#10009: type inference regression when faking injective type families -------------------------------------+------------------------------------- Reporter: aavogt | Owner: Type: bug | Status: new Priority: high | Milestone: 7.10.2 Component: Compiler (Type | Version: 7.10.1-rc1 checker) | Keywords: Resolution: | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | Blocking: Blocked By: | Differential Revisions: Related Tickets: #10226 | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones <simonpj@…>): In [changeset:"a1275a762ec04c1159ae37199b1c8f998a5c5499/ghc"]: {{{ #!CommitTicketReference repository="ghc" revision="a1275a762ec04c1159ae37199b1c8f998a5c5499" Improve improvement in the constraint solver This regrettably-big patch substantially improves the way in which "improvement" happens in the constraint solver. It was triggered by trying to crack Trac #10009, but it turned out to solve #10340 as well. The big picture, with several of the trickiest examples, is described in Note [The improvement story] in TcInteract. The major change is this: * After solving we explicitly try "improvement", by - making the unsolved Wanteds into Deriveds - allowing Deriveds to rewrite Deriveds This more aggressive rewriting "unlocks" some extra guess-free unifications. * The main loop is in TcInteract.solveSimpleWanteds, but I also ended up refactoring TcSimplify.simpl_loop, and its surrounding code. Notably, any insolubles from the Givens are pulled out and treated separately, rather than staying in the inert set during the solveSimpleWanteds loop. There are a lot of follow-on changes * Do not emit generate Derived improvements from Wanteds. This saves work in the common case where they aren't needed. * For improvement we should really do type-class reduction on Derived constraints in doTopReactDict. That entailed changing the GenInst constructor a bit; a local and minor change * Some annoying faffing about with dropping derived constraints; see dropDerivedWC, dropDerivedSimples, dropDerivedInsols, and their Notes. * Some substantial refactoring in TcErrors.reportWanteds. This work wasn't strictly forced, but I got sucked into it. All the changes are in TcErrors. * Use TcS.unifyTyVar consistently, rather than setWantedTyBind, so that unifications are properly tracked. * Refactoring around solveWantedsTcM, solveWantedsAndDrop. They previously guaranteed a zonked result, but it's more straightforward for clients to zonk. }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10009#comment:22> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler