On Wed, Oct 1, 2014 at 10:36 AM, Gesh hseG <gesh@gesh.uni.cx> wrote:

> 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 <dave@zednenem.com>
<http://www.eyrie.org/~zednenem/>