May 2022 - Haskell-Cafe

[ANN] cabal-clean (new tool)
by Andreas Abel 19 Aug '22

19 Aug '22

=== Wondering where your disk space goes with too many Haskell projects compiled with too many GHC versions? === https://hackage.haskell.org/package/cabal-clean https://github.com/andreasabel/cabal-clean `cabal-clean` is a small executable that removes outdated cabal build artifacts from `dist-newstyle`. A build tree is considered outdated if there is one with either - a newer version of the package you are building, or - a newer minor version of the GHC compiler. In contrast to `cabal clean`, the tool `cabal-clean` retains the latest build trees for each GHC major version. Thus, it is geared towards multi-GHC development. Example: (self-applied to cabal-clean's build directory) --- 3.7M ghc-8.8.4 /cabal-clean-0.1.20210815 --- 3.5M ghc-8.10.6/cabal-clean-0.1.20210815 --- 3.7M ghc-9.0.1 /cabal-clean-0.1.20210815 --- 3.5M ghc-8.10.6/cabal-clean-0.1.20210924 +++ 3.6M ghc-8.10.7/cabal-clean-0.1.20210924 +++ 3.7M ghc-9.0.1 /cabal-clean-0.1.20210924 The directories prefixed with --- will be deleted, the ones with +++ retained. Disclaimers: 1. cabal-clean does not check whether deletion will cause dangling references like broken symlinks etc. (Could happen if you created a symlink to an executable that resides in one of the deleted build directories.) 2. cabal-clean does not read nor utilize information from the .cabal file. 3. Calls `du` ("disk usage"), so, might not work on Windows. Enjoy, Andreas

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Re: [Haskell-cafe] Should there be a haskell template for inclusion polymorphism in Haskell?
by Olaf Klinke 05 Jun '22

05 Jun '22

> So my central questions for discussions are: > > - Is inclusion polymorphism something we should use at all in Haskell? > - These boiler plate code maybe better be generated using Haskell template > instead, is there one already, or should > there be one? Maybe one of the authors of a sub-typing library can share a more informed opinion on this. My view as a mathematician: A type being a subtype of another is a relation between types. Relations between types is what multi-parameter type classes is about. Hence it seems that this is how subtypes in Haskell ought to be done. For example, in the attenuation and records-sop package, the latter of which is based on genercis-sop, which depends on Template Haskell. So without having used any of the mentioned packages, I believe your second question can be answered with "yes". Regarding the first question, we should probably ask ourselves how much type-safety we're sacrificing for the convenience we gain. The Num example you gave shows one possible danger: While a subtype relation may hold mathematically, the implementiation details may cause this relation to fail in reality. Example: All floating point numbers are rationals, aren't they? So there should be a subtype relation. But how to convert NaN, -infinity or -0 of type Double to a pair of Integers? Olaf

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Hiding type variables from the signature?
by Ignat Insarov 02 Jun '22

02 Jun '22

Hello café, hope you all doing well! I have this code [1]. Its purpose is to provide an `fmap` that works at any depth. Here is the gist of the gist: ```Haskell fmapz ∷ forall seed fruit seed' fruit' peels recursive. Squeezish seed seed' fruit fruit' peels recursive ⇒ (seed → seed') → fruit → fruit' fmapz function = stretch . fmap @(Squeezed (Strip seed fruit)) function . squeeze ``` As the examples show, it works, and even has some type inference. However, the type signature is unwieldy. It seems like it should have only 4 variables, but I have to also do some type level book-keeping for which the other 2 variables are needed. These book-keeping type variables are determined by type families from the other 4, so surely I can hide them inside a type synonym or something like that? I tried giving a quantified constraint, like so: ```Haskell type Squeezish ∷ ★ → ★ → ★ → ★ → Constraint type Squeezish seed seed' fruit fruit' = forall peels recursive. Squeezish' seed seed' fruit fruit' peels recursive class Squeezish' seed seed' fruit fruit' (peels ∷ [★ → ★]) (recursive ∷ Bool) instance ( peels ~ Strip seed fruit , ForAll peels Functor , Squeezy seed' fruit' peels recursive , Squeezy seed fruit peels recursive ) ⇒ Squeezish' seed seed' fruit fruit' peels recursive ``` — But then the constraints from the context of `Squeezish'` fail to make it to the use site and numerous errors arise. What can I do to solve this problem? Or is it theoretically impossible? [1]: https://gist.github.com/kindaro/6056f753bcf356ced96b817fee72533c

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ML Family Workshop 2022: DEADLINE EXTENSION
by Benoit Montagu 31 May '22

31 May '22

To increase your chances of submitting your work to the ML workshop, *the submission deadline is extended by a week*. The new deadline is Friday 10th June (AoE). A quick reminder: - The workshop does not have proceedings, making it the perfect venue to run some ideas with the community or present some work in progress within a friendly environment. - The work load as an author is low: submissions are only 3 pages long (excluding references) - YOU have the power to make the ML workshop a success! - You have one more full week to submit to <https://ml2022.hotcrp.com/> (please register your submission early!) - All the details are here: <https://icfp22.sigplan.org/home/mlfamilyworkshop-2022#Call-for-Presentations> - The ML workshop is colocated with ICFP 2022 <https://icfp22.sigplan.org/> -- Benoît Montagu, chair of the ML workshop 2022

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Haskell Symposium 2022: Abstract Deadline Extended to June 1st
by Nadia Polikarpova 29 May '22

29 May '22

The Haskell'22 abstract deadline has been extended to *Wednesday, June 1st*. Please register your submissions by this date! The full submission deadline is still *Friday, June 3rd*. Please find the updated CFP below. Nadia Polikarpova (Haskell'22 chair) ========================================================================= ACM SIGPLAN CALL FOR SUBMISSIONS Haskell Symposium 2022 Ljubljana, Slovenia Thu 15 -- Fri 16 September, 2022 http://www.haskell.org/haskell-symposium/2022/ ========================================================================= The ACM SIGPLAN Haskell Symposium 2022 will be co-located with the 2022 International Conference on Functional Programming (ICFP). Differently from previous years, Haskell'22 will use a single-track submission process (that is, we will only have the regular track and no early track). 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. Like an experience report and a functional pearl, tutorials should make 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 ================== 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 2022 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 --------- Abstract submission: 1 June 2022 (Wed) Paper submission: 3 June 2022 (Fri) Notification: 1 July 2022 (Fri) Deadlines are 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://haskell22.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 ================= Lennart Augustsson Epic Games Jean-Philippe Bernardy University of Gothenburg Iavor Diatchki Galois, Inc Michael Gale Tweag William Hallahan Yale University Makoto Hamana Gumma University Andrew Hirsch MPI SWS John Hughes Chalmers University of Technology Alexis King Tweag James Koppel Massachusets Institute of Technology Rudy Matela Channable Dominic Orchard University of Kent Nadia Polikarpova (chair) University of California, San Diego Eric Seidel Bridgewater Associates Satnam Singh Groq Wouter Swierstra Utrecht University Hiroshi Unno University of Tsukuba Marco Vassena Utrecht University Dimitrios Vytiniotis DeepMind If you have questions, please contact the chair at: npolikarpova(a)ucsd.edu =========================================================================

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Research fellow position at University of Bristol
by Meng Wang 28 May '22

28 May '22

Dear Haskellers, The programming languages group at Bristol is recruiting a research fellow to join our dynamic research group https://bristolpl.github.io/. Our research is ranked highly internationally and there is a strong focus on Haskell. We welcome functional programmers anywhere in the world to apply! (Another similar post at the level of research associate is also available.) https://www.bristol.ac.uk/jobs/find/details/?jobId=274355&jobTitle=Research… Meng Wang, PhD (Oxon) Senior Lecturer of Programming Languages Head of PL research group School International Director SCEEM (CS, EEE, EMath) School, University of Bristol PS. A selection of recent papers can be found below. They should give a good indication of the type of PL research conducted at Bristol: - Perera et al. (2022). Linked visualisations via Galois dependencies. POPL 2022. 10.1145/3498668 - Xie et al. (2022). Staging with Class. POPL 2022. 10.1145/3498723 - Yamaguchi et al. (2021). Synbit: Synthesizing Bidirectional Programs using Unidirectional Sketches. OOPSLA 2021. 10.1145/3485482 - Qian Z et al. (2021). Client-Server Sessions in Linear Logic. ICFP 2021. 10.1145/3473567 - Jones E & Ramsay S. (2021). Intensional Refinement Datatypes. POPL 2021. 10.1145/3445980 - Gratzer D et al. (2020). Multimodal Dependent Type Theory. LICS 2020. 10.1145/3373718.3394736 - Matsuda K & Wang M. (2020). Sparcl: A Language for Partially-Invertible Computation. ICFP 2020. 10.1145/3409000 - Zhang J et al. (2019). A Study of Bug Resolution Characteristics in Popular Programming Languages. IEEE Transactions on Software Engineering. 10.1109/TSE.2019.2961897 - Abate A et al. (2019). Automated Formal Synthesis of Provably Safe Digital Controllers for Continuous Plants. Acta Informatica, vol 57. 10.1007/s00236-019-00359-1 - Almeida et al. (2019). Machine-Checked Proofs for Cryptographic Standards: Indifferentiability of Sponge and Secure High-Assurance Implementations of SHA-3. CCS 2019. 10.1145/3319535.336321

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Re: [Haskell-cafe] [Haskell] [ANNOUNCE] GHC 9.2.3 released
by Shayne Fletcher 27 May '22

27 May '22

there are already tickets covering this see https://gitlab.haskell.org/ghc/ghc/-/issues/21633 & -- https://gitlab.haskell.org/ghc/ghc/-/issues/21634. i have found that `allow-newer:true` and `resolver: nightly-2022-05-27` in `hadrian/stack.yaml` provides a workaround for the moment. On Fri, May 27, 2022 at 7:08 PM George Colpitts <george.colpitts(a)gmail.com> wrote: > Hi > > https://gitlab.haskell.org/ghc/ghc/-/issues/21506, ghc-9.4.1-alpha1 does > not install on macos, exists on 9.2.3 also. The workaround given in 21506 > works. Do you want a ticket for this? > > Thanks > George > > > On Fri, May 27, 2022 at 4:41 PM Zubin Duggal <zubin(a)well-typed.com> wrote: > >> >> The GHC developers are very happy to at announce the availability of GHC >> 9.2.3. Binary distributions, source distributions, and documentation are >> available at [`downloads.haskell.org`]( >> https://downloads.haskell.org/ghc/9.2.3) >> >> Download Page: https://www.haskell.org/ghc/download_ghc_9_2_3.html >> Blog Post: >> https://www.haskell.org/ghc/blog/20220527-ghc-9.2.3-released.html >> >> This release includes many bug-fixes and other improvements to 9.2.2 >> including: >> >> * A fix to a critical linking bug on Windows causing the 9.2.2 release >> to be unusable (#21196). >> >> * Numerous bug fixes for regressions and other issues in the >> typechecker (#2147, #21515, #20582, #20820, #21130, #21479). >> >> * Correctness bugs in the code generator and compiler backends (#21396, >> #20959, #21465). >> >> * Packaging fixes on many platforms (#20760, #21487, #21402). >> >> * Many others. See the [release notes][] for a full list. >> >> As some of the fixed issues do affect correctness users are encouraged >> to upgrade promptly. >> >> We would like to thank Microsoft Azure, GitHub, IOG, the Zw3rk stake >> pool, Tweag I/O, Serokell, Equinix, SimSpace, and other anonymous >> contributors whose on-going financial and in-kind support has >> facilitated GHC maintenance and release management over the years. >> Finally, this release would not have been possible without the hundreds >> of open-source contributors whose work comprise this release. >> >> As always, do give this release a try and open a [ticket] if you see >> anything amiss. >> >> Happy compiling, >> >> - Zubin >> >> >> [ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new >> [release notes]: >> https://downloads.haskell.org/ghc/9.2.3/docs/html/users_guide/9.2.3-notes.h… >> _______________________________________________ >> ghc-devs mailing list >> ghc-devs(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >> > _______________________________________________ > ghc-devs mailing list > ghc-devs(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > -- Shayne Fletcher

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[Haskell] [ANNOUNCE] GHC 9.2.3 released
by Zubin Duggal 27 May '22

27 May '22

The GHC developers are very happy to at announce the availability of GHC 9.2.3. Binary distributions, source distributions, and documentation are available at [`downloads.haskell.org`](https://downloads.haskell.org/ghc/9.2.3). Download Page: https://www.haskell.org/ghc/download_ghc_9_2_3.html Blog Post: https://www.haskell.org/ghc/blog/20220527-ghc-9.2.3-released.html This release includes many bug-fixes and other improvements to 9.2.2 including: * A fix to a critical linking bug on Windows causing the 9.2.2 release to be unusable (#21196). * Numerous bug fixes for regressions and other issues in the typechecker (#2147, #21515, #20582, #20820, #21130, #21479). * Correctness bugs in the code generator and compiler backends (#21396, #20959, #21465). * Packaging fixes on many platforms (#20760, #21487, #21402). * Many others. See the [release notes][] for a full list. As some of the fixed issues do affect correctness users are encouraged to upgrade promptly. We would like to thank Microsoft Azure, GitHub, IOG, the Zw3rk stake pool, Tweag I/O, Serokell, Equinix, SimSpace, and other anonymous contributors whose on-going financial and in-kind support has facilitated GHC maintenance and release management over the years. Finally, this release would not have been possible without the hundreds of open-source contributors whose work comprise this release. As always, do give this release a try and open a [ticket] if you see anything amiss. Happy compiling, - Zubin [ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new [release notes]: https://downloads.haskell.org/ghc/9.2.3/docs/html/users_guide/9.2.3-notes.h…

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GHC.X.Hackage and GHC's release cycle
by Ben Gamari 27 May '22

27 May '22

Hello all, Recently in the Haskell Foundation's stability working group we have been discussing various practical issues that Haskell users and prospective adopters encounter around the ecosystem. During these discussions the topic of GHC's release schedule and the ecosystem's process migration to new GHC releases has come up repeatedly. To address some of these issues, I have opened a pair of somewhat-related HF Tech Proposals: * the GHC.X.Hackage proposal ([ghc.x.hackage]) seeks to extend the [head.hackage] infrastructure used to test GHC to enable use by library maintainers in porting and testing their libraries on new GHC releases. * the [tick-tock] release proposal seeks to provide better guidance to users needing long release lifecycles by designating every other release as a "long-term support" release, with 18-month backport window. Given the wide applicability of these proposals we hope to gather feedback from a broad swath of the community. If you have questions, opinions, or concerns on either, please leave your feedback on the respective proposal. We look forward to hearing from you. Thanks! - Ben [ghc.x.hackage]: https://github.com/haskellfoundation/tech-proposals/pull/27 [head.hackage]: https://ghc.gitlab.haskell.org/head.hackage/ [tick-tock]: https://github.com/haskellfoundation/tech-proposals/pull/34

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Should there be a haskell template for inclusion polymorphism in Haskell?
by Miao ZhiCheng 27 May '22

27 May '22

Hi Haskellers! Here are some of my thoughts about polymorphism in Haskell especially the inclusion polymorphism, with questions in the end. ~IGNORE THESE LHS BOILER PLATES~ > {-# LANGUAGE GADTs #-} > module Main where > import Unsafe.Coerce ( unsafeCoerce ) ~IGNORE THESE LHS BOILER PLATES~ * Classification of Polymorphism Let's assume that we are following the classification of polymorphism where there are four types of it: ?? Btw, does anyone have a good citation of this classification? I got it from all over the Internet instead: e.g https://www.tutorialspoint.com/types-of-polymorphisms-ad-hoc-inclusion-para… ** Overloading Polymorphism It allows function with same name to act in different manner for different types. ** Coercion Polymorphism Also called as casting sometimes. ** Inclusion Polymorphism Also called as subtyping. This allows to point derived classes using base class pointers and references. ** Parametric Polymorphism Also called early Binding, it allows to use same piece of code for different types. We can get it by using templates. ** Sub Classifications In terms of being ad-hoc or universal: - Overloading and Coercion are ad-hoc, - Inclusion and Parametric are universal. In terms of being compile-time vs run-time: - Coercion, Overloading, Parametric are all compile-time polymorphism, - while only Inclusion is run-time polymorphism. * Different Polymorphism in Haskell In terms of their counter parties in Haskell language: ** Overloading Polymorphismin Haskell I think type classes basically provides such overloading ad-hoc polymorphism. Some thinks so too: https://stackoverflow.com/questions/6636107/how-does-haskell-handle-overloa… > class Fooable a where > foo :: a -> Int > instance Fooable Int where > foo = id > instance Fooable Bool where > foo _ = 42 ** Coercion Polymorphism in Haskell I think Haskell has both `Coercible` & `unsafeCoerce` for handling representational equality in compile time. But I am not sure if c/c++ style of conercion between similar types such as int/float/double is something Haskell would want to inject these magic conversion code ever. ** Parametric Polymorphism in Haskell This is probably what Haskell is best at the most, demonstrated by the classic `map` function definition: > map' :: (a -> b) -> [a] -> [b] > map' _ [] = [] > map' f (x:xs) = f x : map' f xs Note that, one could also combine parametric and overloading together using constraints: > double :: Num a => a -> a > double x = x + x This `double` function is parametric, but constrained only to work with Num type classes, hence their implementations are overloaded. ** Inclusion Polymorphism in Haskell Ok, finally inclusion polymorphism is what I want to focus on and have some questions here. Before that, I do want to mention one observation, inclusion polymorphism probably is most likely the old OOP habits that die hard. For me at least, since I consider myself a recovering OOP run-time polymorphism addict, and my guess is that unless the use case do need run-time polymorphism, e.g. run-time execution layer abstration, it is often better off using other type of polymorphism to achieve the same result. And even Bjarne Stroustrup is flirting with the idea: [[https://www.youtube.com/watch?v=xcpSLRpOMJM][Bjarne Stroustrup - Object Oriented Programming without Inheritance - ECOOP 2015]] Let's think of translating this piece of pseudo OOP style code into haskell. ``` interface Num { Num (+)(Num a, Num b); Num (*)(Num a, Num b); Num abs(Num a); Num negate(Num a); Num signum(Num a); Num fromInteger(Integer); } interface Show { String show(Show a); } class AnyNum<T> implements Num, Show { // trivially implements Num Show interfaces } // Now we can use AnyNum<Int>, AnyNum<Double>, etc. through their Num or Show interfaces. ``` In Haskell though, thanks to the ability of having existential type, AnyNum is actually quite nice to write: > data AnyNum where > MkAnyNum :: (Num a, Show a) => a -> AnyNum But in order to use AnyNum like using interfaces in our pseudo code, we would have to write these boiler plates and in some cases using unsafeCoerce due to not using Typeable run-time information: > instance Num AnyNum where > (+) (MkAnyNum a) (MkAnyNum b) = MkAnyNum $ a + unsafeCoerce b > (*) (MkAnyNum a) (MkAnyNum b) = MkAnyNum $ a + unsafeCoerce b > abs (MkAnyNum a) = MkAnyNum . abs $ a > negate (MkAnyNum a) = MkAnyNum . negate $ a > signum (MkAnyNum a) = MkAnyNum . signum $ a > fromInteger = MkAnyNum . fromInteger > instance Show AnyNum where > show (MkAnyNum a) = show a Here is some test code that demonstrate how distressful unsafeCoerce is: > main = do > let a = MkAnyNum(1 :: Int) > let b = MkAnyNum(2 :: Double) > let c = MkAnyNum(3 :: Int) > print $ show $ a + b > print $ show $ a + c Outputs are: ``` λ> "4611686018427387905" "4" ``` Few observations: - If there is at most one occurance of the usage of the type class, there is no need for the unsafeCoerce. - Otherwise, unsafeCoerce is required, or otherwise those functions could be left to "undefined". So my central questions for discussions are: - Is inclusion polymorphism something we should use at all in Haskell? - These boiler plate code maybe better be generated using Haskell template instead, is there one already, or should there be one? Cheers, Miao

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