#11732: Deriving Generic1 interacts poorly with TypeInType -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: TypeInType, | Generics Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): There is a small problem here: {{{ instance (k ~ *) => Functor (P1 (a :: k)) }}} doesn't type-check. The problem is that this requires "immediate" use of the kind equality. In other words, we would need to give a name to the kind equality in order to cast by it later in the same type. But, last spring, Simon and I decided it would be simpler not to allow binding of kind equalities in types -- we can now bind them only in terms. (See also the first bullet [wiki:DependentHaskell/Phase1#Answers here].) Ignoring this issue, let's consider 1. `instance (k ~ *) => Functor (P2 k)` 2. `instance Functor (P2 *)` I claim that these (if the first were to type-check) instances are entirely equivalent. Normally, inlining an equality constraint changes the meaning of an instance, but not here. The first instance matches `Functor (P2 k)` for any value of `k`. What's interesting is that the second instance, exactly as written, does too. That's because the only possible value for `k` in `Functor (P2 k)` is `*`. Anything else is surely ill- typed. Accordingly, we should be comfortable just using the second instance. And I believe that anytime this sort of instantiation would happen has this property (that anything else would be ill-typed). This all means we can just do nothing about this particular issue. Which is nice. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11732#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler