Day 2: The beauty of formatting

Day 2: The beauty of formatting

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

If you’re familiar with sprintf, you’ll feel right at home with .fmt. If you haven’t heard about sprintf before, or if you’ve heard of it but are a bit fuzzy on the details, you might want to skim the perldoc page. Don’t drown in it, though; it’s longish. Just savour it.

Back to .fmt, sprintf‘s spunky little sister. Here are a few ways to use .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 sprintf. Big deal, right?

What I haven’t told you yet is that .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, mappings):

  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 .fmt for pairs as well, but it works analogously to the one for hashes.

.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 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 too 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 flat 0..9,2 xx 3'

          #
         ###
        #####
       #######
      #########
     ###########
    #############
   ###############
  #################
         ###
         ###
         ###
[*] If you are using Windows, remember you need to switch the quotes around
c:\>perl6.exe -e "say ' 'x 9-$_,'#'x$_*2-1 for flat 0..9,2 xx 3"

13 thoughts on “Day 2: The beauty of formatting

      1. I started out writing it that way, but ran into unexpected trouble with Rakudo. And after I changed it, people told me this version was clearer. Though in retrospect, I think I agree that .kv would have been both more idiomatic and nicer. :)

  1. After the Great List Refactor, the following code works for creating the Christmas tree:

    perl6 -e ‘say ” “x 9-$_, “#”x$_*2-1 for (0..9,2 xx 3).flat’

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