> It seems the first part of your proposal just says
> "Haskellers can no longer expect class instance coherence."
> As such, this is a drastic change that would require all Haskell
> programs to be inspected to see if they crucially rely on coherence
No. Coherence is preserved, since you always use the unique in-scope
dictionary. What *is* lost, however, is global uniqueness, which means
that code that assumes that the same dictionary is used for the same
type across calls to functions must be modified. Arguably, that code
is semantically incorrect to begin with, but that's neither here nor
there.