This evening, I was pondering constructs I regularly use in Perl, especially ones which tend to get overly complicated as the requirements increase.

One such that continues to frustrate me is when writing something like:

my @array = qw(red blue green yellow black white purple brown orange g
+ray);
foreach (@array) {
    do_something_with($_);
}
[download]

Which is simple enough. But if I suddenly realize that do_something_with() ought to know the index of the item in the array:

#  Inputs:  $1 ... color name
#           $2 ... The color index (a new arg)
##
sub do_something_with {
    my ($color, $idx) = @_;
    # For now, just print the color prefixed with its index...
    printf " %2d.  '$color'\n", $idx;
}
[download]

... it wastes time, and extra space, to go back and modify the foreach loop to the equivalent for loop (ignoring for a moment the synonymity of foreach and for):

for (my $i = 0; $i < @array; $i++) {
    my $item  = $array[$i];
    do_something_with($item, $i);
}
[download]

Even a shorter idiom takes an extra line, and isn't quite as "clean" (imo), as the $i has to be defined outside of the loop:

my $i;
foreach (@array) {
    do_something_with($item, $i++);
}
[download]

Then it occurred to me that a state variable (available in Perl 5.10) should work.  Sure enough, the following does exactly as I had hoped:

use feature ":5.10";
my @array = qw(red blue green yellow black white purple brown orange g
+ray);
foreach (@array) {
    do_something_with($item, state i++);
}

# Prints
#  0.  'red'
#  1.  'blue'
#  2.  'green'
#  3.  'yellow'
#  4.  'black'
#  5.  'white'
#  6.  'purple'
#  7.  'brown'
#  8.  'orange'
#  9.  'gray'
[download]

Even a preincrement is simple and cleanly "contained":

for (@array) {
    do_something_with($_, ++ state $i);
}

# Prints
#  1.  'red'
#  2.  'blue'
#  3.  'green'
#  4.  'yellow'
#  5.  'black'
#  6.  'white'
#  7.  'purple'
#  8.  'brown'
#  9.  'orange'
# 10.  'gray'
[download]

This will surely save me future editing time (as long as I'm on a system with Perl 5.10 installed).

Moreover, the idiom works with map just as nicely:

map { do_something_with($_, state $i++) } @array;
[download]

Does anyone else have examples to share, of idioms they've discovered, to decrease the time spent changing functionality in their Perl code?

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