DData revision /equivalence vs equality

Wolfgang Jeltsch wolfgang at jeltsch.net
Wed Mar 17 14:50:53 EST 2004

Am Mittwoch, 17. März 2004 13:30 schrieb Ketil Malde:
> [...]

> > Furthermore, having only an equivalence implies that there is a
> > stronger equality below that cannot be also an instance of Eq.
> I've already said that I think Eq's (==) should be "structural
> equality", which I thought meant that there shouldn't be any
> "deeper" equality.

I think, the name "structural equality" is misleading because it seems to 
imply that equivalent values have to have the same (internal) structure.  If 
values a and b have different internal representations but are 
indistinguishable for the library user, than a == b should hold.

But I totally agree with you that == should mean equality and not anything 
different.  And this is also what The Haskell Report says in § 6.3.1: "The Eq 
class provides equality (==) and inequality (/=) methods."  So a lot of the 
MultiSet/Bag and the bias discussion can be avoided.  DData can and should 
assume that the Eq methods mean exactly what they are supposed to mean: 
equality and inequality.

> [...]

> -kzm


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