Syntactic Changes (Part 2 of 3)

Ternaries

Ternaries in Raku are formed with ?? and !! rather than ? and : in Perl.

say $condition ?  "True" :  "False";  # Perl
say $condition ?? "True" !! "False";  # Raku

It’s really to free up the colon : for a lot of other uses in Raku. But more about that later.

Matching

The most obvious change is that Perl uses =~ to match a string with a regular expression:

# Perl
say "matched" if "foo" =~ /o/;  # matched

In Raku, the more generic ~~ (aka smartmatch) infix operator is used:

# Raku
say "matched" if "foo" ~~ /o/;  # matched

If your regular expression in Perl is simple enough, there’s a good chance it will just work in Raku (as seen above). Whenever you use positional captures, there’s a bit of a gotcha involved.

In Perl, positional captures start with $1:

# Perl
"foo" =~ /(\w)/;say $1;   # f

In Raku, positional captures start at $0. This makes sense if you realize that $0 is syntactic sugar for $/[0], in which the Match variable $/ is used as an array. And indices of arrays start at 0.

# Raku
"foo" ~~ /(\w)/;say $0;   # ｢f｣

Also note that in Raku, any capture is a full Match object, that stringifies with the ｢｣ special quotation marks.

Array and Hash Elements

In Raku, the sigil of an array or a hash does not change, regardless of how it is used. In Perl, the sigil of an array or hash changes depending on the context.

# Perl
my @array = (1,2,3,4,5);
say $array[0];      # 1, sigil changed
say @array[1,2,3];  # 234

Note how in Perl the sigil changed from @ to $ to get a single element out of the array. In Raku, there is "sigil invariance", the sigil never changes:

# Raku
my @array = 1,2,3,4,5;
say @array[0];      # 1, no sigil change
say @array[1,2,3];  # 234

The same for hashes:

# Perl
my %hash = (a => 42, b => 666);
say $hash{a};    # 42, sigil changed
say @hash{a,b};  # 42666

Note that in the case of hashes, a hash in Perl can have 3 different sigils. This has been a large part of newbie confusion in Perl. Which is why Raku does it differently, by not changing the sigil:

# Raku
my %hash = a => 42, b => 666;
say %hash<a>;    # 42, sigil does not change
say %hash<a b>;  # (42 666)

Also note that when specifying hash elements in Raku, the closest thing to using bare words is the < > syntax. This is closest to qw( ) in Perl.

Curly Close at end of line

In Perl, some structures that take a block (such as if and while) do not need a semi-colon after them. While others, such as eval, do need a semi-colon to prevent a syntax error:

# Perl
if ($condition) {
    …   # do something
}       # no semi-colon needed
eval {
    …   # some code that might die
};      # requires a semi-colon

In Raku, the rule with regards to a code block's closing curly brace is that if it is the last non-whitespace character in source code on a line, then a semi-colon will always be assumed to follow it.

This means that all code constructs that take a block, can be written like they are an if:

# Raku
try {
    …   # some code that might die
}       # no semi-colon needed

This also implies that if you create a subroutine that takes a block to execute, you do not have to specify a semi-colon either:

# Raku
sub call-times ($times, &code) {
    code() for 1 .. $times;
}
call-times 5, {
    say “hello world”;
}       # no semi-colon needed either

Summary

In this blog post the different syntax for ternaries, the different indexing on matches, the sigil invariance on arrays and hashes, and the standard behaviour of curly close brackets at the end of a line were covered.