Version 4 (modified by ross@…, 8 years ago) (diff)


Overlapping Instances

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Brief Explanation

Instances are allowed to overlap, as long as there is a most specific one in any case. Contexts of instances play no part in testing for overlapping.



Hugs compares instance declarations, while GHC delays tests until the instances are used.

  • Hugs considers instances to be overlapping if they are unifiable, e.g.
    instance C Int b
    instance C a Bool
    Similarly the "most specific" test is applied to instances (though not as cleverly as it could be).
  • GHC only requires -fallow-overlapping-instances if it needs to resolve a constraint that overlaps with both, e.g. C Int Char is fine, but C a Bool overlaps. Similarly a most specific instance is needed only for constraints encountered in type checking.

An alternative to what GHC implements would be to declare whether a class may be overlapping on a class by class basis, perhaps with something like

class overlapping Foo a where


  • useful on occasion.


  • including a new module can silently change behavior of a program.
  • makes it very easy for incorrect programs to still typecheck
  • can cause confusing error messages
  • can sometimes be simulated with the extra-method trick used in the Show class of the prelude for showing lists of characters differently than lists of other things.