#9063: Default associated type instances are too general -------------------------------------+------------------------------------ Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 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): Can you give an example for instances? There is a design question here. For a start, the associated type should really mention at least one of the class variables. This should be prohibited: {{{ class C a where type F b :: * }}} Second, the default instances should mention the same class variable in the same position, not some pattern. For example {{{ class C a where type F a b :: * type instance F a Int = Int -- OK type instance F a Bool = Char -- OK type instance F Int a = Bool -- Definitely not OK! type instance F [a] b = Char -- Not OK }}} Reason: the default instances are simply templates, from which you can create the real `type instance` in the instance decl. For example {{{ instance C Float -- type instance F Float Int = Int -- type instance F Float Bool = Char }}} How would one instantiate the "not-ok" defaults here?? We are not currently checking any of this, nor is it documented. [http://www.haskell.org/ghc/docs/7.8.2/html/users_guide/type-families.html #assoc-decl-defs Current manual entry] Before moving to fix it, let's just check that we agree. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9063#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler