#9171: Can't match type family with kind polymorphism ----------------------------+---------------------------------------------- Reporter: | Owner: MikeIzbicki | Status: new Type: bug | Milestone: Priority: normal | Version: 7.8.2 Component: Compiler | Operating System: Unknown/Multiple Keywords: | Type of failure: GHC rejects valid program Architecture: | Test Case: Unknown/Multiple | Blocking: Difficulty: Unknown | Blocked By: | Related Tickets: | ----------------------------+---------------------------------------------- Create a module with the following code: {{{ data Base type family GetParam (p::k1) (t::k2) :: k3 type instance GetParam Base t = t }}} It loads into GHCi just fine. But when we run: {{{ ghci> :t undefined :: GetParam Base (GetParam Base Int) }}} we get an error {{{ <interactive>:1:14: Couldn't match expected type ‘GetParam Base (GetParam Base Int)’ with actual type ‘GetParam Base (GetParam Base Int)’ NB: ‘GetParam’ is a type function, and may not be injective The type variable ‘k0’ is ambiguous In the ambiguity check for: GetParam Base (GetParam Base Int) To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In an expression type signature: GetParam Base (GetParam Base Int) In the expression: undefined :: GetParam Base (GetParam Base Int) }}} If we modify the module by either 1) set `k3` to `k2`, `k1`, or any concrete kind or 2) set `k2` to any concrete kind then the ghci snippet will type check just fine. I assume that this is a mistake, and that the original code should work just fine. If there is some reason, however, why the program would be unsound, then the error message should be made a bit more informative. The claim is that the kind `k0` is ambiguous, but it doesn't appear anywhere in my code! In my full program, it took me a long time to narrow down that this was the source of the problem. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9171 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler