In Haskell, the class Foldable is standalone i.e.
class Foldable t where ...
However, in Frege:
class Functor t => Foldable t where ...
Why was this constraint introduced? What ill-formed programs/ideas can it rule out?
It prevented me from defining an instance Foldable HashSet, where HashSet is a custom data type which wraps Data.HashMap to provide a set-based interface. (It can be worked around by applying toList first, so it's not a serious issue.)
I can only guess that it used to be the case that Haskell's Foldable did have Functor as a superclass once and that Frege inherited the Foldable code at this time. Could've been 2012 or so.
I'm not sure I understand your concern as Data.HashMap is indeed a Functor. But anyway, I invite you to design a patch and make a pull request.