#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: -------------------------------------+------------------------------------ Comment (by simonpj): I discussed with Joachim. The "apartness" idea definitely doesn't work. Consider {{{ newtype Fix1 x = F1 (x (Fix1 x)) newtype Fix2 x = F2 (x (Fix2 x)) }}} and try solving `Coercible (Fix1 (Either Age)) (Fix2 (Either Int))`. Everything is fully known, and always apart, yet we get a loop. Moreover, analysing the definitions to find loop breakers (as GHC does now) fails in the presence of type functions {{{ newtype Bad a = B (F a) type instance F Int = Bad Int }}} Conclusion: the depth-bound idea is the only real option. We also need to prevent the construction of recursive dictionaries, but we can use the depth of a constraint to stop that too; do not solve a constraint of depth n by using a constraint of lower depth. This is rather conservative, but it's enough to prevent recursive dictionary construction. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8503#comment:19 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler