On Wed, Oct 1, 2014 at 10:36 AM, Gesh hseG
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.
How is that neither here nor there? Haskell says that types belong to
classes, and each (type) instance of a class has a specific implementation.
The fact that GHC implements this using dictionaries is irrelevant. It’s
perfectly valid to implement type classes with partial evaluation, or
matching on type parameters, or even C++-style templates.
In other words: no, coherency is not preserved. You can’t change the
semantics of classes and then say that programs built around the old
semantics were wrong all along.
--
Dave Menendez