At 12:30 AM -0700 7/14/06, Darren Duncan wrote:
If $a === $b means what I think it does, then I believe that a not-premature implementation optimization of === would be that it always $a := $b if it was returning true, so that any future === of $a and $b or aliases thereof could short-circuit with a =:= test even if they weren't created as aliases, and Perl would be automatically more memory efficient without those extra storage copies.


Sorry, I stated some things badly in that previous email, mainly the "$a := $b" part, which is technically incorrect, so I will try and clarify what I meant to say.

What I propose concerning non-premature === optimizing is a system where, at any time that two appearances of the same immutable value are compared with ===, they are immediately consolidated into a single appearance. Or at least $a.value := $b.value occurs immediately, and garbage collection of the second and now unreferenced copy happens whenever it would happen.

For illustration:

 $a = 'hello'; # one copy of the value 'hello' in memory
 $b = 'hello'; # a second copy of the value 'hello' elsewhere in memory
 $c = 'world'; # one copy of 'world' in memory
 $a === $b; # now only one copy of 'hello' is in memory, $a and $b point to
 $a === $c; # nothing changes, as values are different
 $b = $c; # now only $a points to 'hello', $b and $c point to one 'world'

I of course did not mean for the actual symbol $a := $b to happen, only what they point to internally.

Of course, the above example could be constant folded to one copy of 'hello' at compile time, but my illustration is meant to be for situations where $a and $b are declared or set far apart, possibly from run-time input values, so the folding happens at run time.

What I meant with the =:= shortcut then, is that $a.value =:= $b.value could return true following the above run of $a === $b.

Sorry for any confusion.

FYI, I plan to explicitly illustrate the principle in my next Set::Relation update, since its types are immutable, so that any operations which involve comparing two relations or tuples or headings or values therein with === will have a side-effect of consolidating them if they are equal. Later on, if this happens at the language level, I would less likely have to do it myself.

-- Darren Duncan

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