TypeFamilies vs. FunctionalDependencies & type-level recursion
anthony_clayden at clear.net.nz
Thu May 24 15:00:19 CEST 2012
<oleg at ...> writes:
> I don't think Overlapping Instances will be in Haskell' any time soon
> since there are doubts about the soundness. Overlapping
> instances are clearly unsound with type functions. Whether they are
> sound with functional dependencies is not clear, but there are warning
I have now worked through that post in detail, thank you. And replied (on the
cafe http://www.haskell.org/pipermail/haskell-cafe/2012-May/101417.html )
As SPJ says there, I don't expect there's any real difference in the fundeps
approach compared to type families. And as a matter of taste, I find type
families more easy to understand and reason about, and more *functional*.
But I don't see in SPJ's post any real doubts about soundness, just
restrictions that would have to be imposed. He concludes "I believe that
if ..., then overlap of type families would be fine."
The only onerous restriction is that overlapping instances would have to be in
a single module. And I don't think that is needed under my proposal to dis-
As a matter of interest, how would the TTypeable approach address those
examples? Particularly the existentials (examples 3 and 4). How would it look
inside the GADTs to discharge the constraints (or apply the type functions)?
I notice example 4 (and 1) 'exploits' separate compilation/imported
overlapping instances to arrive at unsoundness. How does TTypeable handle
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