Day 13 – String Interpolation and the Zen Slice

So you think you know all about string interpolation in Perl 6?

Well, especially coming from Perl 5, you may find some things that do not work exactly as you’re used to. The simple examples all work, as it seems:

my $a = 42;
say 'value = $a'; # value = $a
say "value = $a"; # value = 42

my @a = ^10;
say 'value = @a'; # value = @a
say "value = @a"; # value = @a HUH??


In earlier versions of Perl 5 (or was it Perl 4?), this gave the same result. At one point, it was decided that arrays would be interpolated in double quoted strings as well. However, this caused quite a few problems with double quoted texts with email addresses in them: you would need to escape each @ (if you were using strict, which you of course should have). And if you forgot, and there was an array that just happened to have the same name as the user part of an email address, you would be in for a surprise. Or if you didn’t use strict, you would suddenly have the text without the email address. But then again, you got what you asked for.

So how can we make this work in Perl 6?

Introducing the Zen slice

The Zen slice on an object, returns the object. It’s like you specify nothing, and get everything. So what does that look like?

my @a = ^10;
say "value = @a[]"; # value = 0 1 2 3 4 5 6 7 8 9


You will have to make sure that you use the right indexers for the type of variable that you’re interpolating.

my %h = a => 42, b => 666;
say "value = %h{}"; # value = a 42 b 666


Note that the Zen slice on a hash returns both keys and values, whereas the Zen slice on an array only returns the values. This seems inconsistent, until you realize that you can think of a hash as a list of Pairs.

The Zen slice only really exists at compile time. So you will not get everything if your slice specification is an empty list at runtime:

my @a;
my %h = a => 42, b => 666;
# a slice, but not a Zen slice:
say "value = %h{@a}"; # value =


So the only way you can specify a Zen slice, is if there is nothing (but whitespace) between the slice delimiters.

The Whatever slice

The * ( Whatever ) slice is different. The Whatever will just fill in all keys that exist in the object, and thus only return the values of a hash.

my %h = a => 42, b => 666;
say "value = %h{*}"; # value = 42 666


For arrays, there isn’t really any difference at the moment (although that may change in the future when multi-dimensional arrays are fleshed out more).

Interpolating results from subs and methods

In double quoted strings, you can also interpolate subroutine calls, as long as they start with an ‘&‘ and have a set of parentheses (even if you don’t want to pass any arguments):

sub a { 42 }
say "value = &a()"; # value = 42


But it doesn’t stop at calling subs: you can also call a method on a variable as long as they have parentheses at the end:

my %h = a => 42, b => 666;
say "value = %h.keys()"; # value = b a


And it doesn’t stay limited to a single method call: you can have as many as you want, provided the last one has parentheses:

my %h = a => 42, b => 666;
say "value = %h.perl.EVAL.perl.EVAL.perl()"; # value = ("b" => 666, "a" => 42).hash


Interpolating expressions

If you want to interpolate an expression in a double quoted string, you can also do that by providing an executable block inside the string:

say "6 * 7 = { 6 * 7 }"; # 6 * 7 = 42


The result of the execution of the block, is what will be interpolated in the string. Well, what really is interpolated in a string, is the result of calling the .Str method on the value. This is different from just saying a value, in which case the .gist method is called. Suppose we have our own class with its own .gist and .Str methods:

class A {
    method Str { "foo" }
    method gist { "bar" }
}
say "value = { A }"; # value = foo
say "value = ", A;   # value = bar


Conclusion

String interpolation in Perl 6 is very powerful. As you can see, the Zen slice makes it easy to interpolate whole arrays and hashes in a string.

In this post I have only scratched the surface of string interpolation. Please check out Quoting on Steroids in a few days for more about quoting constructs.