G'day anaconda_wly,

Here's a breakdown of what's happening with: 

my $op = @_ ? ($seen{lc($field)}++ ? 'PUSH' : 'SET') : 'GET';

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You've got a nested ternary operator here. Firstly, looking at the outer ternary. You've said you understand that when @_ is empty (i.e. FALSE), $op is assigned 'GET'. So, if @_ is not empty (i.e. TRUE), $op is assigned whatever ($seen{lc($field)}++ ? 'PUSH' : 'SET') evaluates to.

"($seen{lc($field)}++ ? 'PUSH' : 'SET')" is the nested ternary. If ($seen{lc($field)}++ is TRUE, 'PUSH' will be assigned to $op; if it's FALSE, then 'SET' will be assigned to $op.

"$seen{lc($field)}++" uses the postfix increment operator. This means that the value is returned first; afterwards the increment is performed. If the '++' was at the front (the prefix increment operator), the increment would be performed first and the incremented value would be returned. A simple code example might explain this better: 

$ perl -Mstrict -Mwarnings -le '
    my $x = 1;
    print $x++;
    print $x;

    my $y = 2;
    print ++$y;
'
1
2
3

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$x starts as 1; $x++ evaluates to 1 for its print statement; afterwards, it's incremented to 2 as print $x shows. $y starts as 2; ++$y evaluates to 3 for its print statement, i.e. the increment is done first.

Back to "$seen{lc($field)}++". If %seen doesn't have a key lc($field), then $seen{lc($field)} will evaluate to FALSE (i.e. $op = 'PUSH'); after that evaluation, $seen{lc($field)} is incremented with the key being created (that's called autovivification) and the value being set at 1 (FALSE in numeric context is zero; 0 + 1 = 1). If %seen does have a key lc($field), its value will be at least 1 and $seen{lc($field)} will evaluate to TRUE (i.e. $op = 'SET'); $seen{lc($field)} will then be incremented again.

Using a hash called %seen (or similar name) and autovivifying keys (with a postfix increment) as they are seen is a common idiom. It would be worth your while studying this closely and ensuring you fully understand everything that's going on here: it's something you're likely to see often.

For wantarray, the context is the calling context; it's got nothing to do with the context inside the executing subroutine. The following may suffice to explain this: 

my @return_values = function(@args); # wantarray TRUE
my $return_value  = function(@args); # wantarray FALSE

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-- Ken

In reply to Re: What's this line means in HTTP::headers? by kcott
in thread What's this line means in HTTP::headers? by anaconda_wly

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