The Pearls of Raku, Issue 3: tr, TR, and StrDistance

Hello! Today, let me show three small but interesting things that you might skip when reading the documentation to the Raku programming language (if you did read it :-).

Table of Contents

tr ad TR

There is a pair of transliteration (or transformation) operators in Raku: tr and TR . As you may guess from the name, they have something in common. But there are also some differences.

The lower case tr performs an in-place substitution:

my $s = 'Hello, World!';

$s ~~ <strong>tr</strong>/o/ó/;

say $s; # Helló, Wórld!

The target string is updated after the operation.

Unlike that, TR does not change the string and returns the new string as a result.

my $s = 'Hello, World!';

my $r = <strong>TR</strong>/o/ó/ <strong>with</strong> $s;

say $s; # Hello, World!
say $r; # Helló, Wórld!

Note that you cannot simply write my $r = $s ~~ TR/o/ó/ , but instead you need to set the topic first, for example,using with . You can do the same with the lowercase version, too:

tr/o/ó/, .say with $s; # Helló, Wórld!

StrDistance

OK, what happens if you save the result of matching with tr as in the following snippet?

my $s = 'Hello, World!';

<strong>my $r = $s ~~ tr/o/ó/;</strong>

Unlike the probable expectation, you do not simply get the new string in $r . The type of that object is StrDistance .

say $r.WHAT; # (StrDistance)

When being coerced to a string, it gives you the string—the modified string after the replacement in our case. When it is coerced to an integer, you get the number of replacements happened:

say <strong>~</strong>$r; # Helló, Wórld!
say <strong>+</strong>$r; # 2

The variable also keeps both original and modified versions of the string:

say $r.before;
say $r.after;

put vs get

The third thing to cover in this post is to see the difference between put and say , when they are called with the StrDistance object.

The difference between put and say is that put internally calls .Str on an object, while say calls .gist .

From the previous section, we know that if a StrDistance is converted to a string, it gives a string, so in our example, you see the updated string after the transliteration:

<strong>put</strong> $r; # Helló, Wórld!

With say , you see more details about the contents:

<strong>say</strong> $r; 
# StrDistance.new(before => "Hello, World!", after => "Helló, Wórld!")