#11457: Run type-checker plugins before GHC's solver -------------------------------------+------------------------------------- Reporter: gridaphobe | Owner: Type: feature request | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 8.0.1-rc1 checker) | Resolution: | Keywords: plugin 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 dfeuer): We need to intertwine the various aspects, and I don't know how that works with the plugin infrastructure. For example, if we have (hypothetically) {{{#!hs import Prelude.Extras {-# SuppressInstance Functor ((,) a) #-} foo :: (p -> q) -> Lift1 ((,) x) p -> Lift1 ((,) x) q foo = fmap }}} GHC will recognize a `Functor (Lift1 ((,) x))` constraint. We need the usual constraint solver to run far enough to determine that this requires `Functor ((,) x)` before we can step in and reject the solution. ---- Note that the suppression regime has not yet gone far enough for a full-on proposal yet. Some things people might want: 1. Suppress or warn about an instance unconditionally. This is clearly the simplest case, and covers the immediate demand. 2. Suppress or warn about an instance if a certain constraint *can* be satisfied. This seems a bit shady, but it may have applications. 3. Suppress or warn about use of an instance if a certain constraint *cannot* be satisfied. That is, add that constraint to the instance temporarily. This might be desirable if an instance is *implementable* without the given constraint, but doesn't really make *sense* without it. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11457#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler