Could something like this work for you? Here, a ~ b and you have access to
b:
case (IsListType :: IsListType [a], Proxy :: Proxy a) of
(_, proxy :: Proxy b) -> asProxyTypeOf undefined proxy :: b
On Thu, Aug 10, 2017 at 4:16 PM, Wolfgang Jeltsch
Hi!
GHC has some support for referring to existentially quantified type variables. For example, consider the following definition:
data SomeList = forall a . SomeList [a]
With the ScopedTypeVariables extension, you can get hold of the concrete type a used in a SomeList value:
f :: SomeList -> SomeList f (SomeList [x :: a]) = SomeList xs where
xs :: [a] xs = [x, x]
However, this approach does not work if there are no fields whose type involves the existentially quantified type variable. Consider, for example, the following definition:
data IsListType a where
IsListType :: IsListType [b]
For each type T, we can match a value of type IsListType T against the pattern IsListType. If this succeeds, we know that T is a list type, but we do not have access to the respective element type.
Is there a way to determine existentially quantified type variables like the b in the above example?
All the best, Wolfgang _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.