On Fri, Sep 23, 2016 at 3:19 AM, Simon Peyton Jones
This is an example of https://ghc.haskell.org/trac/ghc/ticket/12088.
Interesting. Is this case also an example, or is it a non-feature? class C t where type K t :: Type type T t :: K t -> Type m :: t -> T t a min.hs:21:17: error: • Type constructor ‘K’ cannot be used here (it is defined and used in the same recursive group) • In the kind ‘K t -> Type’ Failed, modules loaded: none. GHC accepts this if K t is moved outside of C.
The “type instance T List” declaration actually depends on the “type instance K List” declaration; the latter must be typechecked before the former. But this dependency is absolutely unclear. There’s a long discussion on the thread. Bottom line: we don’t know a solid automated way to spot this kind of problem, so I think we are going to ask for programmer assistance. In this case, we’d put a “separator” after the “type instance K List” decl, to explain that it must be done first:
type instance K List = Type
===========
type instance T List = []
Currently you have to write $(return []) to get the separator, but I think we’ll add a special separator.
Yes, this works. Thanks.
It would be disappointing if this is the best we can do, but I guess other
dependent languages don’t need to deal with open type families and
everything being potentially mutually recursive.
--
Dave Menendez