `availableSpace - length xs'. > > > > There are indeed two types involved here, as the foundation package > > points out: relative offsets (like a tangent space?) and absolute > > counts. Think of NominalDiffTime versus UTCTime, or the two > > interpretations of vectors as points/movements in space. > > > > In this light one could regard the current length as > > "the relative offset of the end of the list" which can readily be > > subtracted from another relative offset. In mathematical terms: Int the > > free group over the monoid of cardinal lengths. > > Hm, interesting point. > > If we do embrace that viewpoint, then I'd say we should go all the way > and interpret indices modulo (non-negative) structure size! This makes > (safe) indexing total (for non-empty structures) and allows things like > xs !! (-1) == last xs as in Perl and some other languages. Unsafe > indexing (as in the vector package) could remain as is for performance > critical code. > > Cheers > Ben I like Haskell particularly for not being like Matlab, where virtually any well-bracketed indexing syntax does produce a result, but not necessarily what you intended or expected. Besides, for some structures length is expensive but index is not so much, so I wonder whether one can modulo-index into a lazy container without evaluating its entire spine. I just wanted to understand/justify why Int is so convenient, and the Time-DiffTime analogy sprang to mind. Following the free group route, one immediately sees that there is an embedding from cardinalities to offsets, but no inverse. We could, of course, bring other categories into the picture such as ordered sets. Then there is an embedding-projection pair between Int and Natural, where all Ints < 0 are mapped to 0. The base functions such as take and drop adhere to this interpretation. Olaf

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Serious bug with Natural in GHC 9.0.1
by Sylvain Henry 05 Mar '21

05 Mar '21

Hi Café, I've made a mistake while implementing constant folding for Natural numbers in GHC 9.0.1 that hasn't been caught before the release. It's a rather serious bug as any number converted into `Natural` with `fromIntegral` can be truncated to Word range if a bogus rule is triggered (see #19345 and #19411 for more details). It is already fixed in HEAD and the fix will be present in GHC 9.0.2 which should be released soon. In the meantime you can check if the rule is triggered with `-ddump-rule-firings`. If it shows that the following rule has fired, then your code may be badly modified: |Rule fired: fromIntegral/Int->Natural (GHC.Real)| (the issue is that despite its name the rule isn't correctly constrained to convert Ints into Natural, but any numeric type...). Sorry about that! Sylvain #19345: https://gitlab.haskell.org/ghc/ghc/-/issues/19345 #19411: https://gitlab.haskell.org/ghc/ghc/-/issues/19411

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naturally, length :: a -> Int
by Johannes Waldmann 04 Mar '21

04 Mar '21

Dear Cafe - let's fight Int. In a lot of code I write (for research, teaching, production), a lot of numbers are in fact natural numbers - because I am counting (e.g., number of elements in a collection) or pointing into what I just counted (e.g., Data.Set.elemAt). But Haskell's "most natural" type is - Int. At least that's my impression from current usage in libraries and teaching examples (see functions length, replicate, ... ). I've developed a horror of reading and writing "length something :: Int". As I will be teaching an introductory class on FP soon, I am looking for ways to handle this. One option is presented in Bird/Gibbons: Algorithm Design with Haskell, which has "type Nat = Int" on page 6. Actually, that's the very first code example that appears in the book! The footnote there refers to a remark in Data.Word that has since been dropped? Now there is Numeric.Natural, (cf. https://mail.haskell.org/pipermail/libraries/2014-November/024203.html ) and numeric literals are already overloaded, so we can just use that? Well, yes and no - we still have to write a type conversion each time we use some library function that insists on Int, or, write cumbersome "genericLength". Also (but that is the lesser concern), Numeric.Natural is for arbitrary-length numbers (corresponding to Integer), while I might sometimes know that numbers are representable in a machine word, and I don't want to pay for the implied pattern match (data Natural = NatS Word | NatJ BigNat). I am willing to pay for underflow checks (on subtraction), and for overflow checks (if there's an upper bound). If I want to live risky (no checks) I could use Data.Word. So, what do you do? (Yes, Peano numerals ...) Or, how do we convince ourselves (and our students) of keeping the current status? NB: here is another practical argument for Natural (from production code, an auto-grading system for student homework) We use datatype-generic parsers heavily. For data Foo = Foo { ... :: Nat ... }, bad input will be rejected by the parser already, and I need no extra checks later on. Less code! I sometimes even use "Pos(itive)". That's all fine and dandy - until I call some library function... - J.W.

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