January 2022 - Haskell-Cafe

Recursive constraints with type families
by Dmitriy Matrosov 26 Jan '22

26 Jan '22

Hi. I've stumbled upon a simple problem with recursive type family: > {-# LANGUAGE RankNTypes #-} > {-# LANGUAGE KindSignatures #-} > {-# LANGUAGE TypeApplications #-} > {-# LANGUAGE TypeOperators #-} > {-# LANGUAGE PolyKinds #-} > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE StandaloneKindSignatures #-} > {-# LANGUAGE ScopedTypeVariables #-} > {-# LANGUAGE UndecidableInstances #-} > > import GHC.TypeLits > import Data.Proxy > import Data.Kind > > type C :: Nat -> Constraint > type family C x where > C 0 = KnownNat 0 > --C x = (KnownNat x, C (x - 1)) > C x = (KnownNat x, C 0) > --C x = KnownNat x > > --type C2 x = (KnownNat x, C2 1) > > showNat :: forall (n :: Nat). C n => Proxy n -> String > showNat _ = show $ natVal (Proxy @n) and the error is Prelude> :r [1 of 1] Compiling Main ( constraint-type-family.lhs, interpreted ) constraint-type-family.lhs:31:22: error: • Could not deduce (KnownNat n) arising from a use of ‘natVal’ from the context: C n bound by the type signature for: showNat :: forall (n :: Nat). C n => Proxy n -> String at constraint-type-family.lhs:30:3-56 Possible fix: add (KnownNat n) to the context of the type signature for: showNat :: forall (n :: Nat). C n => Proxy n -> String • In the second argument of ‘($)’, namely ‘natVal (Proxy @n)’ In the expression: show $ natVal (Proxy @n) In an equation for ‘showNat’: showNat _ = show $ natVal (Proxy @n) | 31 | > showNat _ = show $ natVal (Proxy @n) | ^^^^^^^^^^^^^^^^^ If i write just C x = KnownNat x everything works fine. I guess, the reason is UndecidableInstances, but why and then, how can i build constraints recursively and make it work? Thanks. PS. Initially, this 'showNat' was something like countDown :: forall (n :: Nat). C n => Proxy n -> String countDown _ = let x = natVal (Proxy @n) in if x > 0 then show x ++ countDown (Proxy @(n - 1)) else "end" but it does not work.

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Missing feature of Coercible, or necessary caution?
by Tom Ellis 25 Jan '22

25 Jan '22

Consider the following program. {-# LANGUAGE TypeApplications #-} import Data.IntMap (IntMap, mapKeysMonotonic, Key) import Data.Coerce (coerce) newtype MyMap = MyMap (IntMap Bool) doesn'tWork :: (Key -> Key) -> MyMap -> MyMap doesn'tWork = coerce mapKeysMonotonic works :: (Key -> Key) -> MyMap -> MyMap works = coerce (mapKeysMonotonic @Bool) mapKeysmonotonic has type (Key -> Key) -> IntMap a -> IntMap a and we attempt to coerce it to type (Key -> Key) -> MyMap -> MyMap since MyMap is a newtype around IntMap Bool this ends up generating the constraint Coercible a Bool and there is no way to specify that constraint given that a doesn't otherwise appear in the top-level type. Hence doesn'tWork will not type check. If I force the type of a to be Bool using a TypeApplication then it does type check (works). But is this a missing feature of GHC's Coercible system? Since a is not otherwise constrained is it not valid for GHC to choose it to be Bool ? Coercible instances contain now observable content so no observable behaviour could depend on the particular choice of Coercible a Bool instance, and choosing a ~ Bool is the natural one. Is there some violation of type safety or otherwise broken behaviour that could arise from choosing arbitrary Coercible instances on type variables that are not otherwise constrained? Tom

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Guards on variable bindings
by Tom Ellis 24 Jan '22

24 Jan '22

According to GHC there are multiple declarations of bar, but not of foo. I don't understand. Why is it not valid to have multiple clauses for a variable binding? Tom bar | False = () bar = () foo _ | False = () foo _ = ()

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why OverloadedTuples extension is missing?
by Daneel Yaitskov 17 Jan '22

