#14728: Is (GeneralizedNewtypeDeriving + associated type classes) completely bogus? -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.2 checker) | Keywords: deriving, Resolution: | TypeFamilies 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 RyanGlScott): Replying to [comment:3 goldfire]:
Here's the correct type instance:
{{{ type instance forall (a :: Type) (x :: Identity a). T (Identity a) x = T a ('runIdentity x) }}}
where I've used `'` to use the term-level `runIdentity` function in a type.
This requires Dependent Haskell, I presume?
I don't think this would be impossible to support. Currently, the `deriving` mechanism produces HsSyn. I suppose that makes it easier w.r.t. inferring contexts and such. But suppose we could write the RHS of type instances in Core, and use `HsCoreTy` to embed it into HsSyn. Then, `runIdentity` is just a cast by the axiom induced by the `Identity` newtype.
One correction: `deriving` only uses HsSyn for derived //methods//. It does in fact generate `Core` for the RHS of type instances. So what you describe might be possible today? (I'm not sure I follow the other details, so it's hard for me to say.)
This would need to produce
{{{ type instance forall (a :: Type) (x :: Maybe (Identity a)). S (Identity a) x = S a (x |> g) where g :: Maybe (Identity a) ~ Maybe a g = Maybe axIdentity }}}
`axIdentity`? What sorcery is this?
Actually, as I'm writing this all up, I realize that `FamInstEnv.normaliseType` is behind the times here. It, too, needs to take all of these challenges into account in order to produce a well- kinded output. I'll post a new bug to this effect.
Is it worth doing all of these here, for GND? Probably not. And I think
Interesting. Is there a program that exhibits this bug (that doesn't leverage this GND business)? the idea of "just don't allow this" may be best. However, it's good to know that we ''could'' do this if we wanted. Indeed. And we could quite easily change this design in the future, so I'm not too worried about being conservative for now. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14728#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler