WAT: opCmp and opEquals woes

[Fool via Digitalmars-d]

On Monday, 28 July 2014 at 00:23:36 UTC, H. S. Teoh via 
Digitalmars-d wrote:
> On Sun, Jul 27, 2014 at 07:04:08PM +0000, Fool via
>> Defining opEquals only makes sense if a user wants to replace 
>> equality
>> by some equivalence relation (different from equality).
>
> Not necessarily. The user type may be implemented in a way where
> member-wise binary comparison is not the correct implementation 
> of
> equality. For example, it could be a tree structure implemented 
> by
> integer indices into a backing array of nodes. There is no way 
> the
> compiler could know, in general, how to correctly compare two 
> instances
> of such a structure, since the bit-level representation of two 
> equal
> objects may be completely different, yet they represent 
> equivalent
> trees. You're still implementing equality, but it's equality 
> that's not
> the same as binary equality.

I think we agree except for a subtle difference in defining 
equality and equivalence. In my personal language there is a 
single equality but there are many equivalences.


> The problem with imposing these kinds of restrictions, is that 
> they are
> generally not enforceable (at least, not without significantly 
> crippling
> legitimate use cases). At some point, we have to stop 
> babysitting the
> programmer and trust that he's competent enough to not try to 
> subvert
> the language to make it do stuff it wasn't intended to do. As 
> somebody
> once said:
>
> 	Unix was not designed to stop people from doing stupid things,
> 	because that would also stop them from doing clever things.
> 	-- Doug Gwyn
>
> We're not talking about Unix here, but the same principle 
> applies.

I agree.


>> Please excuse my lack of creativity: in presence of opCmp I 
>> cannot see
>> a single sensible use case for defining a.opEquals(b) 
>> different from
>> a.opCmp(b) == 0.
>
> Floating-point numbers? ;-)

Thank you for pushing me there! It's true.

So D has to separate opEquals and opCmp since otherwise a user 
could not define floating-point 'equality' and 'comparison' 
himself in the same way as it is defined by the language.

I'm convinced know. :-)

Thanks!