advent blog post about .fmt

Unwrapping the second gift brought to you by Perl 6 this Advent, we find... a
method named C<.fmt>.

If you're familiar with C<sprintf>, you'll feel right at home with C<.fmt>. If
you haven't heard about C<sprintf> before, or if you've heard of it but are a
bit fuzzy on the details, you might want to skim the L<perldoc
page|http://perldoc.perl.org/functions/sprintf.html>. Don't drown in it,
though; it's longish. Just savour it.

Back to C<.fmt>, C<sprintf>'s spunky little sister. Here are a few ways to use
C<.fmt> to format strings and integers.

  say 42.fmt('%+d')                # '+42'
  say 42.fmt('%4d')                # '  42'
  say 42.fmt('%04d')               # '0042'
  say :16<1337f00d>.fmt('%X')      # '1337F00D'  

All this is good and well, but not really more than a shorter method form of
C<sprintf>. Big deal, right?

What I haven't told you yet is that C<.fmt> is overloaded, and works
differently on arrays (or more precisely, lists):

  say <huey dewey louie>.fmt       # 'huey dewey louie'
  say <10 11 12>.fmt('%x')         # 'a b c'
  say <1 2 3>.fmt('%02d', '; ')    # '01; 02; 03'

Similarly, it's overridden on hashes (or rather, maps):

  say { foo => 1, bar => 2 }.fmt   # 'foo     1
                                   #  bar     2'
  
  say { Apples => 5, Oranges => 10 }.fmt('%s cost %d euros')
                                   # 'Apples cost 5 euros
                                   #  Oranges cost 10 euros'
  
  say { huey => 1, dewey => 2, louie => 3 }.fmt('%s', ' -- ')
                                   # 'huey -- dewey -- louie'

The way hashing works may give your output a different order than the ones
shown above. Oh, and there's an overloaded C<.fmt> for pairs as well, but
it works analogously to the one for hashes.

C<.fmt> is a useful little tool to have when you want to change some value,
or an array or a hash of values, into to some given format. It's like
C<sprintf>, but tailored to Do What You Mean for arrays and hashes, too.

There's only one risk in all of this: Perl 6 might soil the reputation of
the Perl family of languages by simply being to darn readable. In order
to counter this risk, I leave a small parting gift in the form of a
simple-but-dense Christmas tree printing Perl 6 one liner:

  $ perl6 -e 'say " "x 9-$_,"#"x($_*2-1)for 0..9,2xx 3'
  
          #
         ###
        #####
       #######
      #########
     ###########
    #############
   ###############
  #################
         ###
         ###
         ###