On Wed, 17 Mar 2004, Ketil Malde wrote:
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.
Taking this serious we would also have to use "deriving Eq" and never any weaker definition of Equality. As I explain in http://www.stud.tu-ilmenau.de/~robertw/dessy/fun/principles.html the problem can really only be understood by considering abstract data types: Equality must be so strong that all exported functions can't distinguish equal elements. What you do internally doesn't matter. As the above document explains (in the last section, by the way), this view of Abstract Data Types will also make any "biasing" in (e.g.) Set operations superfluous. In DData you say "as a concrete implementation Data.Set may specify biasing", but in the Abstract Collection Standard, we don't have that excuse any more. When looking at the standard you will see that Abstraction actually looses you much stuff, since Abstraction is always about _leaving things out_. The benefit is that the things (properties) that are left in are much more general. Of course an implementation of the Abstract Collections can strengthen the specifications (e.g. to include biasing), but giving that anyone relying on this, can't use other implementations any more, it suddenly becomes much less attractive. Surely the Gentle Introduction into Haskell with Collections must explain more of Abstract Data Types and the Laws of Programming. Robert