I agree with your points on (B), and this would be personally least  
surprising to me, given the other existing methods (it is mutable,  
after all :)
I'm just a sample size of one, though.

Dr Matthew Berryman
Defence and Systems Institute
SPRI Building
University of South Australia
Mawson Lakes  SA  5095
t +61 8 8302 5882
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CRICOS Provider Number: 00121B

On 29/10/2009, at 7:40 PM, Sean Luke wrote:

> Not hearing a peep from the peanut gallery :-( I'm inclined to make
> the (B) modifications.  Going once, going twice?
> Sean
> On Oct 27, 2009, at 7:41 PM, Sean Luke wrote:
>> I'm facing a tough decision on Double2D which could affect the
>> entire community and so I'm interested in some feedback.
>> I've been going through the physics2d engine and doing some cleanup
>> and one thing that could really help in that cleanup is to add some
>> functions to Double2D which are presently only in MutableDouble2D,
>> but to do so in such a way that Double2D's version would have
>> somewhat different semantics.
>> MutableDouble2D has four kinds of ways of "adding":
>> MutableDouble2D m1, m2, m3;  // 2d vectors
>> double v;  // a scalar
>> m1.addIn(m2);   // m1 <- m1 + m2, return m1
>> m1.addIn(v);    // m1 <- m1 + v (at all slots in m1), return m1
>> m1.add(m2, m3); // m1 <- m2 + m3, return m1
>> m1.add(m2, v);  // m1 <- m2 + v (at all slots in m1), return m1
>> The use of "add" in this context is unfortunate, I know.  It's going
>> to cause problems in a second.  But originally my idea was to enable
>> stuff like this:
>> m1 = new MutableDouble2D().add(m2, m3).multiplyIn(v).normalize();
>> Which does
>> 	m1 <- normalize((m2 + m3) * v)
>> ... but doesn't do any new allocations at all, because at each step
>> we just overwrite the variables inside the MutableDouble2D we  
>> created.
>> Okay, so that's cute.  The problem comes when I want to add similar
>> functionality to Double2D.  I can't implement an "addIn" method
>> because Double2D is IMMUTABLE.  Instead, I'd do something like this:
>> Double2D d1, d2, d3;
>> double v;
>> d1 = d2.add(d3).mutiply(v).normalize();
>> This does the same thing but at each step a new Double2D is
>> created.  For example,
>> d1.add(d2);	// new d <- d1 + d2, return d
>> That's a good, easy to understand functional style which is much
>> less convoluted than the MutableDouble2D approach, BUT it allocates
>> lots of Double2Ds, which isn't particularly efficient, though it's
>> not horrible.  So it's a useful functionality to have in Double2D.
>> The problem is that the semantics are somewhat different than
>> MutableDouble2D's semantics, in which the original object is
>> OVERWRITTEN.  This is particularly obvious in the normalize()
>> method, which in MutableDouble2D normalizes the actual
>> MutableDouble2D, but for Double2D would produce a new Double2D (it's
>> have to).
>> Also MutableDouble2D's add(...) method, for example, takes two
>> arguments and has totally different semantics than Double2D's
>> add(...) method would.
>> I'm trying to nail down what options I have.  One choice I have been
>> mulling over is to add methods to Double2D like add(d) [note one
>> argument], multiply(v), etc., and then also create similar
>> MutableDouble2D methods with the same names.  But the question is
>> how the MutableDouble2D methods should work.  Should they (A)
>> produce NEW MutableDouble2D instances or (B) overwrite the existing
>> MutableDouble2D instance, like other MutableDouble2D methods
>> presently do?
>> (A) is more semantically consistent with the proposed new Double2D
>> methods.
>> (B) is semantically consistent with the existing MutableDouble2D
>> methods.
>> I'm trying to follow the principle of least surprise but I don't
>> know which of these would have less surprise.   normalize() in
>> particular will *have* to be case (B).  There's no way around it.
>> Which has been nudging me to wards doing (B).   A third option would
>> be to just create Double2D methods and not create ANY equivalent
>> MutableDouble2D methods.
>> The decision made here will have a long-standing effect on use of
>> these classes, and they're so integral to MASON that I want to be
>> very very careful.  Backward compatability will be retained but I am
>> concerned about making things weird in the future.
>> So I'd really appreciate some opinions on the matter.
>> Sean