May 2018 - Haskell-Cafe

Re: [Haskell-cafe] How to expose if a constraint is satisfiable
by Anthony Clayden 09 May '18

09 May '18

Thanks David, Oliver, I'll leave it so Clinton whether that helps his use case. I see the write-ups for both the Constraint Unions package and ifCxt mentioned on the reddit thread talk about needing large amounts of boilerplate. That is, an instance for each class and type that effectively tells the class has an instance for that type. So that's the same as the AdvancedOverlap I linked to -- from Oleg + SPJ. IIUC, what Clinton's asking is: since there are already instances for `Show Int`, `Ord Float`, etc (declared in the Prelude), why can't ghc 'just know' that and expose it to type-level logic that can say: if there's `Show a` do this, otherwise do that? AntC > On *Tue May 8 22:29:20 UTC 2018*, David Feuer wrote: > Maybe look at https://www.reddit.com/r/haskell/comments/6k86je/constraint_unions_bringing… > (relating to the repository Oliver links to) and note also Edward Kmett's response. Eurrm. Could somebody explain that response using familiar language. I think the `:-` operator might be referring to Edward's & David's 'constraints' package on github, for which I've asked if there's human-facing documentation. Also asked if it'll be superseded by the 'Qualified Constraints' new development. >> On Tue, May 8, 2018 at 9:52 AM, Oliver Charles <ollie at ocharles.org.uk <http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe>> wrote: >>* I've only skimmed the thread, so sorry if this is a red herring, but could *>>* this be helpful?*

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Scoped Type Variables discussion forum [was: open up the issues tracker on ghc-proposals]
by Anthony Clayden 08 May '18

08 May '18

This thread is a discussion about discussions, not the discussion itself ;-) I'm cc'ing to the cafe; but I'd prefer replies to come to glasgow-haskell-users. >> I can volunteer to at least scrape together all the objections to ScopedTypeVariables as currently. It's not yet a proposal, so not on github. Start a wiki page? A cafe thread? (It'll get lost.) A ghc-users thread? (It'll get ignored.) > ... don’t care what forums or list or whatever. As long as it’s collated and such > It could even be on the prime issue tracker for prime proposals. Just that it’s written down :) Thanks Carter, but I understand Haskell Prime to be to assess mature/stable proposals (preferably already delivered as extensions). This discussion is at first going to be more exploratory: * likes and dislikes about ScopedTypeVariables as currently. * confusions experienced by users (especially newbies) -- although absolute newbies wouldn't be using it(?), so intermediates? * feedback from those teaching Haskell. * wild ideas for possible alternative designs. * possible improvements to the current design. * I think we're all agreed that ScopedTypeVariables should have been in Haskell from the beginning; but it wasn't, so now we have to worry about backwards compatibility for programs that worked around the omission. Or do we? What code would break? How much pain would that cause? * anything else? > We have lots of forums, but your point is that certain sorts of discussions never get going with the right audience – you especially point to “confused beginners”. > ... It’s quite a challenge because beginners tend not to be vocal, and yet they are a crucial set of Haskell users. Every Haskell user started as a beginner. On this particular topic, there's plenty of confused people asking questions on StackOverflow. (Heads up: they're especially asking why they need explicit `forall` whereas in reguar Haskell that 'intermediates' see, the forall is implicit.) Can other people point me to questions/likes/dislikes on other forums? Reddit for example. If you've read this far, you now understand what we're trying to cover. It's going to be random/varied thoughts at first, then perhaps coalescing to an approach or two. At that point a formal proposal on github proper; and the random stuff might be interesting background but will essentially get archived/thrown away. I do agree with David's suggestion that github Issue tracker looks like a suitable solution. We can write formatted code and text. We can add links and references. What do others think? Joachim has opened up Issues tracker, as a try-out. If using it doesn't work out, that's fine and in keeping with my "thrown away" above. Also where else should I post links to this message to 'advertise' the thread? I don't reddit much, so if that's suitable, please someone post there. Thank you AntC

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How to expose if a constraint is satisfiable
by Clinton Mead 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: --- 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 05 May '18

05 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 04 May '18

04 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 04 May '18

04 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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