Proposal: Instance apartness guards

Anthony Clayden anthony_clayden at
Wed Nov 8 09:29:57 UTC 2017

Surprisinlgy little comment on this proposal.
Perhaps it landed when yous were busy elsewhere.
I'd like to push it to the committee soon;
this is an invitation for more feedback.

Richard E wrote a nice brief summary

Thank you

----- Original Message Follows -----
> After years of pondering this idea (in various forms), 
> and several rounds of discussion on several forums, 
> I've written it up.
> "This proposal tackles the thorny topic of Overlapping
> instances,
>  for both type classes and Type Families/Associated types,
>  by annotating instance heads with type-level apartness
> Guards.
>  Type-level disequality predicates appear in Sulzmann &
> Stuckey 2002;
>  in the type-level ‘case selection’ in HList 2004;
>  and in various guises in Haskell cafe discussions in
> following years.
>  This proposal builds on the apartness testing implemented
>  as part of the Closed Type Families work."
> All feedback welcome.
> AntC

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