Re: [FWP] Testing list?

[Uri Guttman <uri@sysarch.com>]

>>>>> "PS" == Peter Scott <Peter@PSDT.com> writes:

  >> so you have a scalar and array context being applied to @a at the same
  >> time. not nice.

  PS> I don't see why that should be a show-stopper.   This is really about 
  PS> 'propagating context', which happens, for instance, in the hook operator:

  PS>          @c = $scalar_condition ? @a : @b;

  PS> And so the above can be written as

  PS>          @a = @a ? @a : (1, 2);

but we are discussing ||= and not ?:

this is illegal:

perl -e '@a ||= 2'
Can't modify array deref in logical or assignment at -e line 1, at EOF
Execution of -e aborted due to compilation errors.

while this works:

perl -e '@a = @a || 2 ; print "@a\n"'
2

and that is not what you want to happen in the former case (if it was
legal).

the definition of ||= requires the lvalue be readable as an rvalue of
the same type. but assigning to @a is not the same as reading @a in a
scalar context. that is the problem with arrays and ||=. it is not a
problem of propogating context but that || evaluates in scalar context
only. and the assignment propogates an array context. that is a
contradiction so it is an illegal operation.

||= can't expand the first into the second. it is inherently a execution
shortcut as well as a syntactical one. think about its origins in c,
where an early compiler would generate longer code for
	
	i = i + n ;

than

	i += n ;

it was done to make the code more efficient as much as syntactic
shortcuts. they knew that they could add a value to a location without
reading in the location first. knowing pdp-11 assembly really helps when
understanding c since many of its odd (for its time) features come from
that instruction set (++,--,+=,-= *foo, etc. are all mapped directly to
one pdp-11 instruction).

uri

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