Here is something interesting I found through playing around with map, my favorite Perl toy. I've not the faintest if this could be useful in a more general way or if it's just, well, interesting crap...

Here's how it emerged:

Just as an exercise, I wanted to generate a list of strings with a specific length from every possible combination of a list of characters.

But not with for loops, but with nested maps! In its simplest form, with 2 chars and strings with a length of two:

my @a = qw( a b );
my @b = map { my $x = $_;
              map { $x . $_ } @a;
            } @a;
[download]

This produces aa, ab, ba, bb.

Adding characters is easy:

my @a = qw( a b c );
[download]

This produces aa, ab, ac, ba, bb, bc, ca, cb, cc.

But increasing the length of the string is not so easy. I did this by adding another nested map:

my @b = map { my $x = $_;
              map { my $y = $_;
                    map { $x . $y . $_ } @a;
                  } @a;
            } @a;
[download]

which, with qw( a b ), produces aaa, aab, aba, abb, baa, bab, bba, bbb. This is, of course, stupid.

One way to solve this problem is with a recursive function:

sub f
{ my ( $n, @a ) = @_;
  croak "illegal value for n: $n" if $n < 1;
  return @a if $n == 1;
  map { my $x = $_;
        map { $x . $_ ) } f( $n - 1, @a );
      } @a;
}
[download]

Now you could make it more general by adding a coderef parameter, string concatenation being only a special case needed in the original example:

sub f
{ my ( $n, $g, @a ) = @_;
  croak "illegal value for n: $n" if $n < 1;
  return @a if $n == 1;
  map { my $x = $_;
        map { $g->( $x, $_ ) } f( $n - 1, $g, @a );
      } @a;
}
[download]

So we have a function f that produces a list b by applying a function g to all possible combinations of a list a by calling itself recursively n times.

Is this useful? Or could it be further transformed so that it yields something useful?

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