[Haskell-cafe] Closed classes

[Eric Seidel]

The work on instance chains[1] seems related, but I'm not very familiar
with it, so it might just be superficial.

[1]: http://web.cecs.pdx.edu/~mpj/pubs/instancechains.pdf

On Wed, Feb 17, 2016, at 14:21, David Feuer wrote:
> I was thinking about ~two notions of "closed class" yesterday, and I'm
> curious if anyone's done any work on either concept. In each case, the
> class definition is followed by *all* of its instances, and the instances
> are checked *in order* (rather than based on specificity and
> OVERLAPS/OVERLAPPABLE pragmas).
> 
> No backtracking:
> 
> If the instance head matches, GHC commits to it. Associated types are
> treated as closed type families, and would work just the same (I don't
> think any significant extension to the closed type family mechanism would
> be required). This seems to make a very nice parallel to the usual open
> classes with open associated types. And it lets you combine overlapping
> instances with associated types without (I believe) risking type safety.
> 
> Backtracking:
> 
> GHC does not commit to the instance until it has satisfied the instance
> constraints. This lets instance writers offer multiple alternative
> instance
> constraints. Associated types would be a good bit trickier. One option
> would be to require all instances with the same head to share a type/data
> instance. The other (much more invasive) option would be to allow the
> instance chosen to guide the type selection, which would push the
> backtracking into the type checker.
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