#9123: Emit quantified Coercible constraints in GeneralizedNewtypeDeriving -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 7.8.2 checker) | Keywords: Roles, Resolution: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: quantified- | constraints/T9123{,a} Blocked By: 15290 | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): If we wanted to do this (and I'm waiting for Simon to say that we don't), the quantified constraint would be only an output from the constraint solver, not an input. The constraint solver works over a tree of constraints, where (roughly) interior nodes are givens (which can bring skolems into scope for their descendants) and leaves are wanteds. When inferring, we build up the tree of constraints and then simplify it to another tree that logically entails the first one. When we can simplify no more, we (roughly) look at the tree. If it's the kind of tree we like to quantify over (`Eq a`: yes. `Int ~ Bool`: no. An implication: no, today), convert it back to a normal constraint and quantify. To infer quantified constraints, all we have to do is change what trees we like to quantify over. Some of the action is in `TcSimplify.approximateWC` which looks to find simple (i.e. non-implication) constraints to quantify over. It will have to be adapted to sometimes return implication constraints. Then, the code in `TcDerivInfer.simplifyDeriv` will have to be taught to handle the implication constraints. I don't think it will be all that invasive, but we might find that we're accepting more programs than we like. See Note [Exotic derived instance contexts] in TcDerivInfer. Perhaps this helps start you off... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9123#comment:61 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler