[Haskell-cafe] Fwd: Increasing Haskell modularity

[Tom Ellis]

On Wed, Oct 01, 2014 at 05:36:49PM +0300, Gesh hseG 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.

What sort of code is that exactly?  I'm aware there are things like
`Data.Set` which crucially rely on global uniquess of instances to maintain
invariants.  Are there any other sorts of functionality where this is
needed?

Tom