recursive equal, and firstDifference functions

[Dan]

On Tuesday, 19 March 2013 at 21:11:12 UTC, Jonathan M Davis wrote:
> Lots of stuff uses ==, toHash, etc. That's the way the language 
> is designed.

Agreed

> Defining your types without defining those properly just isn't 
> going to work for
> a _lot_ of stuff. Creating an external function to compare 
> objects or generate
> hashes or anything like that is just going to cause problems, 
> because only
> your stuff would use it.

Ok - you are making strong statement and I don't see why it is 
the case.

Here is an example that does work for hashing.
Note: R implements its own hash and just prints to show nothing 
up the sleeve.
Other classes (S,T) have no toHash implemented directly at all - 
yet you can see u1 and u3 resolve to the same hash and u2 a 
different, as you would expect.

-------------------------
import std.stdio;
import opmix.mix;
import pprint.pp;

struct R {
   string r;
   hash_t toHash() const nothrow {
     try {
       writeln("toHash called on r");
     } catch(Exception) {
     }
     return typeid(string).getHash(&r);
   }
}

struct S {
   R r;
   string s;
   string[int] si;
}

struct T {
   S s;
}

struct U {
   mixin ToHash;
   T t;
}

void main() {
   U u1 = U(T(S(R("a"), "foo", [1:"goo"])));
   U u2 = U(T(S(R("a"), "foo".idup)));
   U u3 = U(T(S(R("a"), "foo".idup, [1:"goo".idup])));

   writeln(deepHash(u1));
   writeln(deepHash(u2));
   writeln(deepHash(u3));

   writeln(pp(u1));

   int[U] aa;
   aa[u1] = 100;
   writeln("Is u1 in aa ", u1 in aa);
   writeln("Is u2 in aa ", u2 in aa);
   aa[u2] = 100;
   writeln("Is u2 in now aa ", u2 in aa);

}

----------------------- OUTPUT --------------------------
toHash called on r
614624
toHash called on r
582446
toHash called on r
614624
{
  (U).t = {
   (T).s = {
    (S).r = {
     (R).r = "a"
    }
    (S).s = "foo"
    (S).si = {
     (K(1)[0] =>
      V("goo")),
    }
   }
  }
}
toHash called on r
toHash called on r
Is u1 in aa 7F2C7E556FC0
toHash called on r
Is u2 in aa null
toHash called on r
toHash called on r
Is u2 in now aa 7F2C7E556F40
----------------------------------------------------------------------


> The built-in stuff wouldn't, and the standard library
> wouldn't. For instance, AAs require that opEquals and toHash be 
> defined, and no
> matter how clever your external functions for comparing objects 
> or generating
> hashes are, they're not going to work with the built-in AAs. 
> Any type which is
> going to work with the built-in AAs must define opEquals and 
> toHash.
>

The above works with the built-in AAs.
Please offer an example.

Thanks
Dan