#12821: Find the incredible vanishing [The improvement story] -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by bgamari): It looks like ddbb97d00fdbc5870a4076ed15af8e607b161cb2 is the culprit. Simon, do you think you can clean up the remaining references? Also, it looks like the original note nicely described the goal of improvement and had some useful examples for newcomers. It's not clear that any of the notes that replaced it are quite so approachable to newcomers. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12821#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#12821: Find the incredible vanishing [The improvement story] -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones ): In [changeset:"1eec1f21268af907f59b5d5c071a9a25de7369c7/ghc" 1eec1f21/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="1eec1f21268af907f59b5d5c071a9a25de7369c7" Another major constraint-solver refactoring This patch takes further my refactoring of the constraint solver, which I've been doing over the last couple of months in consultation with Richard. It fixes a number of tricky bugs that made the constraint solver actually go into a loop, including Trac #12526 Trac #12444 Trac #12538 The main changes are these * Flatten unification variables (fmvs/fuvs) appear on the LHS of a tvar/tyvar equality; thus fmv ~ alpha and not alpha ~ fmv See Note [Put flatten unification variables on the left] in TcUnify. This is implemented by TcUnify.swapOverTyVars. * Don't reduce a "loopy" CFunEqCan where the fsk appears on the LHS: F t1 .. tn ~ fsk where 'fsk' is free in t1..tn. See Note [FunEq occurs-check principle] in TcInteract This neatly stops some infinite loops that people reported; and it allows us to delete some crufty code in reduce_top_fun_eq. And it appears to be no loss whatsoever. As well as fixing loops, ContextStack2 and T5837 both terminate when they didn't before. * Previously we generated "derived shadow" constraints from Wanteds, but we could (and sometimes did; Trac #xxxx) repeatedly generate a derived shadow from the same Wanted. A big change in this patch is to have two kinds of Wanteds: [WD] behaves like a pair of a Wanted and a Derived [W] behaves like a Wanted only See CtFlavour and ShadowInfo in TcRnTypes, and the ctev_nosh field of a Wanted. This turned out to be a lot simpler. A [WD] gets split into a [W] and a [D] in TcSMonad.maybeEmitShaodow. See TcSMonad Note [The improvement story and derived shadows] * Rather than have a separate inert_model in the InertCans, I've put the derived equalities back into inert_eqs. We weren't gaining anything from a separate field. * Previously we had a mode for the constraint solver in which it would more aggressively solve Derived constraints; it was used for simplifying the context of a 'deriving' clause, or a 'default' delcaration, for example. But the complexity wasn't worth it; now I just make proper Wanted constraints. See TcMType.cloneWC * Don't generate injectivity improvement for Givens; see Note [No FunEq improvement for Givens] in TcInteract * solveSimpleWanteds leaves the insolubles in-place rather than returning them. Simpler. I also did lots of work on comments, including fixing Trac #12821. }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12821#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler