On Fri, Sep 23, 2016 at 3:19 AM, Simon Peyton Jones <simo...@microsoft.com> wrote:
> 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 <d...@zednenem.com> <http://www.eyrie.org/~zednenem/>
_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
