On Fri, Sep 23, 2016 at 3:19 AM, Simon Peyton Jones <simonpj@microsoft.com> wrote:

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 <dave@zednenem.com>
<http://www.eyrie.org/~zednenem/>