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Call for Papers: PACMPL issue ICFP 2021
by Sam Tobin-Hochstadt 02 Feb '21

02 Feb '21

PACMPL Volume 5, Issue ICFP 2021 Call for Papers Accepted papers to be invited for presentation at The 26th ACM SIGPLAN International Conference on Functional Programming To Be Held Virtualy http://icfp21.sigplan.org/ ### Important dates Submissions due: 2 March 2021 (Tuesday) Anywhere on Earth https://icfp21.hotcrp.com Author response: 20 April (Tuesday) - 23 April (Friday) 17:00 UTC Notification: 7 May (Friday) Final copy due: 30 June (Wednesday) Conference: 22 August (Sunday) - 27 August (Friday) ### About PACMPL Proceedings of the ACM on Programming Languages (PACMPL <https://pacmpl.acm.org/>) is a Gold Open Access journal publishing research on all aspects of programming languages, from design to implementation and from mathematical formalisms to empirical studies. Each issue of the journal is devoted to a particular subject area within programming languages and will be announced through publicized Calls for Papers, like this one. ### Scope [PACMPL](https://pacmpl.acm.org/) issue ICFP 2021 seeks original papers on the art and science of functional programming. Submissions are invited on all topics from principles to practice, from foundations to features, and from abstraction to application. The scope includes all languages that encourage functional programming, including both purely applicative and imperative languages, as well as languages with objects, concurrency, or parallelism. Topics of interest include (but are not limited to): * Language Design: concurrency, parallelism, and distribution; modules; components and composition; metaprogramming; macros; pattern matching; type systems; type inference; dependent types; session types; gradual typing; refinement types; interoperability; domain-specific languages; imperative programming; object-oriented programming; logic programming; probabilistic programming; reactive programming; generic programming; bidirectional programming. * Implementation: abstract machines; virtual machines; interpretation; compilation; compile-time and run-time optimization; garbage collection and memory management; runtime systems; multi-threading; exploiting parallel hardware; interfaces to foreign functions, services, components, or low-level machine resources. * Software-Development Techniques: algorithms and data structures; design patterns; specification; verification; validation; proof assistants; debugging; testing; tracing; profiling; build systems; program synthesis. * Foundations: formal semantics; lambda calculus; program equivalence; rewriting; type theory; logic; category theory; monads; continuations; control; state; effects; names and binding; program verification. * Analysis and Transformation: control flow; data flow; abstract interpretation; partial evaluation; program calculation. * Applications: symbolic computing; formal-methods tools; artificial intelligence; systems programming; distributed systems and web programming; hardware design; databases; XML processing; scientific and numerical computing; graphical user interfaces; graphics and multimedia; GPU programming; scripting; system administration; security. * Education: teaching introductory programming; parallel programming; mathematical proof; algebra. Submissions will be evaluated according to their relevance, correctness, significance, originality, and clarity. Each submission should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. The technical content should be accessible to a broad audience. PACMPL issue ICFP 2021 also welcomes submissions in two separate categories — Functional Pearls and Experience Reports — that must be marked as such when submitted and that need not report original research results. Detailed guidelines on both categories are given at the end of this call. Please contact the associate editor if you have questions or are concerned about the appropriateness of a topic. ### Preparation of submissions **Deadline**: The deadline for submissions is **Tuesday, March 2, 2021**, Anywhere on Earth (<https://www.timeanddate.com/time/zones/aoe>). This deadline will be strictly enforced. **Formatting**: Submissions must be in PDF format, printable in black and white on US Letter sized paper, and interpretable by common PDF tools. All submissions must adhere to the "ACM Small" template that is available (in both LaTeX and Word formats) from <https://www.acm.org/publications/authors/submissions>. There is a limit of **25 pages for a full paper or Functional Pearl** and **12 pages for an Experience Report**; in either case, the bibliography will not be counted against these limits. Submissions that exceed the page limits or, for other reasons, do not meet the requirements for formatting, will be summarily rejected. Supplementary material can and should be **separately** submitted (see below). See also PACMPL's Information and Guidelines for Authors at <https://pacmpl.acm.org/authors.cfm>. **Submission**: Submissions will be accepted at <https://icfp21.hotcrp.com/> Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. **Author Response Period**: Authors will have a 72-hour period, starting at 17:00 UTC on **Tuesday, April 20, 2021**, to read reviews and respond to them. **Supplementary Material**: Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. This supplementary material should **not** be submitted as part of the main document; instead, it should be uploaded as a **separate** PDF document or tarball. Supplementary material should be uploaded **at submission time**, not by providing a URL in the paper that points to an external repository. Authors are free to upload both anonymized and non-anonymized supplementary material. Anonymized supplementary material will be visible to reviewers immediately; non-anonymized supplementary material will be revealed to reviewers only after they have submitted their review of the paper and learned the identity of the author(s). **Authorship Policies**: All submissions are expected to comply with the ACM Policies for Authorship that are detailed at <https://www.acm.org/publications/authors/information-for-authors>. **Republication Policies**: Each submission must adhere to SIGPLAN's republication policy, as explained on the web at <http://www.sigplan.org/Resources/Policies/Republication>. ### Review Process This section outlines the two-stage process with lightweight double-blind reviewing that will be used to select papers for PACMPL issue ICFP 2021. We anticipate that there will be a need to clarify and expand on this process, and we will maintain a list of frequently asked questions and answers on the PACMPL issue website to address common concerns. **PACMPL issue ICFP 2021 will employ a two-stage review process.** The first stage in the review process will assess submitted papers using the criteria stated above and will allow for feedback and input on initial reviews through the author response period mentioned previously. As a result of the review process, a set of papers will be conditionally accepted and all other papers will be rejected. Authors will be notified of these decisions on **May 7, 2021**. Authors of conditionally accepted papers will be provided with committee reviews along with a set of mandatory revisions. After four weeks (June 4, 2021), the authors will provide a second submission. The second and final reviewing phase assesses whether the mandatory revisions have been adequately addressed by the authors and thereby determines the final accept/reject status of the paper. The intent and expectation is that the mandatory revisions can be addressed within four weeks and hence that conditionally accepted papers will in general be accepted in the second phase. The second submission should clearly identify how the mandatory revisions were addressed. To that end, the second submission must be accompanied by a cover letter mapping each mandatory revision request to specific parts of the paper. The cover letter will facilitate a quick second review, allowing for confirmation of final acceptance within two weeks. Conversely, the absence of a cover letter will be grounds for the paper’s rejection. **PACMPL issue ICFP 2021 will employ a lightweight double-blind reviewing process.** To facilitate this, submitted papers must adhere to two rules: 1. **author names and institutions must be omitted**, and 2. **references to authors' own related work should be in the third person** (e.g., not "We build on our previous work ..." but rather "We build on the work of ..."). The purpose of this process is to help the reviewers come to an initial judgement about the paper without bias, not to make it impossible for them to discover the authors if they were to try. Nothing should be done in the name of anonymity that weakens the submission or makes the job of reviewing the paper more difficult (e.g., important background references should not be omitted or anonymized). In addition, authors should feel free to disseminate their ideas or draft versions of their paper as they normally would. For instance, authors may post drafts of their papers on the web or give talks on their research ideas. ### Information for Authors of Accepted Papers * As a condition of acceptance, final versions of all papers must adhere to the new ACM Small format. The page limit for the final versions of papers will be increased by two pages to help authors respond to reviewer comments and mandatory revisions: **27 pages plus bibliography for a regular paper or Functional Pearl, 14 pages plus bibliography for an Experience Report**. * Authors of accepted submissions will be required to agree to one of the three ACM licensing options: open access on payment of a fee (**recommended**, and SIGPLAN can cover the cost as described next); copyright transfer to ACM; or retaining copyright but granting ACM exclusive publication rights. Further information about ACM author rights is available from <http://authors.acm.org>. * PACMPL is a Gold Open Access journal, and authors are encouraged to publish their work under a CC-BY license. Gold Open Access guarantees permanent free online access to the definitive version in the ACM Digital Library, and the recommended CC-BY option also allows anyone to copy and distribute the work with attribution. Gold Open Access has been made possible by generous funding through ACM SIGPLAN, which will cover all open access costs in the event authors cannot. Authors who can cover the costs may do so by paying an Article Processing Charge (APC). PACMPL, SIGPLAN, and ACM Headquarters are committed to exploring routes to making Gold Open Access publication both affordable and sustainable. * ACM offers authors a range of copyright options, one of which is Creative Commons CC-BY publication; this is the option recommended by the PACMPL editorial board. A reasoned argument in favour of this option can be found in the article [Why CC-BY?](https://oaspa.org/why-cc-by/) published by OASPA, the Open Access Scholarly Publishers Association. * ACM Author-Izer is a unique service that enables ACM authors to generate and post links on either their home page or institutional repository for visitors to download the definitive version of their articles from the ACM Digital Library at no charge. Downloads through Author-Izer links are captured in official ACM statistics, improving the accuracy of usage and impact measurements. Consistently linking to the definitive version of an ACM article should reduce user confusion over article versioning. After an article has been published and assigned to the appropriate ACM Author Profile pages, authors should visit <http://www.acm.org/publications/acm-author-izer-service> to learn how to create links for free downloads from the ACM DL. * The official publication date is the date the papers are made available in the ACM Digital Library. This date may be up to *two weeks prior* to the first day of the conference. The official publication date affects the deadline for any patent filings related to published work. * Authors of each accepted submission are invited to attend and be available for the presentation of that paper at the conference. The schedule for presentations will be determined and shared with authors after the full program has been selected. ### Artifact Evaluation Authors of papers that are conditionally accepted in the first phase of the review process will be encouraged (but not required) to submit supporting materials for Artifact Evaluation. These items will then be reviewed by an Artifact Evaluation Committee, separate from the paper Review Committee, whose task is to assess how the artifacts support the work described in the associated paper. Papers that go through the Artifact Evaluation process successfully will receive a seal of approval printed on the papers themselves. Authors of accepted papers will be encouraged to make the supporting materials publicly available upon publication of the papers, for example, by including them as "source materials" in the ACM Digital Library. An additional seal will mark papers whose artifacts are made available, as outlined in the ACM guidelines for artifact badging. Participation in Artifact Evaluation is voluntary and will not influence the final decision regarding paper acceptance. ### Special categories of papers In addition to research papers, PACMPL issue ICFP solicits two kinds of papers that do not require original research contributions: Functional Pearls, which are full papers, and Experience Reports, which are limited to half the length of a full paper. Authors submitting such papers should consider the following guidelines. #### Functional Pearls A Functional Pearl is an elegant essay about something related to functional programming. Examples include, but are not limited to: * a new and thought-provoking way of looking at an old idea * an instructive example of program calculation or proof * a nifty presentation of an old or new data structure * an interesting application of functional programming techniques * a novel use or exposition of functional programming in the classroom While pearls often demonstrate an idea through the development of a short program, there is no requirement or expectation that they do so. Thus, they encompass the notions of theoretical and educational pearls. Functional Pearls are valued as highly and judged as rigorously as ordinary papers, but using somewhat different criteria. In particular, a pearl is not required to report original research, but, it should be concise, instructive, and entertaining. A pearl is likely to be rejected if its readers get bored, if the material gets too complicated, if too much specialized knowledge is needed, or if the writing is inelegant. The key to writing a good pearl is polishing. A submission that is intended to be treated as a pearl must be marked as such on the submission web page, and should contain the words "Functional Pearl" somewhere in its title or subtitle. These steps will alert reviewers to use the appropriate evaluation criteria. Pearls will be combined with ordinary papers, however, for the purpose of computing the conference's acceptance rate. #### Experience Reports The purpose of an Experience Report is to help create a body of published, refereed, citable evidence that functional programming really works -- or to describe what obstacles prevent it from working. Possible topics for an Experience Report include, but are not limited to: * insights gained from real-world projects using functional programming * comparison of functional programming with conventional programming in the context of an industrial project or a university curriculum * project-management, business, or legal issues encountered when using functional programming in a real-world project * curricular issues encountered when using functional programming in education * real-world constraints that created special challenges for an implementation of a functional language or for functional programming in general An Experience Report is distinguished from a normal PACMPL issue ICFP paper by its title, by its length, and by the criteria used to evaluate it. * Both in the papers and in any citations, the title of each accepted Experience Report must end with the words "(Experience Report)" in parentheses. The acceptance rate for Experience Reports will be computed and reported separately from the rate for ordinary papers. * Experience Report submissions can be at most 12 pages long, excluding bibliography. * Each Experience Report accepted to the PACMPL issue will be presented at the conference, but depending on the number of Experience Reports and regular papers accepted, authors of Experience reports may be asked to give shorter talks. * Because the purpose of Experience Reports is to enable our community to accumulate a body of evidence about the efficacy of functional programming, an acceptable Experience Report need not add to the body of knowledge of the functional-programming community by presenting novel results or conclusions. It is sufficient if the Report states a clear thesis and provides supporting evidence. The thesis must be relevant to the PACMPL issue, but it need not be novel. The review committee will accept or reject Experience Reports based on whether they judge the evidence to be convincing. Anecdotal evidence will be acceptable provided it is well argued and the author explains what efforts were made to gather as much evidence as possible. Typically, more convincing evidence is obtained from papers which show how functional programming was used than from papers which only say that functional programming was used. The most convincing evidence often includes comparisons of situations before and after the introduction or discontinuation of functional programming. Evidence drawn from a single person's experience may be sufficient, but more weight will be given to evidence drawn from the experience of groups of people. An Experience Report should be short and to the point: it should make a claim about how well functional programming worked on a particular project and why, and produce evidence to substantiate this claim. If functional programming worked in this case in the same ways it has worked for others, the paper need only summarize the results; the main part of the paper should discuss how well it worked and in what context. Most readers will not want to know all the details of the project and its implementation, but the paper should characterize the project and its context well enough so that readers can judge to what degree this experience is relevant to their own projects. The paper should take care to highlight any unusual aspects of the project. Specifics about the project are more valuable than generalities about functional programming; for example, it is more valuable to say that the team delivered its software a month ahead of schedule than it is to say that functional programming made the team more productive. If the paper not only describes experience but also presents new technical results, or if the experience refutes cherished beliefs of the functional-programming community, it may be better to submit it as a full paper, which will be judged by the usual criteria of novelty, originality, and relevance. The associate editor will be happy to advise on any concerns about which category to submit to. ### ICFP Organizers General Chair: Sukyoung Ryu (KAIST, South Korea) Accessibility Co-Chairs: Lindsey Kuper (UC Santa Cruz, USA), Kathrin Stark (Princeton University, USA) Artifact Evaluation Co-Chairs: Gabriel Scherer (INRIA Saclay, France), Brent Yorgey (Hendrix College, USA) Industrial Relations Co-Chairs: Alan Jeffrey (Roblox, USA), Simon Marlow (Facebook, England) Programming Contest Co-Organisers: Alex Lang, Jasper Van der Jeugt (Fugue, Switzerland) Publicity and Web Chair: Sam Tobin-Hochstadt (Indiana University, USA) Student Research Competition Chair: Anders Miltner (University of Texas, USA) Student Volunteer Co-Chairs: Lily Bryant (University of British Columbia, Canada), Jaemin Hong (KAIST, South Korea), Hanneli Tavante (McGill University, Canada) Video Co-Chairs: Leif Andersen (Northeastern University, USA), Benjamin Chung (Northeastern University, USA) Workshops Co-Chairs: Leonidas Lampropoulos (University of Maryland, USA), Zoe Paraskevopoulou (Princeton University, USA) ### PACMPL Volume 5, Issue ICFP 2021 Associate Editor: Ronald Garcia, (University of British Columbia, Canada) Review Committee: Zena Ariola (University of Oregon, USA) Stephanie Balzer (Carnegie Mellon University, USA) Matteo Cimini (UMass Lowell, USA) Youyou Cong (Tokyo Institute of Technology, Japan) Harley Eades (University of Augusta, USA) Andrew Gordon (Microsoft Research & University of Edinburgh, United Kingdom) Benjamin Greenman (Brown University, USA) Arjun Guha (Northeastern University, USA) Jurriaan Hage (Utrecht, The Netherlands) Favonia (University of Minnesota, USA) Suresh Jagannathan (Purdue University, USA) Patricia Johann (Appalachian State University, USA) Ralf Jung (Max Planck Institute, Germany) Ekaterina Komendantskaya (Heriot-Watt, Scotland) Leonidas Lampropoulos (University of Maryland, USA) Kazutaka Matsuda (Tohoku University, Japan) Akimasa Morihata (Univeristy of Tokyo, Japan) Stefan Muller (Illinois Institute of Technology, USA) Max New (Wesleyan University, USA) Rishiyur Nikhil (Bluespec , USA) Cyrus Omar (University of Michigan, USA) Brigitte Pientka (McGill University, Canada) Norman Ramsey (Tufts University, USA) Christine Rizkallah (University of New South Wales, Australia) Taro Sekiyama (National Institute for Informatics, Japan) Eijiro Sumii (Tohoku University, Japan) Amin Timany (Aarhus University, Denmark) Mitchell Wand (Northeastern University, USA) Steve Zdancewic (University of Pennsylvania, USA)

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PhD position at IMDEA Software Institute
by Niki Vazou 02 Feb '21

02 Feb '21

Hey Haskellers, Applications are invited for a PhD position at the IMDEA Software Institute, Madrid, Spain. The selected candidate will work under my supervision on the theory and practise of refinement types. Application deadline is March 15. More information are available here: https://software.imdea.org/open_positions/2021-02-phd-refinements.html Best, Niki Vazou

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Project builds with stack, not with cabal. How to diagnose?
by Han Joosten 01 Feb '21

01 Feb '21

Dear Café, I have been building my project for quite some time now using Stack. That works fine. However, I also want to make sure cabal users can build it from source. Recently I have been trying to set this up using github actions. I use a matrix to build both with stack and with cabal. I use the same version of ghc. A recent build can be seen here: https://github.com/AmpersandTarski/Ampersand/runs/1756887612 I guess that the error is due to the fact that the used packages to build are somehow different between the stack build and the cabal build. Is there some way to find out what those differences are? Any help would be appreciated! Thanks Han Joosten

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PSA: IRC #haskell-ide-engine has moved to #haskell-language-server
by Alan & Kim Zimmerman 29 Jan '21

29 Jan '21

Given haskell-ide-engine is retired, we have moved the IRC channel to something more appropriate. Alan

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Compilation of large data
by Dušan Kolář 29 Jan '21

29 Jan '21

Dear Café, How can I compile large data in the source code? The data structure contains a few numerical and String constants and a long list of structures (containing two short lists of Strings). If I use ghci, it's fine, no problem (until some size). When I want to compile then I run out of memory (32GB, plus 32GB swap) on arch linux. Yes, machine with 256GB solved temporarily the problem, but only temporarily. The data is generated (about 10.000 elements for the list and more). Should it be read from the file during the runtime? Should the structure be somehow decoupled (to small lists, each of them in a separate file)? Or there is no way, how to make it work... :-( If I use different input, a bit smaller, which I can compile, the resulting binary is rather small, less than 7MB. Regards Dušan

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Take over maintainership binary search
by Jappie Klooster 28 Jan '21

28 Jan '21

Hi, I wish to take over the maintainership of the binary search package. The company I work for (riskbook) currently uses binary search. However the current version doesn't build from hackage because ghc changed how setting files are used, so I made a PR simply deleting that: https://github.com/nushio3/binary-search/pull/3 That has been several months ago and now I'm running against the same issue. There hasn't been a from the author (nushio3), and the email address CC'ed in here are full according to gmail. With regards, Jappie Klooster Riskbook | Software engineer e jappie(a)riskbook.com p +31644237437 Riskbook is a Limited Company (Company No. 10475714) registered at 1 Adamson Drive, Horsehay, Telford, TF4 3UJ, United Kingdom. All contents of this email should be treated as private and confidential unless otherwise stated.

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explicit type for "local" functions
by Jean-Marc Alliot 27 Jan '21

27 Jan '21

Just a short and probably simple question. Let's consider the following function: collatz n = let coll (n,cpt) | n==1 = (1,cpt) | even n = coll (div n 2,cpt+1) | odd n = coll (div (n*3+1) 2,cpt+1) in snd (coll (n,0)) It is easy to set explicitly the type of "collatz" by writing before the function definition: collatz :: Integer->Int But can I set explicitly the type of the "local" function "coll" ? Thanks

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Re: [Haskell-cafe] A quick question about distribution of finite maps.
by Olaf Klinke 26 Jan '21

26 Jan '21

> I find the idea that finite sets and functions may be > represented in Haskell very attractive in general Okay, so the Map was a red herring. It is only one possible implementation of the mathematical concept you are after. To be more clear, let's use Map with capital M whenever we mean the data structure and "map" or "function" whenever we mean the mathematical concept. What you really want is functions between finite sets. Maps do not, in general, represent functions in the Control.Category sense. That got me hooked onto the Kleisli thing. Classical case of nerd-sniping. But you want to deviate from Control.Category and that's perfectly fine because we know the math works. > This establishes a Cartesian closed category of somewhat idealized > entities. Can > the realization of this category in Haskell be perfected to exclude > pathologies > by construction? I think you need dependent types, but let's try. import Data.Map (Map) import qualified Data.Map.Strict as Map import Data.Set (Set) import qualified Data.Set as Set -- Note: One could also define -- FiniteFunction a b = (Set a, a -> b) -- hence the Map in your OP was misleading. newtype FiniteFunction a b = FMap {asMap :: Map a b} domain :: FiniteFunction a b -> Set a domain = Map.keysSet . asMap codomain :: Ord b => FiniteFunction a b -> Set b codomain = Set.fromList . Map.elems . asMap unsafeRunFiniteFunction :: Ord a => FiniteFunction a b -> a -> b unsafeRunFiniteFunction = (assertDomain.) . flip Map.lookup . asMap where assertDomain (Just b) = b assertDomain Nothing = error "argument outside of domain" -- | @(unsafeRunFiniteFunction f) `restrictedTo` (domain f) = f@ restrictedTo :: (a -> b) -> Set a -> FiniteFunction a b f `restrictedTo` dom = FMap (Map.fromSet f dom) -- | (.) of the category. -- 'id' on dom is @id `restrictedTo` dom@ compose :: (Ord a, Ord b, Ord c) => FiniteFunction b c -> FiniteFunction a b -> FiniteFunction a c compose (FMap g) (FMap f) = FMap (fmap g' f) where g' b = case Map.lookup b g of Nothing -> error "type mismatch between domain and codomain" Just c -> c {-- Now to the CCC structure. At this point we run into a problem: Categorically, if f :: A -> (C^B) and we uncurry it, then the domain of (uncurry f) is a square, the cartesian product of A and B. Trying to curry a function whose domain is a proper subset of the cartesian product of A and B should be a type error in my opinion, but Haskell's type system can not express this. I imagine failure of this property may lead to unexpected results when used in conjunction with 'compose'. But maybe currying functions with non-square domain is a desirable feature. If so, remove assertUncurried below. --} -- | @'isSquare' setOfPairs = True@ iff @setOfPairs@ -- is of the form -- @Set.fromList [(a,b) | a <- setA, b <- setB]@. isSquare :: (Ord a, Ord b) => Set (a,b) -> Bool isSquare setOfPairs = let fstProj = Set.map fst setOfPairs sndProj = Set.map snd setOfPairs in all (\a -> all (\b -> (a,b) `Set.member` setOfPairs) sndProj) fstProj isUncurried :: (Ord a, Ord b) => FiniteFunction (a,b) c -> Bool isUncurried = isSquare . domain -- | Beware: we can not express in the type system that -- for all keys @a@ the 'domain' :: Set b of the value under @a@ -- is the same Set, which is a prerequisite for the CCC operation. -- Hence we can not guarantee that the resulting 'domain' 'isSquare'. uncurryFiniteFunction :: (Ord a, Ord b) => FiniteFunction a (FiniteFunction b c) -> FiniteFunction (a,b) c uncurryFiniteFunction = FMap . Map.foldMapWithKey unc . asMap where unc a (FMap bc) = Map.mapKeysMonotonic ((,) a) bc -- | likewise, if the 'domain' of the uncurried function -- fails 'isSquare' then the result is not a well-defined -- higher-order 'FiniteFunction'. curryFiniteFunction :: (Ord a, Ord b) => FiniteFunction (a,b) c -> FiniteFunction a (FiniteFunction b c) curryFiniteFunction f@(FMap ab_c) = assertUncurried (FMap a_bc) where filterFst a = Map.foldMapWithKey (\ab c -> if fst ab == a then Map.singleton (snd ab) c else Map.empty) ab_c as = Map.foldMapWithKey (\(a,_) _ -> Set.singleton a) ab_c a_bc = Map.fromSet (FMap . filterFst) as assertUncurried = if isUncurried f then id else error "not an uncurried function"

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Puzzling "Reduction stack overflow" error message
by CASANOVA Juan 24 Jan '21

