#9242: Implement {-# OVERLAPPABLE #-} and {-# INCOHERENT #-} pragmas -------------------------------------+------------------------------------- Reporter: simonpj | Owner: diatchki Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.8.2 Component: Compiler | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by simonpj): Just for completeness, here is the proposed specification again, with the fixes above, except for the remarks about incoherence in comments 22-24: {{{ The willingness to be overlapped or incoherent is a property of the instance declaration itself, controlled as follows: * An instance is "incoherent" if it has an INCOHERENT pragama, or if it appears in a module compiled with -XIncoherentInstances. * An instance is "overlappable" if it has an OVERLAPPABLE or OVERLAPS pragama, or if it appears in a module compiled with -XOverlappingInstances, or if the instance is incoherent. * An instance is "overlapping" if it has an OVERLAPPING or OVERLAPS pragama, or if it appears in a module compiled with -XOverlappingInstances, or if the instance is incoherent. compiled with -XOverlappingInstances. Now suppose that, in some client module, we are searching for an instance of the target constraint (C ty1 .. tyn). The search works like this. * Find all instances I that match the target constraint; that is, the target constraint is a substitution instance of I. These instance declarations are the candidates. * Find all non-candidate instances that unify with the target constraint. Such non-candidates instances might match when the target constraint is further instantiated. If all of them are incoherent, proceed; if not, the search fails. * Eliminate any candidate IX for which both of the following hold: * There is another candidate IY that is strictly more specific; that is, IY is a substitution instance of IX but not vice versa. * Either IX is overlappable or IY is overlapping. * If only one candidate remains, pick it. Otherwise if all remaining candidates are incoherent, pick an arbitrary candidate. Otherwise fail. }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9242#comment:25 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler