recursive equal, and firstDifference functions

[John Colvin]

On Tuesday, 19 March 2013 at 11:46:14 UTC, monarch_dodra wrote:
> On Tuesday, 19 March 2013 at 10:08:43 UTC, Jonathan M Davis 
> wrote:
>> On Tuesday, March 19, 2013 09:25:43 timotheecour wrote:
>>> we need a std.algorithm.equalRecurse(T1,T2)(T1 a, T2 b) that
>>> compares recursively a and b;
>>> 
>>> its behavior should be:
>>> 
>>> if opEqual is defined, call it
>>> else, if its a range, call std.algorithm.equal (ie compare nb
>>> elements, then each element for equality)
>>> else, if it's a class/struct, make sure types are same and 
>>> call
>>> it recursively on each field.
>>> else if it's a numerical type, call "=="
>>> else (is there an else?)
>>> 
>>> just as std.algorithm.equal, we should have
>>> equalRecurse([1],[1.0]);
>>
>> If you want recursive equal, then do equal!equal. Granted, 
>> that's only one
>> level of recursion, but how many levels deep are you really 
>> going to have your
>> ranges? And you have to get to == eventually anyway in order 
>> to compare the
>> deepest elements. Going beyond a range of ranges is likely to 
>> be quite rare,
>> and when it does happen, you can simply nest equal as many 
>> times as you need.
>>
>> - Jonathan M Davis
>
> "equal!equal(RoR1, RoR2)"
>
> That looks cute, but I think it says something about how 
> powerful and expressive D can be, while being compile-time 
> optimized. It's those little things that still amaze me about D.

and then:

template NumDimensions (T) {
	static if(is(ElementType!T == void))
		const NumDimensions = 0;
	else
		const NumDimensions = 1 + NumDimensions!(ElementType!T);
}

bool rec_equal(R0, R1)(R0 r0, R1 r1)
	if(NumDimensions!R0 == NumDimensions!R1)
{
	mixin("return " ~ replicate("equal!", NumDimensions!(R0)-1) ~ 
"equal(r0, r1);");
}

obviously it requires some more checks, but it works nicely 
(except if you feed it two integer literals, in which case the 
compiler throws an out of memory error!).