#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

#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 gridaphobe): Replying to [comment:2 adamgundry]:

It should be easy to run the plugins first and permit them to reject dictionary constraints that have arisen directly from the source, but it's not clear that is very useful because many constraints arise during solving.

Shouldn't plugins get access to the extra constraints as part of the inner loop in `solveSimpleWanteds`? Oh I see, `solveSimples` has its own loop that greedily solves the constraints that it can, bummer.. We could change `solveSimples` to not grab constraints from the work-list directly, but to just solve the constraints it's given. Then any new work- list items would have to go through the whole pipeline, including plugins. I don't know what the performance implications would be though. But even then, it would still be hard to ensure that our plugin gets to reject the dictionary constraint, what if another plugin runs first and solves it? Hrm..

(Moreover, this would not work for equality constraints, because those are handled eagerly by the unifier.)

Right, but to be fair, I can't think of a reason I, as a user, would want to reject an equality constraint that can be proven. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11457#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11457: Run type-checker plugins before GHC's solver -------------------------------------+------------------------------------- Reporter: gridaphobe | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.0.1-rc1 checker) | Keywords: Resolution: | TypeCheckerPlugins Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by adamgundry): * keywords: plugin => TypeCheckerPlugins -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11457#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler