October 2014 - ghc-devs

Re: Call Arity, oneShot, or both
by David Feuer 26 Oct '14

26 Oct '14

> There is also the option of combining both. Then we do not get the > regression, but still the improvement for fft2: I *definitely* think we should leave Call Arity in place by default unless and until something strictly better comes along. One very nice feature is that it works for a lot of user-written code of various kinds without the user having to do *anything* special. oneShot seems more limited in applicability, for use primarily in library code. So I would personally think that it should be added, with an option, of course, to turn it off. I would also go for documenting it as experimental and provisional. David

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old-time.git/tests/time004.hs is DST-sensitive :-/
by Herbert Valerio Riedel 26 Oct '14

26 Oct '14

Hi *, if anyone wonders, why TEST=time004 suddenly fails: it's sensitive to DST (and depends on your system's TZ-config): http://git.haskell.org/packages/old-time.git/blob/HEAD:/tests/time004.hs For me, the comparison this unit-test checks now suddenly fails, because the DST-switch occurs on different days in 2012 and 2013 in my TZ, i.e. length "Sun Oct 27 10:56:42 CET 2013" /= length "Sat Oct 27 11:56:42 CEST 2012" Does anyone happen to know what the rationale behind this 'time004' unit test was? Cheers, hvr

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Improving specialization, redux
by David Feuer 24 Oct '14

24 Oct '14

I spoke with Simon today, and I think I have a bit of a better idea now of what's going on with specialization, and why it sometimes fails to specialize things as much as it could. Apparently, the replacement of (sel @ type dict) by sel.type is accomplished by the use of a rewrite rule generated by the specializer when it specializes an overloaded function using that (or something similar to that, anyway). The trouble is that as things are currently done, there's no way in general to get the right replacement to stick on the RHS of the rule when such forms arise in other contexts. The type checker constructs dictionaries in type checking, but afterwards we can't produce new ones. The most likely way to solve this is probably to (somehow) decouple dictionary creation and selection from type checking so as to expose those facilities to later phases. The concept would be something vaguely like this: instead of creating rewrite rules, the specializer would instead check whether the appropriate dictionary already existed, and create it if not. The simplifier would then check for a dictionary every time it encountered a class method whose instance is determined. No, I have no idea how much work such a change would involve.

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Making GHCi awesomer?
by Christopher Done 24 Oct '14

24 Oct '14

Good evening, So I’ve been working on Haskell user-facing tooling in general for some years. By that I mean the level of Emacs talking with Haskell tools. I wrote the interactive-haskell-mode (most functionality exists in this file <https://github.com/haskell/haskell-mode/blob/master/haskell-process.el#L1> ). which launches a GHCi process in a pipe and tries very earnestly to handle input/output with the process reasonably. For Emacs fanciers: Written in Elisp, there’s a nice command queue that you put commands onto, they will all be run on a FIFO one-by-one order, and eventually you’ll get a result back. Initially it was just me using it, but with the help of Herbert Riedel it’s now a mode on equal footing with the venerable inferior-haskell-mode all ye Emacs users know and love. It’s part of haskell-mode and can be enabled by enabling the interactive-haskell-mode minor mode. For years I’ve been using GHCi as a base and it’s been very reliable for almost every project I’ve done (the only exceptions are things like SDL and OpenGL, which are well known to be difficult to load in GHCi, at least on Linux). I think we’ve built up a good set of functionality <https://github.com/haskell/haskell-mode/wiki/Haskell-Interactive-Mode> purely based on asking GHCi things and getting it to do things. I literally use GHCi for everything. For type-checking, type info, I even send “:!cabal build” to it. Everything goes through it. I love my GHCi. Now, I’m sort of at the end of the line of where I can take GHCi. Here are the problems as I see them today: 1. There is no programmatic means of communicating with the process. I can’t send a command and get a result cleanly, I have to regex match on the prompt, and that is only so reliable. At the moment we solve this by using \4 (aka ‘END OF TRANSMISSION’). Also messages (warnings, errors, etc.) need to be parsed which is also icky, especially in the REPL when e.g. a defaulted Integer warning will mix with the output. Don’t get me started on handling multi-line prompts! Hehe. 2. GHCi, as a REPL, does not distinguish between stdout, stderr and the result of your evaluation. This can be problematic for making a smooth REPL UI, your results can often (with threading) be interspersed in unkind ways. I cannot mitigate this with any kind of GHCi trickery. 3. It forgets information when you reload. (I know this is intentional.) 4. Not enough information is exposed to the user. (Is there ever? ;) 5. There is a time-to-market overhead of contributing to GHCi — if I want a cool feature, I can write it on a locally compiled version of GHC. But for the work projects I have, I’m restricted to given GHC versions, as are other people. They have to wait to get the good features. 6. This is just a personal point — I’ve like to talk to GHCi over a socket, so that I can run it on a remote machine. Those familiar with Common Lisp will be reminded of SLIME and Swank. Examples for point 4 are: - Type of sub-expressions. - Go to definition of thing at point (includes local scope). - Local-scope completion. - A hoogle-like query (as seen in Idris recently). - Documentation lookup. - Suggest imports for symbols. - Show core for the current module. - Show CMM for the current module, ASM, etc. SLIME can do this. - Expand the template-haskell at point. - The :i command is amazingly useful, but programmatic access would be even better.¹ - Case split anyone? - Etc. ¹I’ve integrated with it in Emacs so that I can C-c C-i any identifier and it’ll popup a buffer with the :i result and then within that buffer I can drill down further with C-c C-i again. It makes for very natural exploration of a type. You’ve seen some of these features in GHC Mod, in hdevtools, in the FP Haskell Center, maybe some are in Yi, possibly also in Leksah (?). So in light of point (5), I thought: I’ve used the GHC API before, it can do interactive evaluation, why not write a project like “ghc-server” which encodes all these above ideas as a “drop-in” replacement for GHCi? After all I could work on my own without anybody getting my way over architecture decisions, etc. And that’s what I did. It’s here <https://github.com/chrisdone/ghc-server>. Surprisingly, it kind of works. You run it in your directoy like you would do “cabal repl” and it sets up all the extensions and package dependencies and starts accepting connections. It will compile across three major GHC versions. Hurray! Rub our hands together and call it done, right? Sadly not, the trouble is twofold: 1. The first problem with this is that every three projects will segfault or panic when trying to load in a project that GHCi will load in happily. The reasons are mysterious to me and I’ve already lugged over the GHC API to get to this point, so that kind of thing happening means that I have to fall back to my old GHCi-based setup, and is disappointing. People have similar complaints of GHC Mod & co. “Getting it to work” is a deterrant. 2. While this would be super beneficial for me, and has been a good learning experience for “what works and what doesn’t”, we end up with yet another alternative tool, that only a few people are using. 3. There are just certain behaviours and fixes here and there that GHCi does that take time to reproduce. So let’s go back to the GHCi question: is there still a development overhead for adding features to GHCi? Yes, new ideas need acceptance and people have to wait (potentially a year) for a new feature that they could be using right now. An alternative method is to do what Herbert did which is to release a “ghci-ng” <http://hackage.haskell.org/package/ghci-ng> which sports new shiny features that people (with the right GHC version) will be able to compile and use as a drop-in for GHCi. It’s the same codebase, but with more stuff! An example is the “:complete” command, this lets IDE implementers do completion at least at the REPL level. Remember the list of features earlier? Why are they not in GHCi? So, of course, this got me thinking that I could instead make ghc-server be based off of GHCi’s actual codebase. I could rebase upon the latest GHC release and maintain 2-3 GHC versions backwards. That’s certainly doable, it would essentially give me “GHCi++”. Good for me, I just piggy back on the GHCi goodness and then use the GHC API for additional things as I’m doing now. But is there a way I can get any of this into the official repo? For example, could I hack on this (perhaps with Herbert) as “ghci-ng”, provide an alternative JSON communication layer (e.g. via some —use-json flag) and and socket listener (—listen-on ), a way to distinguish stdout/stderr (possibly by forking a process, unsure at this stage), and then any of the above features (point 4) listed. I make sure that I’m rebasing upon HEAD, as if to say ghci-ng is a kind of submodule, and then when release time comes we merge back in any new stuff since the last release. Early adopters can use ghci-ng, and everyone benefits from official GHC releases. The only snag there is that, personally speaking, it would be better if ghci-ng would compile on older GHC versions. So if GHC 7.10 is the latest release, it would still be nice (and it *seems* pretty feasible) that GHC 7.8 users could still cabal install it without issue. People shouldn’t have to wait if they don’t have to. Well, that’s everything. Thoughts? Ciao!

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RE: Does D351 Unwiring-Integer patch now look like you envisioned?
by Simon Peyton Jones 24 Oct '14

24 Oct '14

I replied, but failed to press "Submit" (which is several screenfuls away). Sorry The Maybe DataCon idea looks right to me. FWIW I *hate* the way that TidyPgm is forced to predict what CorePrep will do. I've created https://ghc.haskell.org/trac/ghc/ticket/9718 to explain. Simon | -----Original Message----- | From: Herbert Valerio Riedel [mailto:hvriedel@gmail.com] | Sent: 24 October 2014 08:48 | To: Simon Peyton Jones | Subject: Does D351 Unwiring-Integer patch now look like you | envisioned? | | Hello Simon, | | I was wondering if | | https://phabricator.haskell.org/D351 | | looks the way you expected, and more specifically I'd like some | feedback on the `DataCon` vs. `Maybe Id` comment at | | https://phabricator.haskell.org/D351#8682 | | Thanks! | hvr

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Reading type families from interface files
by Jan Stolarek 23 Oct '14

23 Oct '14

Devs, Is there a plumbing for returning declarations (not instances) of type families from an interface file? Janek

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Current description of Core?
by Sophie Taylor 23 Oct '14

23 Oct '14

Hi, Is the current description of Core still System FC_2 (described in https://www.seas.upenn.edu/~sweirich/papers/popl163af-weirich.pdf)?

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Avoiding the hazards of orphan instances without dependency problems
by David Feuer 22 Oct '14

22 Oct '14

Orphan instances are bad. The standard approach to avoiding the orphan hazard is to always put an instance declaration in the module that declares the type or the one that declares the class. Unfortunately, this forces packages like lens to have an ungodly number of dependencies. Yesterday, I had a simple germ of an idea for solving this (fairly narrow) problem, at least in some cases: allow a programmer to declare where an instance declaration must be. I have no sense of sane syntax, but the rough idea is: {-# InstanceIn NamedModule [Context =>] C1 T1 [T2 ...] #-} This pragma would appear in a module declaring a class or type. The named module would not have to be available, either now or ever, but attempting to declare such an instance in any module *other* than the named one would be an error by default, with a flag -XAllowForbiddenInstancesAndInviteNasalDemons to turn it off. The optional context allows multiple such pragmas to appear in the type/class-declaring modules, to allow overlapping instances (all of them declared in advance).

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RE: D202: Injective type families
by Simon Peyton Jones 22 Oct '14

22 Oct '14

Devs I now know how to use 'arc patch' to get a Phab ticket onto my disk. But if I edit the code, can I make a git commit and upload it back to D202? That would be akin to sharing a branch with (in this case Jan) the author. How do I do that? It is often more direct than making comments. Simon ________________________________________ From: noreply(a)phabricator.haskell.org [noreply(a)phabricator.haskell.org] Sent: 22 October 2014 14:18 To: Simon Peyton Jones Subject: [Differential] [Updated, 486 lines] D202: Injective type families jstolarek updated this revision to Diff 961. jstolarek added a comment. The implementation of parser and renamer is finished (subject to revisions of course). The parser recognizes injectivity declaration for closed and open type families and for associated types. Renamer checks whether the injectivity condition is well-formed. If it's not then it prints out informative (hopefully) error message. I've made the decission to require that type variables on the RHS of injectivity condition should be placed in exactly the same order as they were declared in type family head. Thanks to this the implementation of renaming is linear in the numer of type variables. Also, SynTyCon stores a list of Bools and so the functions are allowed to be injective only in some arguments. What remains to be done is: 1. type checking of injectivity (ie. whether the function declared as injective really is injective). 2. extending constraint solver with knowledge about injectivity REPOSITORY rGHC Glasgow Haskell Compiler CHANGES SINCE LAST UPDATE https://phabricator.haskell.org/D202?vs=960&id=961 BRANCH T6018-injective-type-families REVISION DETAIL https://phabricator.haskell.org/D202 AFFECTED FILES compiler/deSugar/DsMeta.hs compiler/hsSyn/Convert.lhs compiler/hsSyn/HsDecls.lhs compiler/iface/BuildTyCl.lhs compiler/iface/IfaceSyn.lhs compiler/iface/MkIface.lhs compiler/iface/TcIface.lhs compiler/parser/Parser.y.pp compiler/parser/RdrHsSyn.lhs compiler/prelude/TysPrim.lhs compiler/rename/RnSource.lhs compiler/rename/RnTypes.lhs compiler/typecheck/TcCanonical.lhs compiler/typecheck/TcInstDcls.lhs compiler/typecheck/TcInteract.lhs compiler/typecheck/TcTyClsDecls.lhs compiler/typecheck/TcTypeNats.hs compiler/types/CoAxiom.lhs compiler/types/FamInstEnv.lhs compiler/types/TyCon.lhs compiler/vectorise/Vectorise/Type/Env.hs REPLY HANDLER ACTIONS Reply to comment, or !reject, !abandon, !reclaim, !resign, !rethink, !unsubscribe. To: jstolarek, simonpj, goldfire, austin Cc: thomie, goldfire, simonmar, ezyang, carter, Mikolaj

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Re: [commit: ghc] master: Reify data family instances correctly. (e319d6d)
by Joachim Breitner 22 Oct '14

22 Oct '14

Hi Richard, Travis is complaining about your commit: Actual stderr output differs from expected: --- ./th/T9692.stderr 2014-10-21 13:22:53.783212762 +0000 +++ ./th/T9692.comp.stderr 2014-10-21 13:52:36.314083513 +0000 @@ -1,2 +0,0 @@ -data family T9692.F (a_0 :: k_1) (b_2 :: k_3) :: * -data instance T9692.F GHC.Types.Int x_4 = T9692.FInt x_4 *** unexpected failure for T9692(normal) https://s3.amazonaws.com/archive.travis-ci.org/jobs/38604151/log.txt It maybe be due to a missing flushing of stdout; I added that line and will let you know if that was not it. Greetings, Joachim Greetings, Joachim Am Dienstag, den 21.10.2014, 13:21 +0000 schrieb git(a)git.haskell.org: > Repository : ssh://git@git.haskell.org/ghc > > On branch : master > Link : http://ghc.haskell.org/trac/ghc/changeset/e319d6d2704edc2696f47409f85f4d4ce… > > >--------------------------------------------------------------- > > commit e319d6d2704edc2696f47409f85f4d4ce58a6cc4 > Author: Richard Eisenberg <eir(a)cis.upenn.edu> > Date: Mon Oct 20 15:36:57 2014 -0400 > > Reify data family instances correctly. > > Summary: > Fix #9692. > > The reifier didn't account for the possibility that data/newtype > instances are sometimes eta-reduced. It now eta-expands as necessary. > > Test Plan: th/T9692 > > Reviewers: simonpj, austin > > Subscribers: thomie, carter, ezyang, simonmar > > Differential Revision: https://phabricator.haskell.org/D355 > > > >--------------------------------------------------------------- > > e319d6d2704edc2696f47409f85f4d4ce58a6cc4 > compiler/typecheck/TcSplice.lhs | 10 +++++++++- > 1 file changed, 9 insertions(+), 1 deletion(-) > > diff --git a/compiler/typecheck/TcSplice.lhs b/compiler/typecheck/TcSplice.lhs > index bb6af8c..e952a27 100644 > --- a/compiler/typecheck/TcSplice.lhs > +++ b/compiler/typecheck/TcSplice.lhs > @@ -1338,8 +1338,16 @@ reifyFamilyInstance (FamInst { fi_flavor = flavor > DataFamilyInst rep_tc -> > do { let tvs = tyConTyVars rep_tc > fam' = reifyName fam > + > + -- eta-expand lhs types, because sometimes data/newtype > + -- instances are eta-reduced; See Trac #9692 > + -- See Note [Eta reduction for data family axioms] > + -- in TcInstDcls > + (_rep_tc, rep_tc_args) = splitTyConApp rhs > + etad_tyvars = dropList rep_tc_args tvs > + eta_expanded_lhs = lhs `chkAppend` mkTyVarTys etad_tyvars > ; cons <- mapM (reifyDataCon (mkTyVarTys tvs)) (tyConDataCons rep_tc) > - ; th_tys <- reifyTypes lhs > + ; th_tys <- reifyTypes (filter (not . isKind) eta_expanded_lhs) > ; return (if isNewTyCon rep_tc > then TH.NewtypeInstD [] fam' th_tys (head cons) [] > else TH.DataInstD [] fam' th_tys cons []) } > > _______________________________________________ > ghc-commits mailing list > ghc-commits(a)haskell.org > http://www.haskell.org/mailman/listinfo/ghc-commits > -- Joachim “nomeata” Breitner mail(a)joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata(a)joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata(a)debian.org

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