Perl 6: Scalar, Array and Hash interpolation

Abstract

The second entry of the newsletter (the first was Perl 6: Arrays with unique values) in which you'll see how to interpolate variables and expressions in a string.

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This entry was first sent out as part of the . Visit .

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Welcome back to the Perl 6 Tricks and Treats

This is the second message in 2 days already but don't worry, I don't think I'll be able to keep up the frequency.

The list does not have an archive, can be found on my web site.

I'd like to thank Thomas Maier who pointed out a bug in the installation instructions I gave. That's awesome. That means at least he tried to follow them.

Anyway, I won't send his fix now, I'll let you all find it yourself; :-)

Besides, - Rakudo project lead - also contacted me and pointed me to the current most up-to date set of instructions they are maintaining on the .

I am really happy that some people pointed out flaws in my first posting already. Keep it that way!

Scalar, Array and Hash interpolation

If you place a scalar variable in a double quoted string Perl 6 will put the value of the scalar in the the string just as it happens in Perl 5.

    use v6;

    my $name = "Foo";
    say "Hello $name, how are you?";
    

And the output will look like this:

    Hello Foo, how are you?

Arrays and Hashes are not interpolated in the same way, but there is a much more extended way for interpolating such variables.

Within a string anything you put in curly braces will be interpolated so if you have an array you can say:

    my @names = ;
    say "Hello {@names}, how are you?";
    

Output:

    Hello Foo Bar Moo, how are you?

The problem here is that based on that output you don't know if there were 3 elements in that array or two "Foo Bar" and "Moo" or just one: "Foo Bar Moo".

Interpolating expressions

But that's not a real problem, actually the curly braces in the double quoted string allow you to put any expression between them, the expression will be evaluated and the result will be interpolated: So you can say:

    say "Hello { join(', ', @names) } how are you?";

And the output will look like:

    Hello Foo, Bar, Moo how are you?

While this still does not exactly show how many values are there, it provide a much bigger flexibility. As a side note, for the object oriented people out there, you could write the above code as follows:

    say "Hello { @names.join(', ') } how are you?";

Which will yield the same output as above.

Debugging prints

For ultimate debugging purposes, the best way to print is of course using the .perl method of the array:

    say "Names: {@names.perl}";

That will print

    Names: ["Foo", "Bar", "Moo"]

But what if you'd also like to see the name of your variable as well. Then you can rely on the fact that arrays are not normally interpolated in strings and write:

    say "@names = {@names.perl}";

That will print

    @names = ["Foo", "Bar", "Moo"]

Just expressions

    say "Take 1+4";

will print

    Take 1+4

as expected but as I wrote you can put any expression in the curly braces, so you can also write

    say "Take {1+4}";

that will print

    Take 5

I wonder when will it print .

Interpolating Hashes

There is no any specially in interpolating hashes. It works just the same as

    use v6;

    my %phone = (
        foo => 1,
        bar => 2,
    );

    say "%phone = { %phone } ";

will print

    %phone = foo   1
    bar  2

which is not that nice so better use the .perl method and write:

    say "%phone = { %phone.perl } ";

which will print:

    %phone = {"foo" => 1, "bar" => 2}

that's already a usable representation of the data.

Interpolating multidimensional arrays

Just to complete the picture I'd like to show a two dimensional array and how it looks when printed with the .perl method and interpolated in a string.

    use v6;

    my @matrix = (
        [1, 2],
        [3, 4],
    );

    say "@matrix = { @matrix.perl }";

Output:

    @matrix = [[1, 2], [3, 4]]

Getting Perl 6

I am using and recommending the Rakudo implementation of Perl 6 that runs on top of the Parrot Virtual Machine.

See the up-to-date instructions on on the Rakudo web site