24 Jan '21

Hello Haskell Cafe, I almost feel guilty for posting this since I almost never read messages from this list. But I guess others do and do so because they want to so it's an asymmetry that is fine, and which probably won't be like that forever (I'll get to reading them again at a different stage of my life I'm feeling better). I also have very bad experience with asking "complex" Haskell questions in Stack Overflow, with people either not replying or replying stuff that indicates they did not understand the question. Digression aside, I have encountered this error before and it is a nasty one, but this time I am just wildly puzzled by what it's trying to tell me. This is part of a very large program (and I think the huge stack of types and instances is what is hiding the problem here), but I'll try to condense it into a small bit. The following function I have is the one that throws the error: resolve_to_constraints_metacnf :: SOMetaSignature -> SOMetaCNF -> Computation (Maybe SOMetaUnifSystem) resolve_to_constraints_metacnf sig cnf = result where f1 = (ADDirect <$>) :: SOMetaliteral -> SOMetaUnifLiteral; f2 = (f1 <$>) :: [SOMetaliteral] -> [SOMetaUnifLiteral]; f3 = (f2 <$>) :: [[SOMetaliteral]] -> [[SOMetaUnifLiteral]]; ucnf = f3 cnf :: [[SOMetaUnifLiteral]]; resolved = res_computeresolve SOResGreedyFactorH ucnf :: StateT uv Computation (Maybe SOMetaUnifSystem); runstated = runStateT resolved (UnifVar 0); result = fst <$> runstated The error starts as follows: Reduction stack overflow; size = 201 When simplifying the following type: Eq OFunction The rest is the standard for this type of errors. OFunction does not appear explicitly on my function signature, but if we look at the definitions of SOMetaSignature, SOMetaCNF, SOMetaLiteral, SOMetaUnifLiteral and SOMetaUnifSystem, we see they are type aliases for parameterised types, all of which include OFunction as one of their parameters. Here's an example: type SOMetaUnifLiteral = Literal (AtomDependant CAtomPF CTermF LambdaCNF SOPredicate OPredicate OFunction OVariable SOAMVariable SOMVariable UnifVariable) (Yes, I know how tedious it is to have so many type parameters, tell me about it). And perhaps also relevantly, this is the signature of res_computeresolve (which type checks): res_computeresolve :: (ResLiteral l r mt, Heuristics h [Literal l] () (ResStep l) m) => h -> [[Literal l]] -> mt m (Maybe r) (note, might be relevant, that ResLiteral requires mt to be a MonadTrans and MFunctor, which StateT, the type it gets instantiated to in the calling function, is. m gets instantiated to Computation which is a Monad I implemented myself (and works fine).) Now, the natural thing to think is to track the Constraints to find which one is asking for an Eq OFunction and how that works. It definitely is a constraint that I use all over the place. But, here is the puzzling thing. Eq OFunction is clearly implemented. OFunction is a really simple type. Here is the basic definition: data OFunction = OFun Int Int deriving (Eq, Ord) So, why is it having such a hard time proving Eq OFunction, if it is a really simple type with a derived instance of Eq (which has worked thus far without any issues)? My suspicion is that the message is bugged, and the stack overflow is not on that specific instance, but rather on some type variable on a larger instance that it can never entirely instantiate no matter how many constraints it resolves. This is why I added those inline type signatures to the triple fmap, to try to help GHC know what the types should look like. But I am still getting the error, and I really don't know what else to do now. Sorry for the long message, but since I don't know where the error is coming from, I wanted to give some context, and my entire program is tens of thousands of lines of codes by now... I hope people are able to skim and find the relevant bit that tells them what's likely the stupid mistake I am making. Thanks in advance, Juan. The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

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Re: [Haskell-cafe] stack: duplicate modules in library
by Chris Reade 22 Jan '21

22 Jan '21

Solved - thanks to suggestions from Sebastiaan Joosten. A syntax error (missing hyphens) in package.yaml file was creating duplicates in the .cabal file. So nothing to do with cached binary at all. Chris > On 21 Jan 2021, at 13:57, Chris Reade <chrisreade(a)mac.com> wrote: > > I have come across a problem with haskell stack putting duplicate modules in a library. > > On "stack build" I get > > "Cabal-simple_mPHDZzAJ_3.0.1.0_ghc-8.8.4: Duplicate modules in library:..” > > This may have originally been caused when I changed capitalisation of a filename, but changing back did not resolve the problem. > > My question is how do I get rid of the library so I can rebuild? > I tried "stack purge” and also removing ~/.stack with no luck > (the latter causes a new ghc download on stack build, but with the same message afterwards) > > I know the source modules are fine because they all load in "stack ghci” > Is there some other directory that stack/cabal uses for binary cache that I can safely delete? > > Chris > > >

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