Re: A6: argument initializations via //=, ||=, ::=

[Michael Lazzaro]

On Tuesday, March 25, 2003, at 02:19  PM, Damian Conway wrote:
And I don't think that allowing 20 different types of assignment in 
the parameter list of a subroutine actually helps at all. Especially 
since the vast majority of parameters in Perl 6 will be constant.
Twenty types of _initialization_.  :-D

Seriously, tho, I'm not sure I understand the constantness part.

   sub foo($x = 1) {...}     # A6 syntax

I read the above as saying $x is indeed constant, but if it's not 
explicitly placed by the caller, we're going to pretend the caller 
passed us a 1.  Likewise, I read

   sub foo($x //= 1) {...}

as saying the value stored in $x is a constant, but if the caller 
passed an undefined value (or didn't pass anything at all), we're going 
to instead pretend they passed us a (still-constant) 1.  I'm not sure 
why that violates any rules.(?)

As a marginal bonus, perhaps an assertion-style

   sub foo($x //= 1) {...}

optimizes to be faster, runtime, than

   sub foo($x) { $x //= 1; ... }

when passing a constant in $x, e.g C<foo(5)> or C<foo('BLAH')>, since 
it can optimize out the assertion at compile-time?

MikeL