Programming Perl, 3rd Edition

Footers

While $^ ($FORMAT_TOP_NAME) contains the name of the current header format, there is no corresponding mechanism to do the same thing automatically for a footer. Not knowing how big a format is going to be until you evaluate it is one of the major problems. It's on the TODO list.^[2]

Here's one strategy: if you have a fixed-size footer, you can get footers by checking $- ($FORMAT_LINES_LEFT) before each write and then print the footer yourself if necessary.

Here's another strategy; open a pipe to yourself, using open(MESELF, "|-") (see the open entry in Chapter 29) and always write to MESELF instead of STDOUT. Have your child process postprocess its STDIN to rearrange headers and footers however you like. Not very convenient, but doable.

Accessing Formatting Internals

For low-level access to the internal formatting mechanism, you may use the built-in formline operator and access $^A (the $ACCUMULATOR variable) directly. (Formats essentially compile into a sequence of calls to formline.) For example:

$str = formline <<'END', 1,2,3;
@<<<  @|||  @>>>
END

print "Wow, I just stored `$^A' in the accumulator!\n";

Or to create an swrite subroutine that is to write as sprintf is to printf, do this:

use Carp;
sub swrite {
    croak "usage: swrite PICTURE ARGS" unless @_;
    my $format = shift;
    $^A = "";
    formline($format, @_);
    return $^A;
} 

$string = swrite(<<'END', 1, 2, 3);
Check me out
@<<<  @|||  @>>>
END
print $string;

If you were using the FileHandle module, you could use formline as follows ...