#9123: Emit quantified Coercible constraints in GeneralizedNewtypeDeriving -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 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 simonpj):
If we wanted to do this (and I'm waiting for Simon to say that we don't)
I don't! It's pretty much always possible to take all the constraints needed for the constructors to typecheck, and spit them out as the instance context. Bingo - the instance typechecks. But we don't always want to do that. E.g. If we need `Int ~ Bool` we probably don't want to quantify over that. In fact GHC is pretty conservative: `Note [Exotic derived instance contexts]` in `TcDerivInfer`. One could make it less conservative provided you still obeyed the termination conditions. Generally we are pretty conservative, even for ordinary functions in `tcSimplifyInfer`, where we have no termination conditions to worry about. See `pickQuantifiablePreds` in `TcType`. We don't even ''try'' to quantify over a quantified constraint. Before quantifying over quantified constraints in instances, we should consider it for the simpler case of ordinary functions. And I frankly doubt it'll be feasible in practice. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9123#comment:63 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler