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Re^3: What is true and false in Perl?

by tobyink (Abbot)
on Oct 18, 2012 at 02:26 UTC ( #999654=note: print w/replies, xml ) Need Help??

in reply to Re^2: What is true and false in Perl?
in thread What is true and false in Perl?

That's not quite true. Observe:

# Does not say "true". This is as you describe. say "true" if undef, undef; # Still doesn't say true. say "true" if 1, 2, 3, 4, undef;

The reason for this behaviour is that if imposes a scalar context on the expression that acts as its condition. When the comma operator is used in a scalar context it is not a list constructor and instead evaluates the left argument, discards it and then evaluates the right argument (it basically acts like the semicolon but with a different precedence).

Thus this:

say "true" if 1, 2, 3, 4, undef;

Is essentially the same as:

sub x { 1; 2; 3; 4; return undef; } say "true" if x();
perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

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