Well, this version seems to work:
import future, typetraits
type
Functor[A] = concept f
type MatchedGenericType[T] = stripGenericParams(f.type)[T]
# `f` will be a value of a type such as `Option[T]`
# `MatchedGenericType` will become the `Option` type
get(f) is A # f.val is a private member of Option
# The Functor should provide a way to obtain
# a value stored inside it
type T = typed
# `auto` is too broad here, it includes `untyped` which does not
match
# the return value of a closure (argument 2 of map)
map(f, A -> T) is MatchedGenericType[T]
# And it should provide a way to map one instance of
# the Functor to a instance of a different type, given
# a suitable `map` operation for the enclosed values
import options
echo Option[int] is Functor # prints true
