Structs

Waldemar Horwat waldemar at google.com
Tue Jun 1 18:15:57 PDT 2010


Brendan:

To clear up something that misled me when reading the first few messages of this struct thread:

I assume that the proposal is that:

const TA =
    Array.newTypedArray(fixed_length,
                        Object.newStructType({x:"u32", y:"u32", z:"u32",
                                              r:"u8", g:"u8", b:"u8", a:"u8"}));
let a = new TA(...);

b = a[i];
a[i].x += 1;

Now b.x also reflects the new value of a[i].x; i.e. b is an alias, not a mutable copy.  The same thing would happen if one had mutated b.x.

That leads to a couple plausible meanings of b === a[i]:

1.  Pointer equality:  True if and only if mutating one's contents would mutate the other's.

2.  Recursive value equality:  True if and only if all of their constituent elements are ===.  I hope that you can't have things like aliases themselves as first-class values inside structs, as that could create cycles (although you could still make === work in that case if you really wanted to).

Bit-by-bit equality is not desirable in the presence of NaN's and signed zeroes.  If we go with choice 2, the struct {x:17, y:NaN} should not be === to {x:17, y:NaN}.  Using bit equality, it might or might not be ===, which then could be used to discriminate among different kinds of NaN's.  There is currently nothing in the language with that power and I'd prefer to keep it that way.

    Waldemar


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