#13408: Consider inferring a higher-rank kind for type synonyms -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: closed => new * resolution: wontfix => Comment: I talked with goldfire today about this issue, but in a slightly different context. The issue this time was that I had: {{{#!hs data A :: Type -> Type data B a :: A a -> Type }}} And I wanted to define a type synonym alias for `B`: {{{#!hs type C = B }}} This time, we came to the opposite conclusion of comment:2 — that is, GHC //should// be able to accept this. There are two main reasons why: * Unlike in the original example, in which the RHS of a type synonym was attempting to partially apply a type synonym (which is problematic), there is no problem with partially applying a data type constructor such as `B`. * The implementation might be easier than what goldfire had originally thought when writing comment:2. In the code that kind-checks type synonym declarations, we would need to check that the type synonym is the sole declaration in a mutually recursive group. (That is to say, no funny business with recursion is going on.) If this holds true, then we could grab the kind from the RHS (even if it's fancy, like the kind of `B :: forall a -> A a -> Type`) and use that to infer the kind of the whole type synonym. Having this work is especially important in the context of this particular example since there is currently no way to write the kind `forall a -> A a -> Type` in source Haskell. (If there were a way to write this, then `type C = (B :: forall a -> A a -> Type)` would already be a suitable workaround.) goldfire, please correct me if I got any details wrong here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13408#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler