Re: [ID 20000128.001] $a = 1,,,,,,,,,

[Larry Wall <larry@wall.org>]

cook@sightpath.com writes:
: Perl fails to reject this program:
: 
:  $a = 1,,,,,,,,

It also fails to reject these:

    $a = (1,,,,,,,,)
    $a = 1,2,3,4,5,6,7,8,9,
    $a = 1,,,,,,,,,10,

: The superfluous commas just seem to be ignored.

Yes.  This is a recursive consequence of the rule that any list can end
with a comma, which is ignored.  And the fact that lists don't require
parens around them unless required by precedence.  So don't take the
absence of parentheses for the absence of a list.

So your example is parsing as all these sublists:

    ($a = 1),(),(),(),(),(),(),(),()

which, after you interpolate all the null lists, is just the list

    ($a = 1)

which is, of course, no different from

    $a = 1

: The camel says these are wrong (so I'm told; p528?):

That's talking specifically about putting a comma between a filehandle
and subsequent arguments.

:  print $a, if s/x/y/;
:  print, if s/x/y/;
: 
: Yet perl accepts them.

Why not?  There are no filehandles there, as far as Perl is concerned.
Just a list of size 1 and a list of size 0, either of which can end
with a comma.

Actually, now that I think about it, you can't actually start a list
with a comma, so the second one is probably parsing as (print),()
rather than print((),()).  In which case that's a list of size 1 as well,
the first and only element of which is the bare print operator.  After all,
if you said

    print, next if s/x/y/

it would print $_ and then do a next, since next would not be considered
an argument to the print.  That's why we don't allow lists to start with
a comma.

But they may certainly end with one.  And since a list that ends with
a comma is a list, it can in turn end with a comma.  And since a list
that ends with two commas is a list, it can in turn end with a comma.
Und so weiter...

Larry

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References to:

Michael Cook <cook@sightpath.com>

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