The other day I ran into something that I thought was a little odd. First off, we know that the ternary operator can be used as an lvalue as in: 

$flag ? $on : $off = $value;

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And in fact, this works too: 

$flag ? @array1 : @array2 = qw/This that the other/;

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This works too....... 

sub testthis {
    $_[0] = "Hi mom.\n";
}
testthis ( $flag ? $yes : $no );
foreach ( $yes, $no ) {
    print "$_\n" if defined $_;
}

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But things break down in this push example: 

my $condition = int rand 2;
my ( @array1, @array2 );
my $stuff = "This, that, and the other";
push( ( $condition ? @array1 : @array2 ), $stuff );

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That example gives a big fat compilation error.

I wonder if it has anything to do with the fact that neither of these succeed in modifying the parameter: 

# First example; nothing gets assigned to @array;
my @array;
sub testthis {
    @_ = qw/This that and the other/;
}
testthis ( @array );

# Second example; nothing gets assigned to @array1 or @array2.
# But no compiletime errors are generated either:
my $condition = int rand 2;
sub testthis {
    @_ = qw/This that and the other/;
}
my ( @array1, @array2 );
testthis( $condition ? @array1 : @array2 );

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In a way, it seems like the ternary behavior within push must be related to the behavior of trying to assign to modify the contents of an array passed as a parameter to a sub. But the push version generates a compiletime error, and the non-push version just doesn't perform the modification of the parameter.

Has DWIM broken down? Is this defined behavior? I haven't found the trinary / push behavior documented. Perhaps I've missed something, but it's interesting to ponder anyway.


Dave

In reply to Ternary operator can't be used as first argument of push by davido

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