Here’s how it works: * The rewrite from opi (D m1 … mn) --> mi is done by a BuiltinRule: see MkId.mkDictSelId, and the BuiltinRule that is made there. * At the moment, BuiltinRules are always active (in all phases), see GHC.Core.ruleActivation. To allow them to be selectively active, we’d have to give them a ru_act fiels, like ordinary Rules. That would not be hard. * The phases go * InitialPhase * 2 * 1 * 0 * We could make classop rules active only in phase 1 and 0, say. I don’t know what the consequences would be; running the classop to pick a method out of a dictionary in turn reveals new function applications that might want to work in phase 2, say. * Of course you can always add more phases, but that adds compile time. * Would you want the classop phase to be fixed for every classop? Or controllable for each classop individually. E.g. class C a where { op :: <bype> {-# INLINE [2] op #-} } Here the intent is that, since the pragmas is in the class decl, the pragma applies to the method selector. I remember Conal raising this before, but I’ve forgotten the resolution. I’m entirely open to changes here, if someone is willing to do the work, including checking for consequences. Simon From: ghc-devs <ghc-devs-bounces@haskell.org> On Behalf Of Conal Elliott Sent: 06 March 2020 17:37 To: Christiaan Baaij <christiaan.baaij@gmail.com> Cc: ghc-devs <ghc-devs@haskell.org> Subject: Re: Class op rules Thank you for raising this issue, Christiaan! The current policy (very early class-op inlining) is a major difficulty and the main source of fragility in my compiling-to-categories implementation. I have a tediously programmed and delicately balanced collection of techniques to intercept and transform class ops to non-ops early and then transform back late for elimination, but it doesn't work in all situations. Since class operations roughly correspond to operations in various algebraic abstractions---interfaces with laws---I often want to exploit exactly those laws as rewrite rules, and yet those rules currently cannot be used dependably. - Conal On Fri, Mar 6, 2020 at 7:22 AM Christiaan Baaij <christiaan.baaij@gmail.com<mailto:christiaan.baaij@gmail.com>> wrote: Hello, The other day I was experimenting with RULES and got this warning: src/Clash/Sized/Vector.hs:2159:11: warning: [-Winline-rule-shadowing] Rule "map Pack" may never fire because rule "Class op pack" for ‘pack’ might fire first Probable fix: add phase [n] or [~n] to the competing rule | 2159 | {-# RULES "map Pack" map pack = id #-} The warning seems to suggests two things: 1. "Class op" -> "dictionary projection" are implemented as rewrite rules and executed the same way as other user-defined RULES 2. These rules run first, and you cannot run anything before them Now my question is, is 1. actually true? or is that warning just a (white) lie? If 1. is actually true, would there be any objections to adding a "-1" phase: where RULES specified to start from phase "-1" onward fire before any of the Class op rules. I'm quite willing to implement the above if A) Class op rules are actually implemented as builtin RULES; B) there a no objections to this "-1" phase. Thanks, Christiaan _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org<mailto:ghc-devs@haskell.org> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs