Type families in kind signatures with TypeInType
dave at zednenem.com
Fri Sep 23 18:51:23 UTC 2016
On Fri, Sep 23, 2016 at 3:19 AM, Simon Peyton Jones <simonpj at microsoft.com>
> 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
• 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 at zednenem.com>
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