- Haskell-Cafe - Haskell.org

How to expose if a constraint is satisfiable
by Clinton Mead 09 May '18

09 May '18

It's occurred to me that one could write a class C t which is satisfied whenever (A t) or (B t) is satisfied like so: --- data Satisfied type family IsSatisfiable :: Constraint -> Type class A t class B t class C' t ta tb instance A t => C' t Satisfied tb where ... instance B t => C' t ta Satisfied where ... instance (A t, B t) => C' t Satisfied Satisfied where ... class C' t ( IsSatisfiable (A t)) ( IsSatisfiable (B t)) => C t --- We may need overlapping instances (with all the caveats that come with it) but it should be fine otherwise. "IsSatisfiable" here is defined as follows: IsSatisfiable c = Satisfied -- (if c is satisfiable) IsSatisfiable c = IsSatisfiable c -- (if c is not satisfiable) That's all that's needed. And it seems to be a reasonable type function. I note classes are open, so perhaps it could be dangerous to state that a constraint is not satisfiable (because it might be in another part of a program) but simply not reducing if we can't satisfy the constraint locally should be fine I think. At worst we'll get a compile error, but we shouldn't get inconsistent types. Is there anyway to implement this type function, or alternatively an approach which allows this type of "inherit from two classes" idea?

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Re: [Haskell-cafe] How to expose if a constraint is satisfiable
by Anthony Clayden 08 May '18

08 May '18

> It's occurred to me that one could write a class C t which is satisfied > whenever (A t) or (B t) is satisfied like so: Hi Clinton, this sounds like you might want "Choosing a type-class instance based on the context" https://wiki.haskell.org/GHC/AdvancedOverlap > --- > > data Satisfied > > type family IsSatisfiable :: Constraint -> Type That type family is doing the same job as auxiliary class `ShowPred` on that wiki page. Rather than all the machinery you give for the disjunction, you could use another type family: type family EitherSatisfied :: Type -> Type -> Type instance EitherSatisfied Satisfied tb = Satisfied instance EitherSatisfied ta Satisfied = Satisfied Those two instances do overlap (as you expected); and they exhibit confluence, aka coincident overlap (they produce the same result); so that's fine. But you haven't given any instances for `IsSatisfiable`. How do you expect to get from the Constraint to the `Satisfied` type? You say > IsSatisfiable c = Satisfied -- (if c is satisfiable) What are you going to put for `c`? If you follow that wiki page, you'll need to in effect repeat every instance decl for classes `A, B`: instance A Int where ... type instance IsSatisfiable (A Int) = Satisfied (The wiki page was written before there were type families, so type class `ShowPred` has a Functional Dependency giving a type-level Boolean result.) Your `C t` class becomes class EitherSatisfied ( IsSatisfiable (A t)) ( IsSatisfiable (B t)) ~ Satisfied => C t where ... ---- Nowadays there's a better way: make Associated Types for the two classes, give them a default instance: class A t where type APred t type instance APred t = Satisfied ... class B t where type BPred t type instance BPred t = Satisfied ... Now every instance defined for `A, B` in effect automatically gives you `APred, BPred` instances. Then class EitherSatisifed (APred t) (BPred t) ~ Satisfied => C t where ... AntC

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help with emacs intero
by Dennis Raddle 06 May '18

06 May '18

I started having trouble with Emacs intero mode a week ago. I've tried to get help on the Intero issues list as well as Stack Overflow to no avail. I'm wondering if anyone here knows enough about Intero or Emacs to at least point me to the next debugging step. At this point I don't enough to know what to try or look for. Basically flycheck stopped working, and every time I type a few characters I'm getting "Error while checking syntax automatically: (wrong-type-argument number-or-marker-p "Abort trap: 6") Any clue what this is referring to, or why it would suddenly start last week, when all was fine prior?

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Announce: Haskell Platform 8.4.2
by Gershom B 05 May '18

05 May '18

On behalf of the Haskell Platform team, I'm happy to announce the release of Haskell Platform 8.4.2 Now available at https://www.haskell.org/platform/ This includes GHC 8.4.2, cabal-install 2.2.0.0, and stack 1.7.1, all with substantial improvements since the last platform release. A full list of contents is available at https://www.haskell.org/platform/contents.html Thanks to all the contributors to this release, thanks to all the package and tool maintainers and authors, and a big thanks to the GHC team for all their hard work. There are (still) currently no 32 bit Windows builds available. We're looking into the issues preventing us from building an installer for that platform. The components all appear to work individually in such a case, and can be installed separately by users who so desire. A list of new GHC changes is available at: https://ghc.haskell.org/trac/ghc/blog/ghc-8.4.1-released and https://ghc.haskell.org/trac/ghc/blog/ghc-8.4.2-released A list of cabal changes is available at: https://hackage.haskell.org/package/cabal-install-2.2.0.0/changelog The cabal documentation page is at: https://www.haskell.org/cabal/users-guide/ A list of stack changes is at: https://docs.haskellstack.org/en/stable/ChangeLog/#v171 The stack documentation page is at: https://docs.haskellstack.org/en/stable/README/ Happy Haskell Hacking all, Gershom

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Picking up the CI buildnumber during build
by Magnus Therning 05 May '18

05 May '18

I'm looking for a way to pick up the CI buildnumber from the environment (e.g. $TRAVIS_BUILD_NUMBER) and use it as part of of the output of `my-tool --version`. I don't really know where to start looking, so I'm hoping someone on the list has already done something similar and can point me to a working example :) /M -- Magnus Therning OpenPGP: 0x927912051716CE39 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus I am always doing that which I cannot do, in order that I may learn how to do it. — Pablo Picasso

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Strange error with GADT DataKinds -- needs TypeInType!?
by Tom Ellis 05 May '18

05 May '18

Can anyone explain why the type instance for K is fine but the one for I needs TypeInType? (It does indeed work when I turn on TypeInType.) {-# LANGUAGE GADTs #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} data T a b where K :: b -> T a b I :: T a a type family F (a :: T ka kb) -- Data constructor ‘I’ cannot be used here -- (Perhaps you intended to use TypeInType) -- type instance F 'I = () type instance F ('K b) = ()

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emacs heads up
by Rustom Mody 03 May '18

03 May '18

There seems to be some significant developments in emacs' haskell mode Can I understand?? The run-haskell command is now evidently deprecated What is the new alternative? Or do we just stay with run-haskell? Context: - Teaching Haskell - Using emacs - Need a nice REPL - Not (immediately) getting into in stack cabal etc; though will be needed in due course

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[ANNOUNCE] GHC 8.4.2 released
by Ben Gamari 01 May '18

01 May '18

Hello everyone, The GHC team is pleased to announce the availability of GHC 8.4.2. The source distribution, binary distributions, and documentation for this release are available at https://downloads.haskell.org/~ghc/8.4.2 This release is a bug-fix release, fixing numerous regressions and bugs present in GHC 8.4.1. These include: * A regression resulting in some uses of `Control.Exception.evaluate` to be inappropriately optimised away (see #13930) * A regression resulting in segmentation faults of programs compiled with profiling (#14705) * A bug causing runtime system panics while running programs with retainer profiling (#14947) * The configure scripts now accepts a `--disable-dtrace` option, again allowing GHC to be bootstrapped on FreeBSD (#15040) * The version number of the `base` package has been bumped to 4.11.1.0 to reflect the addition of the `GHC.IO.FixIOException` type. This interface was added in 8.4.1 but the version bump was missed due to an oversight. * Support for DWARF debug information has been significantly improved (#14894, #14779) A more thorough list of the changes in this release can be found in the release notes, https://downloads.haskell.org/~ghc/8.4.2/docs/html/users_guide/8.4.2-notes.… Thanks to everyone who has contributed to developing, documenting, and testing this release! As always, let us know if you encounter trouble. How to get it ~~~~~~~~~~~~~ The easy way is to go to the web page, which should be self-explanatory: http://www.haskell.org/ghc/ We supply binary builds in the native package format for many platforms, and the source distribution is available from the same place. Packages will appear as they are built - if the package for your system isn't available yet, please try again later. Background ~~~~~~~~~~ Haskell is a standard lazy functional programming language. GHC is a state-of-the-art programming suite for Haskell. Included is an optimising compiler generating efficient code for a variety of platforms, together with an interactive system for convenient, quick development. The distribution includes space and time profiling facilities, a large collection of libraries, and support for various language extensions, including concurrency, exceptions, and foreign language interfaces. GHC is distributed under a BSD-style open source license. A wide variety of Haskell related resources (tutorials, libraries, specifications, documentation, compilers, interpreters, references, contact information, links to research groups) are available from the Haskell home page (see below). On-line GHC-related resources ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Relevant URLs on the World-Wide Web: GHC home page http://www.haskell.org/ghc/ GHC developers' home page http://ghc.haskell.org/trac/ghc/ Haskell home page http://www.haskell.org/ Supported Platforms ~~~~~~~~~~~~~~~~~~~ The list of platforms we support, and the people responsible for them, is here: http://ghc.haskell.org/trac/ghc/wiki/Contributors Ports to other platforms are possible with varying degrees of difficulty. The Building Guide describes how to go about porting to a new platform: http://ghc.haskell.org/trac/ghc/wiki/Building Developers ~~~~~~~~~~ We welcome new contributors. Instructions on accessing our source code repository, and getting started with hacking on GHC, are available from the GHC's developer's site run by Trac: http://ghc.haskell.org/trac/ghc/ Mailing lists ~~~~~~~~~~~~~ We run mailing lists for GHC users and bug reports; to subscribe, use the web interfaces at http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-tickets There are several other haskell and ghc-related mailing lists on www.haskell.org; for the full list, see https://mail.haskell.org/cgi-bin/mailman/listinfo Some GHC developers hang out on #haskell on IRC, too: http://www.haskell.org/haskellwiki/IRC_channel Please report bugs using our bug tracking system. Instructions on reporting bugs can be found here: http://www.haskell.org/ghc/reportabug

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3rd CfP: IFL 2018 (30th Symposium on Implementation and Application of Functional Languages)
by Jurriaan Hage 01 May '18

01 May '18

Hello, Please, find below the third call for papers for IFL 2018. Please forward these to anyone you think may be interested. Apologies for any duplicates you may receive. best regards, Jurriaan Hage Publicity Chair of IFL --- ============================================================ ==================== IFL 2018 30th Symposium on Implementation and Application of Functional Languages University of Massachusetts Lowell, MA, USA September 5th-7th, 2018 http://iflconference.org ============================================================ ==================== ### Scope The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2018 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Topics of interest to IFL include, but are not limited to: - language concepts - type systems, type checking, type inferencing - compilation techniques - staged compilation - run-time function specialization - run-time code generation - partial evaluation - (abstract) interpretation - metaprogramming - generic programming - automatic program generation - array processing - concurrent/parallel programming - concurrent/parallel program execution - embedded systems - web applications - (embedded) domain specific languages - security - novel memory management techniques - run-time profiling performance measurements - debugging and tracing - virtual/abstract machine architectures - validation, verification of functional programs - tools and programming techniques - (industrial) applications ### Keynote Speakers * Adam Chlipala, Massachusetts Institute of Technology CSAIL * Arjun Guha, University of Massachusetts Amherst ### Submissions and peer-review Differently from previous editions of IFL, IFL 2018 solicits two kinds of submissions: * Regular papers (12 pages including references) * Draft papers for presentations ('weak' limit between 8 and 15 pages) Regular papers will undergo a rigorous review by the program committee, and will be evaluated according to their correctness, novelty, originality, relevance, significance, and clarity. A set of regular papers will be conditionally accepted for publication. Authors of conditionally accepted papers will be provided with committee reviews along with a set of mandatory revisions. Regular papers not accepted for publication will be considered as draft papers, at the request of the author. Draft papers will be screened to make sure that they are within the scope of IFL, and will be accepted for presentation or rejected accordingly. Prior to the symposium: Authors of conditionally accepted papers and accepted presentations will submit a pre-proceedings version of their work that will appear in the draft proceedings distributed at the symposium. The draft proceedings does not constitute a formal publication. We require that at least one of the authors present the work at IFL 2018. After the symposium: Authors of conditionally accepted papers will submit a revised versions of their paper for the formal post-proceedings. The program committee will assess whether the mandatory revisions have been adequately addressed by the authors and thereby determines the final accept/reject status of the paper. Our interest is to ultimately accept all conditionally accepted papers. If you are an author of a conditionally accepted paper, please make sure that you address all the concerns of the reviewers. Authors of accepted presentations will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full article for the formal post-proceedings. The program committee will evaluate these submissions according to their correctness, novelty, originality, relevance, significance, and clarity, and will thereby determine whether the paper is accepted or rejected. ### Publication The formal proceedings will appear in the International Conference Proceedings Series of the ACM Digital Library. At no time may work submitted to IFL be simultaneously submitted to other venues; submissions must adhere to ACM SIGPLAN's republication policy: http://www.sigplan.org/Resources/Policies/Republication ### Important dates Submission of regular papers: May 25, 2018 Submission of draft papers: July 17, 2018 Regular and draft papers notification: July 20, 2018 Deadline for early registration: August 8, 2018 Submission of pre-proceedings version: August 29, 2018 IFL Symposium: September 5-7, 2018 Submission of papers for post-proceedings: November 7, 2018 Notification of acceptance: December 22, 2018 Camera-ready version: February 10, 2019 ### Submission details All contributions must be written in English. Papers must use the ACM two columns conference format, which can be found at: http://www.acm.org/publications/proceedings-template Authors submit through EasyChair: https://easychair.org/conferences/?conf=ifl2018 ### Peter Landin Prize The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honored article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. ### Organization and Program committee Chairs: Jay McCarthy & Matteo Cimini, University of Massachusetts Lowell, USA Program Committee: * Arthur Chargueraud, Inria, FR * Ben Delaware, Purdue University, USA * Christos Dimoulas, Northwestern University, USA * David Darais, University of Vermont, USA * Dominic Orchard, University of Kent, UK * Ekaterina Komendantskaya, Heriot-Watt University, UK * Garrett Morris, University of Kansas, USA * Heather Miller, EPFL & Northeastern University, CH & USA * Jeremy Yallop, University of Cambridge, UK * Keiko Nakata, SAP Innovation Center Potsdam, DE * Laura Castro, University of A Coruna, ESP * Magnus Myreen, Chalmers University of Technology, SWE * Natalia Chechina, Bournemouth University, UK * Peter Achten, Radboud Universiteit Nijmegen, NL * Peter-Michael Osera, Grinnell College, USA * Richard Eisenberg, Bryn Mawr College, USA * Trevor McDonell, University of New South Wales, AUS * Yukiyoshi Kameyama, University of Tsukuba, JAP ### Venue The 30th IFL is organized by the University of Massachusetts Lowell. The City of Lowell is located at the heart of the Merrimack Valley just 30 miles northwest of Boston. Lowell can be easily reached by train or taxi. See the website for more information on the venue. ### Acknowledgments This call-for-papers is an adaptation and evolution of content from previous instances of IFL. We are grateful to prior organizers for their work, which is reused here. A part of IFL 2018 format and CFP language that describes conditionally accepted papers has been adapted from call-for-papers of OOPSLA conferences.

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<<loop>> as result of SemigroupMonoid changes
by Joey Hess 01 May '18

01 May '18

I tried to follow the instructions at https://prime.haskell.org/wiki/Libraries/Proposals/SemigroupMonoid and I got a <<loop>> when my code runs with ghc 8.2.2. My code looked like this: import Data.Monoid import Prelude data InfoVal v = NoInfoVal | InfoVal v deriving (Show) instance Monoid (InfoVal v) where mempty = NoInfoVal mappend _ v@(InfoVal _) = v mappend v NoInfoVal = v Following the "recommended variant" for compatible code, I changed that too: {-# LANGUAGE CPP #-} import Data.Monoid import qualified Data.Semigroup as Sem import Prelude data InfoVal v = NoInfoVal | InfoVal v deriving (Show) instance Monoid (InfoVal v) where mempty = NoInfoVal #if !(MIN_VERSION_base(4,11,0)) mappend = (<>) #endif instance Sem.Semigroup (InfoVal v) where _ <> v@(InfoVal _) = v v <> NoInfoVal = v This loops because <> comes from Data.Monoid not from Data.Semigroup, so mappend = mappend. Note that I diverged slightly from the instructions to get to this wrong code; I imported Data.Semigroup qualified. Without the qualification, the above code fails to compile, with Ambiguous occurrence ‘<>’ So, the transition instructions don't produce code that ghc 8.2.2 can build, and when the obvious fix is made to get it to compile, it compiles into a loop, that is *not* a loop when compiled with newer versions of ghc. -- see shy jo

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