The `==' operator (Re: [nem-en] Nemerle 0.2.0 released)

[Kamil Skalski]

On Mon, Sep 13, 2004 at 10:03:14PM +0200, Michal Moskal wrote:
> On Mon, Sep 13, 2004 at 08:27:17PM +0200, Kamil Skalski wrote:
> > On Mon, Sep 13, 2004 at 07:04:43PM +0200, Michal Moskal wrote:
> > > On Mon, Sep 13, 2004 at 12:36:17PM -0400, Mark Friedman wrote:
> > > > I'm curious about the rationale for having the '==' operator not
> > > > fallback to reference equality?  I guess that it's not entirely clear
> > > > to me what the '==' operator actually means in Nemerle.  
> > > 
> > > It works the same as in C#, that it is statically resolved whatever class
> > > of the left or right parameter (or any base class thereof) has an overload
> > > on ==. Some classes (like System.String or System.Decimal) have it.
> > > 
> > > However C# has an additional fallback case -- if there is no matching
> > > overload, reference equality is used. That is == checks if objects on
> > > both sides are physically the same.
> > 
> > This description is not clear. Why "or any base class thereof" doesn't
> > refer also to System.Object? Object is base of all the classes, so
> > simply the rule "whatever class of the left or right parameter (or any
> > base class thereof) has an overload" is always satisfied. This way
> > specification of how it works in C# is indeed clear and consitant - 
> > it chooses the nearest possible overload. 
> 
> Nope, System.Object hasn't got == overload ;) It's special case as
> usual.

Nope, it has:

The predefined reference type equality operators are:

bool operator ==(object x, object y);
bool operator !=(object x, object y);

The operators return the result of comparing the two references for
equality or non-equality.

Since the predefined reference type equality operators accept operands
of type object, they apply to all types that do not declare applicable
operator == and operator != members. Conversely, any applicable
user-defined equality operators effectively hide the predefined
reference type equality operators.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/csspec/html/vclrfcsharpspec_7_9_6.asp

Btw, what is the difference betwen
public static @== (x : object, y : object) : bool { x.ReferenceEqulity (y) }
and special case you are refering to?

Kamil Skalski