My workaround is to wrap types of all kinds as kind *:
data WrapType (a :: k)
...or better yet, as its own kind:
data WrappedType = forall a. WrapType a
Now I can make an apples-to-apples comparison of types of different
kinds, e.g. "WrapType []" and "WrapType Bool". All I need now is a way
of applying wrapped types:
type family WrapApply
(f :: WrappedType) (x :: WrappedType) :: WrappedType
type instance WrapApply
(WrapType (f :: ka -> kfa)) (WrapType (a :: ka)) = WrapType (f a)
-- Ashley Yakeley
On 17/09/12 06:05, Richard Eisenberg wrote:
I see what you're getting at, but the problem is more fundamental than just the
lack of a type *. GHC has no notion of equality between kinds other than
syntactic identity. If two kinds are other than syntactically identical, they
are considered distinct. This fact basically renders your approach doomed to
failure. Furthermore, a promoted datatype and the unpromoted datatype are
distinct entities with the same names, so you can't just use a variable both at
the kind level and the type level (variable ka in your final ConstructedT
example). It is not hard to write a Demote type family that computes an
unpromoted datatype from its promoted kind, but that type family will interfere
with type inference.
That's all the bad news. The good news is that some of us are working out how
to extend GHC's rich notion of type equality to the kind level, which would
also allow intermingling of type- and kind-variables. We're still a little ways
out from starting to think about implementing these ideas, but there's a good
chance that what you want will be possible in the (not-so-terribly-long-term)
future.
Richard
On Sep 17, 2012, at 12:41 AM, Ashley Yakeley wrote:
TypeRep does indeed resemble * as a type.
I'm working on a system for reification of types, building on my open-witness
package (which is essentially a cleaner, more Haskell-ish alternative to
TypeRep).
Firstly, there's a witness type to equality of types:
data EqualType :: k -> k -> * where
MkEqualType :: EqualType a a
Then there's a class for matching witnesses to types:
class SimpleWitness (w :: k -> *) where
matchWitness :: w a -> w b -> Maybe (EqualType a b)
Then I have a type IOWitness that witnesses to types. Through a little Template
Haskell magic, one can declare unique values of IOWitness at top level, or just
create them in the IO monad. Internally, it's just a wrapper around Integer,
but if the integers match, then it must have come from the same creation, which
means the types are the same.
data IOWitness (a :: k) = ...
instance SimpleWitness IOWitness where ...
OK. So what I want to do is create a type that's an instance of SimpleWitness that represents types
constructed from other types. For instance, "[Integer]" is constructed from "[]" and
"Integer".
data T :: k -> * where
DeclaredT :: forall ka (a :: ka). IOWitness a -> T a
ConstructedT ::
forall kfa ka (f :: ka -> kfa) (a :: ka). T f -> T a -> T (f a)
instance SimpleWitness T where
matchWitness (DeclaredT io1) (DeclaredT io2) = matchWitness io1 io2
matchWitness (ConstructedT f1 a1) (ConstructedT f2 a2) = do
MkEqualType <- matchWitness f1 f2
MkEqualType <- matchWitness a1 a2
return MkEqualType
matchWitness _ _ = Nothing
But this doesn't work. This is because when trying to determine whether "f1 a1 ~ f2 a1", even though "f1 a1" has the same kind as "f2
a2", that doesn't mean that "a1" and "a2" have the same kind. To solve this, I need to include in "ConstructedT" a witness
to "ka", the kind of "a":
ConstructedT ::
forall kfa ka (f :: ka -> kfa) (a :: ka).
IOWitness ka -> T f -> T a -> T (f a)
matchWitness (ConstructedT k1 f1 a1) (ConstructedT k2 f2 a2) = do
MkEqualType <- matchWitness k1 k2
MkEqualType <- matchWitness f1 f2
MkEqualType <- matchWitness a1 a2
return MkEqualType
Sadly, this doesn't work, for two reasons. Firstly, there isn't a type for *,
etc. Secondly, GHC isn't smart enough to unify two kinds even though you've
given it an explicit witness to their equality.
-- Ashley Yakeley
On 16/09/12 20:12, Richard Eisenberg wrote:
If you squint at it the right way, TypeRep looks like such a type *. I believe
José Pedro Magalhães is working on a revision to the definition of TypeRep
incorporating kind polymorphism, etc., but the current TypeRep might work for
you.
Your idea intersects various others I've been thinking about/working on. What's
the context/application?
Thanks,
Richard
On Sep 16, 2012, at 7:09 PM, Ashley Yakeley wrote:
Now that we have type promotion, where certain types can become kinds, I find
myself wanting kind demotion, where kinds are also types. So for instance there
would be a '*' type, and all types of kind * would be demoted to values of it.
Is that feasible?
-- Ashley Yakeley
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