17 Jan '22

Cafe, There is a group of GHC extensions and complementary classes to make writing Haskell code a bit sweeter such as: - {} - Num (fromIntegral) - OverloadedStrings - IsString - OverloadedLists - IsList So I have an inductive question - why there is no OverloadedTuples? I haven't found a discussion thread about this topic. I could imagine following tuple syntax interpretation: {-# LANGUAGE OverloadedTuples #-} data Foo = FooA Int String | FooB String Foo deriving (Show, Eq, IsTuple) mkFoo :: Foo mkFoo = (1, "hello") mkFoo2 :: Foo mkFoo = ("abc", (1, "hello")) Sometimes expected type name is known without data constructor. So such expression is more concise. -- Best regards, Daniil Iaitskov

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Newbie question
by Terry Phelps 16 Jan '22

16 Jan '22

I'm trying yet again to "get" Haskell. I'm reading Graham Hutton's book "Programming in Haskell", and am working on one of the exercises. Sounds easy enough: Make a function 'last' that returns the last element of a non-empty list. After struggling a while, I looked at the answer: last = head . reverse. I wish I had thought of that. But I kept trying to get my recursive version done, but I can't seem to make ghci happy. (Version 8.10.5, if you care). I wrote: last [] = [] (Yes, I know that's not a non-empty list, but I don't want ghci whining about non-exhaustive patterns). Then I added: last [x] = x And I checked the type: :t last last:: [a] -> [a] Yes, that looks right. There seems to be only one other case: 2 or more elements. So I wrote: last (x:y:xs) = last (y:xs) I ran it: Prelude> last [1,2] *** Exception: <interactive>:2:1-27: Non-exhaustive patterns in function last What the heck is the problem? I've covered every possible list, haven't I? Stranger yet, I check the type again: Prelude> :t last last :: [a] -> t The type has changed. And I don't understand what it means. What's the 't'? I'm sure this is a simple beginner error, but I'm really confused. I don't see how it isn't exhaustive, and I don't see why the 3rd clause (or whatever it's called) caused the type to change. Someone help, please.

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TFPIE 2022: moving to online and extension to Feb 7th
by Elena Machkasova 14 Jan '22

14 Jan '22

Due to rescheduling of the Lambda Days because of COVID, TFPIE, together with TFP, will be held virtually on March 16th 2022, together with TFP which will be held on March 17-18. There is an optional opportunity to present at the Lambda Days July 28-29 2022 in addition to the virtual presentation in March. Updated TFPIE 2022 Call for papers: https://wiki.tfpie.science.ru.nl/TFPIE2022 The submission deadline is extended to February 7th 2022. TFPIE 2022 welcomes submissions describing techniques used in the classroom, tools used in and/or developed for the classroom and any creative use of functional programming (FP) to aid education in or outside Computer Science. Topics of interest include, but are not limited to: FP and beginning CS students FP and Computational Thinking FP and Artificial Intelligence FP in Robotics FP and Music Advanced FP for undergraduates FP in graduate education Engaging students in research using FP FP in Programming Languages FP in the high school curriculum FP as a stepping stone to other CS topics FP and Philosophy The pedagogy of teaching FP FP and e-learning: MOOCs, automated assessment etc. Best Lectures - more details below In addition to papers, we are requesting best lecture presentations. What's your best lecture topic in an FP related course? Do you have a fun way to present FP concepts to novices or perhaps an especially interesting presentation of a difficult topic? In either case, please consider sharing it. Best lecture topics will be selected for presentation based on a short abstract describing the lecture and its interest to TFPIE attendees. The length of the presentation should be comparable to that of a paper. In addition, the speaker can provide commentary on effectiveness or student feedback. Submissions Potential presenters are invited to submit an extended abstract (4-6 pages) or a draft paper (up to 20 pages) in EPTCS style. The authors of accepted presentations will have their preprints and their slides made available on the workshop's website. Papers and abstracts can be submitted via easychair at the following link: https://easychair.org/conferences/?conf=tfpie2022 After the workshop, presenters are invited to submit (a revised version of) their article for the formal review. The PC will select the best articles for publication in the Electronic Proceedings in Theoretical Computer Science (EPTCS). Articles rejected for presentation and extended abstracts will not be formally reviewed by the PC. Important Dates (updated) Submission deadline: *February 7th* 2022, Anywhere on Earth. Notification: *February 11th* 2022 (Note: earlier submissions will receive earlier response) TFPIE Registration Deadline: TBA Workshop: *March 16* 2022 Submission for formal review: *April 29th* 2022, Anywhere on Earth. Notification of full article: June 1st 2022 Camera ready: July 1st 2022 Program Committee Peter Achten, Radboud University, Netherlands Stephen Chang, University of Massachusetts Boston, USA John Hughes, Chalmers University of Technology, Sweden Elena Machkasova (Chair) - University of Minnesota Morris, USA Kristina Sojakova - INRIA, Paris, France Melinda Tóth, Eötvös Loránd University, Budapest, Hungary Keynote speaker: Our keynote talk is "The perfect Functional Programming course" by Peter Achten. Registration information: TFPIE will take place virtually on March 16 2022, together with TFP which will be held on March 17-18. Registration information will be announced shortly. There is no registration fee. Registration and online attendance are mandatory for at least one author of every paper that is presented at the workshop. Only papers that have been presented at TFPIE may be submitted to the post-reviewing process. -- Dr. Elena Machkasova Associate Professor of Computer Science Division of Science and Mathematics University of Minnesota, Morris Office: Sci 2325 (320) 589-6308 http://cda.morris.umn.edu/~elenam/ Pronouns: she/her/hers or any other

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Postdoc positions on Formal Verification for Zero-Trust IoT Systems at Kyoto University and National Institute of Informatics (NII), Japan
by Taro Sekiyama 14 Jan '22

14 Jan '22

[Please distribute, apologies for multiple postings.] Dear All, We are seeking a few postdoc researchers, who works for a project “Zero-Trust IoT Systems by Collaboration of Formal Verification and System Software” by Japan Science and Technology Agency. We'd be grateful if you could spread the word to interested candidates. * Project Description The project aims at the construction of formally verified secure IoT systems that follow the concept of “zero trust architecture”, dubbed ZT-IoT systems. It consists of four research teams and two teams, led by Atsushi Igarashi, Kyoto University, Japan and Taro Sekiyama, National Institute of Informatics, Japan (NII) are investigating applications of formal verification or programming language techniques to the construction of secure IoT systems. The main research topic of Igarashi’s team is centered around the design and theory of security policy engines for ZT-IoT systems, inclucing the design of a language to describe security policies and policy enforcement algorithms and the techniques for verifying policy enforcement algorithms against given security policies. Other team members are Kohei Suenaga and Masaki Waga at Kyoto University. The main research topic of Sekiyama’s team is centered around techniques for monitoring and intervention for ZT-IoT systems. Other team members include Ichiro Hasuo and Shin-ya Katsumata at NII. Although the two teams are based in different places, we collaborate closely with each other. The appointment can start as early as April 2022 (the starting date is negotiable). The contract will initially run until the end of March 2023, with the possibility of annual renewal until the end of the project, which is March 2027 at maximum. Salary will be about 360,000–550,000 JPY/month. Applicants should have a Ph.D in computer science or related fields, and have a strong background in formal verification and/or programming language theory. Due to the project’s nature, they are required to have strong interests in applying theory to practice; they should also be (self-)motivated, dedicated, and able to work both independently and collaboratively. Strong communication skills in oral and written English are required. * Workplace Members of Igarashi’s team will work at Kyoto University, Kyoto, Japan and members of Sekiyama’s team will work at NII, Tokyo, Japan. (Living costs in Japan are not very high nowadays. An estimate is found here https://www.internations.org/go/moving-to-japan/living/the-cost-of-living-i… and we find rent can be cheaper than the cited amount.) * Applications and inquiries Inquiries can be sent to application-zt-iot [at] fos.kuis.kyoto-u.ac.jp, with the subject CREST Job Inquiry. Feel free to ask us any questions on relevance, topics, compensation, etc. We will reply when we see enough relevance. Applications should be made electronically via the following JREC-IN Portal web sites. NII: https://jrecin.jst.go.jp/seek/SeekJorDetail?fn=3&ln=1&id=D122010021&ln_jor=1 Kyoto: https://jrecin.jst.go.jp/seek/SeekJorDetail?fn=3&ln=1&id=D121122219&ln_jor=1 Please upload a pdf including - your brief CV, - short description of research interests (can be very informal and short), - the list of papers (a dblp or Google scholar link will do, for example), - a couple of representative papers (in pdf), and - (preferably) the contact of two references. We will contact you for further material and interview, provided that we find sufficient relevance in your application. Starting dates are negotiable. The positions will remain open until filled. Many thanks, Taro.

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Call for Participation: BOB 2022 (March 11, Berlin or online)
by Stefan Wehr 13 Jan '22

13 Jan '22

================================================================================ BOB 2022 Conference “What happens if we simply use what’s best?” March 11, 2022, Berlin or online 0100+UTC https://bobkonf.de/2022/ Program: https://bobkonf.de/2022/program.html Berlin or online? Fill out our survey: https://bobkonf.de/2022/onsite.html ================================================================================ BOB conference is a place for developers, architects, and decision-makers to explore technologies beyond the mainstream in software development and to find the best tools available to software developers today. Our goal is for all participants of BOB to return home with new insights that enable them to improve their own software development experience. The program features 14 talks and 8 tutorials on current topics: https://bobkonf.de/2022/program.html The subject range includes functional programming, effects, distributed programming, formal methods, generative art, event-driven systems, the human brain, Haskell, Python, Scala, Lua, Clojure, Erlang, Nix, and others. Derek Dreyer will give the keynote talk. NOTE: We will decide on January 17 whether BOB 2022 will take place on-site in Berlin or online. Either way, we are working towards fostering a lively exchange of exciting ideas and enabling meaningful social interactions. We're collecting feedback on the issue here: https://bobkonf.de/2022/onsite.html If you're interested in BOB, please take a minute to fill the survey linked above! Registration will also open on January 17, once the decision for on-site or online has been made.

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[TFP'22] second call for papers: Trends in Functional Programming 2022 moved to March 17-18 online (together with TFPiE)
by Peter Achten 11 Jan '22

11 Jan '22

============== TFP 2022 =================== == MOVING TO ONLINE SYMPOSIUM, NEW DATES == =========================================== 23rd Symposium on Trends in Functional Programming 17-18 March, 2022 Online event https://trendsfp.github.io/index.html Due the pandemic, we have had to make TFP virtual this year. As a result, we've decided to push back the deadlines and conference date by a few weeks. In particular, the pre-symposium deadline for submitting the first version of your paper is now just after the ICFP deadline. == Important Dates == Submission deadline for draft papers Monday 7th March, 2022 Notification for draft submissions Friday 11th March, 2022 Symposium dates Thursday 17th - Friday 18th March, 2022 Submission deadline for post-symposium reviewing Wednesday 6th April, 2022 Notification for post-symposium submissions Friday 27th May, 2022 The Symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming, taking a broad view of current and future trends in the area. It aspires to be a lively environment for presenting the latest research results, and other contributions. == Scope == The symposium recognizes that new trends may arise through various routes. As part of the Symposium's focus on trends we therefore identify the following five article categories. High-quality articles are solicited in any of these categories: * Research Articles: Leading-edge, previously unpublished research work * Position Articles: On what new trends should or should not be * Project Articles: Descriptions of recently started new projects * Evaluation Articles: What lessons can be drawn from a finished project * Overview Articles: Summarizing work with respect to a trendy subject Articles must be original and not simultaneously submitted for publication to any other forum. They may consider any aspect of functional programming: theoretical, implementation-oriented, or experience-oriented. Applications of functional programming techniques to other languages are also within the scope of the symposium. Topics suitable for the symposium include, but are not limited to: * Functional programming and multicore/manycore computing * Functional programming in the cloud * High performance functional computing * Extra-functional (behavioural) properties of functional programs * Dependently typed functional programming * Validation and verification of functional programs * Debugging and profiling for functional languages * Functional programming in different application areas: security, mobility, telecommunications applications, embedded systems, global computing, grids, etc. * Interoperability with imperative programming languages * Novel memory management techniques * Program analysis and transformation techniques * Empirical performance studies * Abstract/virtual machines and compilers for functional languages * (Embedded) domain specific languages * New implementation strategies * Any new emerging trend in the functional programming area If you are in doubt on whether your article is within the scope of TFP, please contact the TFP 2022 program chairs, Wouter Swierstra and Nicolas Wu. == Best Paper Awards == To reward excellent contributions, TFP awards a prize for the best paper accepted for the formal proceedings. TFP traditionally pays special attention to research students, acknowledging that students are almost by definition part of new subject trends. A student paper is one for which the authors state that the paper is mainly the work of students, the students are listed as first authors, and a student would present the paper. A prize for the best student paper is awarded each year. In both cases, it is the PC of TFP that awards the prize. In case the best paper happens to be a student paper, that paper will then receive both prizes. == Instructions to Author == Papers must be submitted at: https://easychair.org/conferences/?conf=tfp22 Authors of papers have the choice of having their contributions formally reviewed either before or after the Symposium. == Post-symposium formal review process == Draft papers will receive minimal reviews and notification of acceptance for presentation at the symposium. Authors of draft papers will be invited to submit revised papers based on the feedback received at the symposium. A post-symposium refereeing process will then select a subset of these articles for formal publication. == Paper categories == Draft papers and papers submitted for formal review are submitted as extended abstracts (4 to 10 pages in length) or full papers (20 pages). The submission must clearly indicate which category it belongs to: research, position, project, evaluation, or overview paper. It should also indicate which authors are research students, and whether the main author(s) are students. A draft paper for which all authors are students will receive additional feedback by one of the PC members shortly after the symposium has taken place. == Format == Papers must be written in English, and written using the LNCS style. For more information about formatting please consult the Springer LNCS web site. == Program Committee == Guillaume Allais University of St Andrews José Manuel Calderón Trilla Galois, Inc. Stephen Chang University of Massachusetts Boston Matthew Flatt University of Utah Jeremy Gibbons University of Oxford Zhenjiang Hu Peking University Mauro Jaskelioff CIFASIS / Universidad Nacional de Rosario Moa Johansson Chalmers University of Technology Shin-ya Katsumata National Institute of Informatics Oleg Kiselyov Tohoku University Bas Lijnse Netherlands Defence Academy / Radboud University Nijmegen Kazutaka Matsuda Tohoku University Nico Naus Virginia Tech Christine Rizkallah University of New South Wales Alejandro Serrano 47 Degrees Amir Shaikhha Ecole Polytechnique Fédérale de Lausanne Aaron Stump University of Iowa Wouter Swierstra (Co-chair) Utrecht University Baltasar Trancón Y Widemann Semantics GmbH Nicolas Wu (Co-chair) Imperial College London Ningning Xie University of Hong Kong

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[ANN] plutus-simple-model - unit test linrary for plutus with resource estimation
by Anton Kholomiov 11 Jan '22

11 Jan '22

Hi! I’d like to announce the unit test library plutus-simple-model <https://github.com/mlabs-haskell/plutus-simple-model> for plutus that is a joy to use [1]. It’s simple and easy to grasp and it can estimate resource usage for TX (size and execution units). Main idea is to implement a simple blockchain model that updates UTXOs inside a pure State monad if TX is valid. To check TX cardano node functions are used so the check is the same as if node would check it and it produces useful stats for resource usage. Benefits comparing to EmulatorTrace to me: - I find resource usage stats are useful to know in production and this is missing in Emulator tests - EmulatorTrace approach makes it hard to query run-time information and with given lib it’s super easy - Having to define separate API and use lookups/constraints seems clumsy to me. It often leads to writing lots of boilerplate code. - it’s much faster to run I’ve tested it successfully in one project and using it for the second one. It has great docs and tutorials. Enjoy! I hope it will improve your testing experience with plutus. [1] plutus-simple-model <https://github.com/mlabs-haskell/plutus-simple-model> https://github.com/mlabs-haskell/plutus-simple-model

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