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Postings from September 2008
Stéphane Payrard cognominal-at-gmail.com |Perl 6| wrote: > I don't understand how = differs with that semantic from := > I would expect that = would make a copy (clone?) of the object. > For a mutable object, I don't know if that copy should be immediate or deffered > by a mechanism of copy on write. Probably that behavior could depend on a > trait of the class that implements to be copied object. > > Not the best illustration, but the only one I have prepared so far: <http://www.dlugosz.com/Perl6/web/assignment/assign-1.png> This shows $x = $y;. The use of := (binding) would affect the leftmost column. my $z := $y; would have $z's symbol table entry point to the same Item as $y, namely #8CD9. Assignment (=) does not clone or copy the object. In the illustration, note that both Items refer to the same Dog (#A829) after assignment. With reference assignment semantics, if you want a copy or clone, you make one as an explicit step. Hence the existence of methods like deepcopy in the base Object of languages with that feature. The copy-on-write behavior you suggest is what Perl 5 does to make operator overloading appear to have value semantics. But Perl 6 does not describe this mechanism in any way. Instead, "value types" are immutable so it doesn't matter that the object is not cloned. --JohnThread Previous | Thread Next