#15593: QuantifiedConstraints: trouble with type family -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 Resolution: duplicate | Keywords: | QuantifiedConstraints, | ConstraintKinds Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #14860 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => closed * resolution: => duplicate * related: => #14860 Comment: Update: GHC HEAD now //does// allow quantified constraints to be headed by `(~)`. Unfortunately, the original example still won't compile due to the fact that the quantified constraint's head mentions a type family: {{{ Bug.hs:11:1: error: • Illegal type synonym family application ‘DeRef (tree xx)’ in instance: DeRef (tree xx) ~ TreeF xx • In the quantified constraint ‘forall xx. DeRef (tree xx) ~ TreeF xx’ In the context: forall xx. DeRef (tree xx) ~ TreeF xx While checking the super-classes of class ‘MuRef1’ In the class declaration for ‘MuRef1’ | 11 | class (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Bug.hs:12:10: error: • Illegal type synonym family application ‘DeRef (tree xx)’ in instance: DeRef (tree xx) ~ TreeF xx • In the quantified constraint ‘forall xx. DeRef (tree xx) ~ TreeF xx’ In the instance declaration for ‘MuRef1 tree’ | 12 | instance (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} This is the subject of # , so I'm going to close this ticket in favor of that one. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15593#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler