On Tue, Aug 23, 2005 at 03:48:03PM -0400, Sam Ruby wrote:
: How will Perl6 evaluate defaults?
:
: Like Python:
:
: global x
: x=1
: def f(p1=x):
: return p1
: x=2
: print f()
:
: or like Ruby:
:
: $x=1
: def f(p1=$x)
: return p1
: end
: $x=2
: puts f()
By default, delayed like Ruby, but you can easily force earlier
evaluation if that's what you want. In general, pseudo-assignment
to declarators is special-cased to happen at the time appropriate to
the scope and lifetime of the declarator, and formal parameters
are no exception. Basically, the container constructor is passed
the closure of what's on the right of the =, and it can choose to
evaluate that closure whenever it wants. For instance,
state $s = $x;
sets $s to the current value of $x only the first time it's executed.
So while formal parameter defaults execute "as late as possible", and
state variable initializations run "just in time", object attribute
defaults actually run "right away", that is, class declaration time
(aka "BEGIN" time), and you have to add braces to delay evaluation
till object init time. Arguably that's a little inconsistent, but
the inconsistency can be viewed as implicit to the declarator, not the
pseudo-assignment syntax, which is just syntactic sugar for a closure.
Larry
