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Re^5: Line numbers
by tobyink (Canon) on Mar 17, 2012 at 00:12 UTC
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That there exists an explanation does not imply that the explanation is obvious.
I understand that light may act as a wave or particle depending on how you study it. However, this fact is not obvious. It is in fact incredibly non-obvious, which is how it took science so long to get there.
That [caller(0)]->[0] works is (at least to me) obvious. That (caller(0))[0] works and caller(0)[0] fails is (again, to me) non-obvious.
perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'
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I understand that light may act as a wave or particle depending on how you study it.
Certainly, but unlike particle physics, Perl comes with a manual.
I'm not claiming that slicing the list returned from a function has syntax that's immediately obvious to everyone, but it's not that obscure once you understand Perl's precedence.
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So you exchange parens for square brackets and add an arrow instead. Sorry, but you really haven't proven your point. They are both non-obvious until you understand how Perl handles precedence and lists. caller(0)[0] fails because you are trying to index a scalar, not a list.
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> fails because you are trying to index a scalar
No,it fails because it's undefined syntax
syntax error ... near ")["
and indexing a list always requires parens!
see perldata
A list value may also be subscripted like a normal array. You must
+put
the list in parentheses to avoid ambiguity.
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Re^5: Line numbers
by LanX (Saint) on Mar 17, 2012 at 00:07 UTC
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Well he rather meant "intuitive", all examples are about precedence.
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Re^5: Line numbers
by gg48gg (Sexton) on Mar 17, 2012 at 01:46 UTC
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Right. The added complexity actually comes from introducing the say function, which takes a list.
say (caller(0))[0];
is expanding to:
say $package, $filename, $line, $subroutine, $hasargs,$wantarray, $eva
+ltext, $is_require, $hints, $bitmask, $hinthash,[0];
This gives you the syntax error:
say [0];
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> is expanding to:
> say $package, $filename, $line, $subroutine, $hasargs,$wantarray, $evaltext, $is +_require, $hints, $bitmask, $hinthash,[0];
nope, and you're showing valid code! After the last comma comes a literal arrayref [0].
DB<100> print $a,[0];
ARRAY(0x84e16b8)
the syntax-error comes from appending [0] unseparated to a function call say().
and nothing is "expanded" at parsing time.
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Thanks for setting me straight Rolf. I was getting an error because I didn't have the say feature enabled. I guess I shouldn't have stated it so matter-of-fact. I was so sure I had it figured out.
I am still a little confused about this and why anonymous' code works...
print '', (caller(0))[0];
and why this gets a syntax error:
print (caller(0))[0];
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Actually, this is wrong!
When you say:
print (caller(0))[0],
print is interpretated as a function (equaivalent to print('hi')[0]), but it works fine if you say:
print '', (caller(0))[0]
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