In the Day 4 box, we saw an interesting implementation for the factorial function:
sub fac(Int $n) {
[*] 1..$n
}
Okay, so how does that work? Opening up today’s Advent box provides some answers!
Perl 6 has a number of different “meta operators” that modify the existing operators to perform more powerful functions.
The square brackets about are an example of the “reduce metaoperator”; they cause an infix operator to become a list operator that acts as though the infix was placed between each pair of elements. For example, the expression
[+] 1, $a, 5, $b
is essentially the same as writing
1 + $a + 5 + $b
This gives us a handy mechanism to “sum all elements in a list”:
$sum = [+] @a; # sum all elements of @a
Most of the infix operators (including user-defined operators) can be placed inside of square brackets to turn them into reductions:
$prod = [*] @a; # multiply all elements of @a
$mean = ([+] @a) / @a; # calculate mean of @a
$sorted = [<=] @a; # true if elements of @a are numerically sorted
$min = [min] @a, @b; # find the smallest element of @a and @b combined
So, in the factorial subroutine above, the expression [*] 1..$n returns the product of multiplying all of 1 through $n together.
Another useful metaoperator is the “hyper” operator. Placing »
and/or « (or the ASCII >> and << equivalents) next to an operator makes it “hyper”, which causes it operate on elements of lists. For example, the following calculates @c as the pairwise addition of the elements in @a and @b:
@c = @a »+« @b;
In Perl 5, we’d generally write something like
for ($i = 0; $i < @a; $i++) {
$c[$i] = $a[$i] + $b[$i];
}
which is quite a bit longer.
As with the square brackets above, we can use hyper on a variety of operators, including user-defined operators:
# increment all elements of @xyz
@xyz»++
# each element of @x is the smaller of @a and @b
@x = @a »min« @b;
We can also flip the angles to enable a scalar to act like an array:
# multiply each element of @a by 3.5
@b = @a »*» 3.5;
# multiply each element of @x by $m and add $b
@y = @x »*» $m »+» $b;
# invert all elements of @x
@inv = 1 «/« @x;
# concatenate @last, @first to produce @full
@full = (@last »~» ', ') »~« @first;
Of course, reductions and hyper operators can be combined in expressions:
# calculate the sum of squares of @x
$sumsq = [+] ( @x »**» 2);
There are many other metaoperators available, including X (cross), R (reverse), S (sequential). In fact, the “in-place” operators such as +=, *=, ~=, are just meta forms created by suffixing an operator with an equals sign:
$a += 5; # same as $a = $a + 5;
$b //= 7; # same as $b = $b // 7;
$c min= $d; # same as $c = $c min $d;
