#8503: New GeneralizedNewtypeDeriving check still isn't permissive enough -------------------------------------+------------------------------------ Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: #8541 -------------------------------------+------------------------------------ Comment (by nomeata): Got that code in a working form. I don’t claim to know the solver well enough to be completely certain that there is not a way left to create a recursive dictionary, but at least the examples that I tried worked. Commits 6f5c183cb798a5d5e1011417f1f211834e4e9215 (GHC) and ec83ce7530af5474a3ad49d7120913c7f22266f0 (testsuite); for convenience, here is the note describing the design: {{{ Note [Preventing recursive dictionaries] We have some classes where it is not very useful to build recursive dictionaries (Coercible, at the moment). So we need the constraint solver to prevent that. We conservativey ensure this property using the subgoal depth of the constraints: When solving a Coercible constraint at depth d, we do not consider evicence from a depth <= d as suitable. Therefore we need to record the minimum depth allowed to solve a CtWanted. This is done in the SubGoalDepth field of CtWanted. Most code now uses mkCtWanted, which initializes it to initialSubGoalDepth (i.e. 0); but when requesting a Coercible instance (requestCoercible in TcInteract), we bump the current depth by one and use that. There are two spots where wanted contraints attempted to be solved using existing constraints; doTopReactDict in TcInteract (in the general solver) and newWantedEvVarNonrec (only used by requestCoercible) in TcSMonad. Both use ctEvCheckDepth to make the check. That function ensures that a Given constraint can always be used to solve a goal (i.e. they are at depth infinity, for our purposes) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8503#comment:29 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler