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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Richard,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">I’ve taken another run at the constraint solver.
<b>It’s on wip/spj-tc-branch3</b> (you’ll need a forced update).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">I think I can safely commit it to HEAD in the light of your earlier comments, so I’ll do that soon, but I thought I’d just give you (and other ghc-devs) a heads-up.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">The two main commits are below. The former is small; the latter is the main event.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="Code">commit 570c3181342386b5cee1862f85a8ebed7d98d712<o:p></o:p></p>
<p class="Code">Author: Simon Peyton Jones <simonpj@microsoft.com><o:p></o:p></p>
<p class="Code">Date: Wed Nov 23 16:00:00 2016 +0000<o:p></o:p></p>
<p class="Code"><o:p> </o:p></p>
<p class="Code"> Allow TyVars in TcTypes<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> Up to now we've had a rule that a TyVar can't apppear in a type<o:p></o:p></p>
<p class="Code"> seen by the type checker; they should all be TcTyVars. But:<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> a) With -XTypeInType it becomes much harder to exclude them;<o:p></o:p></p>
<p class="Code"> see Note [TcTyVars in the typechecker] in TcType.<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> b) It's unnecessary to exculde them; instead we can just treat<o:p></o:p></p>
<p class="Code"> a TyVar just like vanillaSkolemTv.<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> This is what was causing an ASSERT error in<o:p></o:p></p>
<p class="Code"> indexed-types/should_fail/T12041, reported in Trac #12826.<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> That patch allows a TyVar in a TcType. The most significant<o:p></o:p></p>
<p class="Code"> change is to make Var.tcTyVarDetails return vanillaSkolemTv.<o:p></o:p></p>
<p class="Code"> In fact it already did, but (a) it was not documented, and<o:p></o:p></p>
<p class="Code"> (b) we never exploited it. Now we rely on it.<o:p></o:p></p>
<p class="Code"><o:p> </o:p></p>
<p class="Code">commit 6e6d93b8b713339dc5f74e8f93e35d0d94014d63<o:p></o:p></p>
<p class="Code">Author: Simon Peyton Jones <simonpj@microsoft.com><o:p></o:p></p>
<p class="Code">Date: Tue Oct 25 17:41:45 2016 +0100<o:p></o:p></p>
<p class="Code"><o:p> </o:p></p>
<p class="Code"> Another major constraint-solver refactoring<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> This patch takes further my refactoring of the constraint<o:p></o:p></p>
<p class="Code"> solver, which I've been doing over the last couple of months<o:p></o:p></p>
<p class="Code"> in consultation with Richard.<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> It fixes a number of tricky bugs that made the constraint<o:p></o:p></p>
<p class="Code"> solver actually go into a loop, including<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> Trac #12526<o:p></o:p></p>
<p class="Code"> Trac #12444<o:p></o:p></p>
<p class="Code"> Trac #12538<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> The main changes are these<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> * Flatten unification variables (fmvs/fuvs) appear on the LHS<o:p></o:p></p>
<p class="Code"> of a tvar/tyvar equality; thus<o:p></o:p></p>
<p class="Code"> fmv ~ alpha<o:p></o:p></p>
<p class="Code"> and not<o:p></o:p></p>
<p class="Code"> alpha ~ fmv<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> See Note [Put flatten unification variables on the left]<o:p></o:p></p>
<p class="Code"> in TcUnify. This is implemented by TcUnify.swapOverTyVars.<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> * Don't reduce a "loopy" CFunEqCan where the fsk appears on<o:p></o:p></p>
<p class="Code"> the LHS:<o:p></o:p></p>
<p class="Code"> F t1 .. tn ~ fsk<o:p></o:p></p>
<p class="Code"> where 'fsk' is free in t1..tn.<o:p></o:p></p>
<p class="Code"> See Note [FunEq occurs-check principle] in TcInteract<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> This neatly stops some infinite loops that people reported;<o:p></o:p></p>
<p class="Code"> and it allows us to delete some crufty code in reduce_top_fun_eq.<o:p></o:p></p>
<p class="Code"> And it appears to be no loss whatsoever.<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> As well as fixing loops, ContextStack2 and T5837 both terminate<o:p></o:p></p>
<p class="Code"> when they didn't before.<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> * Previously we generated "derived shadow" constraints from<o:p></o:p></p>
<p class="Code"> Wanteds, but we could (and sometimes did; Trac #xxxx) repeatedly<o:p></o:p></p>
<p class="Code"> generate a derived shadow from the same Wanted.<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> A big change in this patch is to have two kinds of Wanteds:<o:p></o:p></p>
<p class="Code"> [WD] behaves like a pair of a Wanted and a Derived<o:p></o:p></p>
<p class="Code"> [W] behaves like a Wanted only<o:p></o:p></p>
<p class="Code"> See CtFlavour and ShadowInfo in TcRnTypes, and the ctev_nosh<o:p></o:p></p>
<p class="Code"> field of a Wanted.<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> This turned out to be a lot simpler. A [WD] gets split into a<o:p></o:p></p>
<p class="Code"> [W] and a [D] in TcSMonad.maybeEmitShaodow.<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> See TcSMonad Note [The improvement story and derived shadows]<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> * Rather than have a separate inert_model in the InertCans, I've<o:p></o:p></p>
<p class="Code"> put the derived equalities back into inert_eqs. We weren't<o:p></o:p></p>
<p class="Code"> gaining anything from a separate field.<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> * Previously we had a mode for the constraint solver in which it<o:p></o:p></p>
<p class="Code"> would more aggressively solve Derived constraints; it was used<o:p></o:p></p>
<p class="Code"> for simplifying the context of a 'deriving' clause, or a 'default'<o:p></o:p></p>
<p class="Code"> delcaration, for example.<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> But the complexity wasn't worth it; now I just make proper Wanted<o:p></o:p></p>
<p class="Code"> constraints. See TcMType.cloneWC<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> * Don't generate injectivity improvement for Givens; see<o:p></o:p></p>
<p class="Code"> Note [No FunEq improvement for Givens] in TcInteract<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> * solveSimpleWanteds leaves the insolubles in-place rather than<o:p></o:p></p>
<p class="Code"> returning them. Simpler.<o:p></o:p></p>
<p class="Code"> <o:p></o:p></p>
<p class="Code"> I also did lots of work on comments.<span style="font-size:12.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
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