June 2018 - Haskell-Cafe

how to prevent 'stack' from installing 'ghc'?
by Johannes Waldmann 10 Jun '18

10 Jun '18

Dear Cafe, 'stack' tries to download and install a 'ghc' binary in case 'ghc' in $PATH does not match what is needed for the resolver given in 'stack.yaml' How can I switch this off completely and reliably? I much rather 'stack' just fails when it detects the mismatch. putting 'system-ghc: true' in 'stack.yaml' and '~/.stack/config.yaml' apparently is not enough. 'stack' does warn before downloading, but sometimes I'm not seeing the warning, e.g., because the whole thing is triggered silently from using 'intero'. I do have several ghc executables (built from source) in /opt/ghc/ghc-$version/bin/ghc and I just need to set PATH accordingly. It would be even better if I could tell 'stack' (once and for all) about these locations. I think I understand why 'stack' does what it does (reproducible builds) but I really want to minimize the number of other people's binaries on my machine. I do use the auto-download feature for easier CI builds - so I agree it can be useful. - J.W.

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removing a cookie via ghcjs
by Michael Litchard 08 Jun '18

08 Jun '18

Using ghcjs, I am able to read a cookie using getCookie. There's no corresponding remove/deleteCookie that I can find. Could I get some assistance in how to remove the same cookie I can see using getCookie?

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TyDe 2018: SUBMISSION DEADLINE EXTENDED TO JUNE 13th.
by Niki Vazou 07 Jun '18

07 Jun '18

Hey all, the TyDe submission deadline got extended for one week! CALL FOR CONTRIBUTIONS: DEADLINE EXTENDED TO JUNE 13th. TyDe 2018: Type-Driven Development https://icfp18.sigplan.org/track/tyde-2018 23-29 Sep, 2018, St. Louis, Missouri (co-located with ICFP) ============================================== *Call for Contributions* We welcome all contributions, both theoretical and practical, on a range of topics including: dependently typed programming; generic programming; design and implementation of programming languages, exploiting types in novel ways; exploiting typed data, data dependent data, or type providers; static and dynamic analyses of typed programs; tools, IDEs, or testing tools exploiting type information; pearls, being elegant, instructive examples of types used in the derivation, calculation, or construction of programs. *Important Dates* **Extended paper submission deadline: Wednesday, June 13, 2018** Extended abstract deadline: Wednesday, June 13, 2018 Author notification: Friday, June 29, 2018 Deadline for camera ready version: August 5, 2018 Workshop: Thursday, September 27, 2018 *Program Committee* - Guillaume Allais, Radboud University Nijmegen, Netherlands - Zena M. Ariola, University of Oregon, USA - David Darais, University of Vermont, USA - Richard Eisenberg, Bryn Mawr College, USA (co-chair) - Jennifer Hackett, University of Nottingham, UK - Shin-ya Katsumata, National Institute of Informatics, Japan - Daan Leijen, Microsoft Research, USA - Shin-Cheng Mu, Academia Sinica, Taiwan - Dominic Orchard, University of Kent, UK - Peter-Michael Osera, Grinnell College, USA - Zoe Paraskevopoulou, Princeton University, USA - Alberto Pardo, Universidad de la Republica, Uruguay - Matthieu Sozeau, University of Paris Diderot, Paris 7, France - Niki Vazou, University of Maryland, USA (co-chair) *Submission details* Submissions should fall into one of two categories: - Regular research papers (12 pages) - Extended abstracts (2 pages) The bibliography will not be counted against the page limits for either category. Regular research papers are expected to present novel and interesting research results, and will be included in the formal proceedings. Extended abstracts should report work in progress that the authors would like to present at the workshop. Extended abstracts will be distributed to workshop attendees but will not be published in the formal proceedings. We welcome submissions from PC members (with the exception of the two co-chairs), but these submissions will be held to a higher standard. Submission is handled through HotCRP: https://tyde18.hotcrp.com/ All submissions should be in portable document format (PDF) and formatted using the ACM SIGPLAN style guidelines: http://www.sigplan.org/Resources/Author/ Note that the ACM SIGPLAN style guidelines have changed from previous years! In particular, submissions should use the new ‘acmart’ format and the two-column ‘sigplan’ subformat (not to be confused with the one-column ‘acmlarge’ subformat!). Extended abstracts must be submitted with the label ‘Extended abstract’ clearly in the title. Best, Niki Vazou

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Usage of `pure`
by PY 07 Jun '18

07 Jun '18

Hello, Dear Cafe! Consider, I have big application with monad and applicative related code. What problems I can hit if I will replace all `return`s with `pure` (in `do`-blocks, anywhere). So, `return` will be totally eliminated. === Best regards, Paul

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Parsing a matrix table with Data.Frames
by Tyson Whitehead 06 Jun '18

06 Jun '18

I have a case where some of my data is stored in CSV matrix form like this example of a table telling whether person i has pet j ,cat,dog,goldfish bill,true,true,false sue,false,true,true fred,false,false,true I can't see any obvious way to read this sort of thing in with the Data.Frames package. Would love to be told differently. Thanks! -Tyson

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Wanted: New Admin for Planet.Haskell.Org
by Gershom B 05 Jun '18

05 Jun '18

Dear all — the planet.haskell.org server is a low-maintenance feed-aggregator that uses the planet venus software to pull together feeds of haskell-related content. I think it is a great resource, and I’ve used it for years. For some time the planet(a)haskell.org address hadn’t been well-monitored, meaning that requests for adding new rss feeds had languished. I’ve gone through and tried to act on the outstanding requests, and will be doing so for the time being. However, I have a lot on my plate, and would like to pass this responsibility on to a volunteer. It basically means, on the occasional recipt of an email, logging into the server, updating a config file, running a command, and logging back out again. If you’re interested in helping out with this, please contact me. Cheers, Gershom

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Re: [Haskell-cafe] No Enum for (,), no Enum or Bounded for Either
by Olaf Klinke 04 Jun '18

04 Jun '18

>Also note that what you're talking about is a special type of objects, >namely > > type BoundedEnum a = (Bounded a, Enum a) -- using ConstraintKinds > >(I'm sure the mathematicians have a better name for this) Indeed the Prelude states no laws for Bounded. But maybe we all agree on this: forall a. fromEnum minBound <= fromEnum a <= fromEnum maxBound With this law and the rule fromEnum.toEnum=id (for those integers that this type's toEnum accepts) we can say that (Bounded a,Enum a) is the class of all types which have a distinguished finite subset. If one assumes further the law toEnum.fromEnum=id (which is violated e.g. by Double) then (Bounded a,Enum a) are precisely the constructively finite types, that is, the types for which one can exhibit an isomorphism with a finite cardinal. Note that in constructive mathematics, a set may be known to be finite in the sense that the assumption it is infinite is absurd, yet one can not name an isomorphism with a finite cardinal. [*] As Jon Fairbairn pointed out, (Int,Int) can be given an Enum instance. The same is true for every type that is build from types that already have an Enum instance using only sum and product. >This is all true, but I was implicitly assuming that Bounded and Enum ought >to agree with Ord. Consider this: From every type's Enum instance you can derive an Ord instance: compare = compare `Data.Function.on` fromEnum -- Olaf [*] In the text [1] by Andrej Bauer I found this funny theorem: Excluded middle is equivalent to the statement that subsets of finite sets are finite. [1] http://www.ams.org/journals/bull/2017-54-03/S0273-0979-2016-01556-4/S0273-0…

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WST 2018 - Call for Participation (July 18-19, 2018)
by Salvador Lucas 04 Jun '18

04 Jun '18

========================================================================== WST 2018 - Call for Participation 16th International Workshop on Termination part of FLoC 2018 July 18-19, 2018, Oxford, United Kingdom http://wst2018.webs.upv.es/ ========================================================================== REGISTRATION (June 6th is the last day for early registration!) http://www.floc2018.org/register/ INVITED SPEAKERS: James Worrell - University of Oxford "Termination Checking and Invariant Synthesis for Affine Programs" Akihisa Yamada - NII Japan "Towards a Unified Method for Termination" PROGRAMME (see also the list of accepted papers attached below): https://easychair.org/smart-program/FLoC2018/WST-program.html TERMINATION AND COMPLEXITY COMPETITION: In 2018, the Termination and Complexity Competition (TERCOMP) will run in parallel with FLoC 2018 as part of the FLoC Olympic Games. The results of the competition will be presented and discussed during a special session at WST 2018. ACCEPTED PAPERS: Eric Hehner. Objective and Subjective Specifications. Guillaume Genestier and Frédéric Blanqui. Termination of Lambda-Pi modulo rewriting using the size-change principle Jera Hensel, Florian Frohn and Jürgen Giesl. Complexity Analysis for Bitvector Programs Alfons Geser, Dieter Hofbauer and Johannes Waldmann. Comparing on Strings: Semantic Kachinuki Order Nachum Dershowitz and Jean-Pierre Jouannaud. GPO: A Path Ordering for Graphs Jose Divasón, Sebastiaan Joosten, René Thiemann and Akihisa Yamada. A Perron-Frobenius Theorem for Jordan Blocks for Complexity Proving Jonas Schöpf and Christian Sternagel. TTT2 with Termination Templates for Teaching Cristina David, Daniel Kroening and Peter Schrammel. Procedure-Modular Termination Analysis Salvador Lucas. Well-founded models in proofs of termination Aalok Thakkar, Balaji Krishnamurthy and Piyush Gupta. Verification of Rewriting-based Query Optimizers Jesús J. Doménech, Samir Genaim and John P. Gallagher. Control-Flow Refinement via Partial Evaluation Alicia Merayo Corcoba and Samir Genaim. Inference of Linear Upper-Bounds on the Expected Cost by Solving Cost Relations Dieter Hofbauer. Embracing Infinity - Termination of String Rewriting by Almost Linear Weight Functions Carsten Fuhs and Cynthia Kop. Improving Static Dependency Pairs for Higher-Order Rewriting

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A law for MonadReader
by Li-yao Xia 03 Jun '18

03 Jun '18

Hello Café, While trying to document laws for mtl classes (https://github.com/haskell/mtl/issues/5) I have been puzzling over the subtle logical relationships between a few variants. Consider the following four possible laws for MonadReader's ask: (1) (ask >> ask) = ask (2) (ask >>= \r1 -> ask >>= \r2 -> return (r1, r2)) = (ask >>= \r -> return (r, r)) (3) (ask >> return ()) = return () Let reader :: MonadReader r m => (r -> a) -> m a reader f = fmap f ask (4) reader is a monad homomorphism from ((->) r) to m, i.e.: reader (\_ -> a) = return a (reader m >>= \x -> reader (k x)) = reader (m >>= k) Question: which ones imply which ones? Note that (1) and (2) do not imply (3) or (4). Intuitively, (1) and (2) say that ask is idempotent ("asking twice is the same as asking once"), but (3) and (4) imply the stronger property of nullipotence ("ask has no side effects"). --- Any help with the following conjectures is welcome (we're assuming the monad laws hold in the first place): A. (1) and (2) are equivalent. I can prove that (2) implies (1). However the converse eludes me. B. (3) and (4) are equivalent. Again, I can prove that (4) implies (3), but I had no success with the converse. C. ((3) or (4)) implies ((1) and (2)) ((3) implies (1)), and ((4) implies ((1) and (2))) are straightforward, but without the above equivalence, the missing bit is ((3) implies (2)) It seems that parametricity plays an important role in the conjectured implications. I didn't manage to apply free theorems to this problem, but perhaps you will have better luck. Regards, Li-yao

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(names for) invariants for Eq and Ord?
by Johannes Waldmann 02 Jun '18

02 Jun '18

Dear Cafe, I am somewhat surprised that the Haskell Standard (*) https://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-127000… does not contain any notation for (intended) properties of Eq and Ord instances, and their relation. For Java (standard library), the API spec uses wording like "consistent with equals" https://docs.oracle.com/javase/10/docs/api/java/lang/Comparable.html I am well aware that semantics (transitivity, etc.) cannot be enforced statically, in either language. But both standards still speak of "total order". Do we (Haskell) need something similar to "consistent with equals"? It depends. E.g., looking at a (random) source in containers (Data.Map.Internal), it seems that it will always use the result of compare, never of (==), on keys. That's not obvious from the docs. I just checked -- I can make a type where (==) = undefined, but write a proper Ord instance, and use it as key type. I am asking this because I will be teaching type classes, with Eq and Ord as examples. I detected the funny situation that the Java specification looks "more mathematical" than the corresponding Haskell one. What will the students think ... - J.W. (*) The Haskell standard is what you find when you scroll down, down, down to the very bottom of https://www.haskell.org/documentation . So, probably not that important...

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