One of the complaints about Perl was always that it lacks a real
case or switch statement. While you could always fake one, people
were not satisfied. Of course if you give them a switch statement
they will find something else....
So in accordance with the Perl 6 design perl will NOT have a case
or switch. It will have a given keyword. It will also behave
differently than the usual case. It will do The Right Thing (tm).
The syntax is quite simple:
given($value) {
when(3) { say "Three"; }
when(7) { say "Seven"; }
when(9) { say "Nine"; }
default { say "None of the expected values"; }
}
We are taking the value in $value and comparing it to the
values within the when() statements. When we find one that
matches, the block after the when() is executed AND the
given statement is terminated. That is, no more when()
is checked.
If non of the when() cases fit then the (optional)
default {} block is executed.
Let's see another example:
given($value) {
when(/^\d+$/) { say "digits only"; }
when(/^\w+$/) { say "Word characters"; }
when(/^[a-zA-Z0-9.-]+$/) { say "Domain namish"; }
default { say "None of the expected"; }
}
It is very similar but now we have regular expressions instead
of fixed values in the when() statement. Each regex on its
turned is tried against the value. When one of them matches
its block is executed and the given() statement is terminated.
Yes, what you are suspecting is right. The when() statements
are actually applying ~~, the smart match operator.
I have already written about it earlier in Smart Matching in Perl 5.10.
So you can use any kind of value in the when() statement.
A number will check numeric equality using ==, a string will use
eq, a regex will try to match the given value and if you
supply a subroutine reference then Perl will call that subroutine
using the given value as a parameter and check the true-ness
of the return value.
See this example:
given($value) {
when(10) {
say "Number 10";
}
when([11, 23, 48]) {
say "In the list";
}
when(/^\d+$/) {
say "digits only";
}
when(\&is_number) {
say "Is number";
}
default {
say "None of the above";
}
}
sub is_number {
return $_[0] =~ /\d/ and $_[0] =~ /^[+-]?\d*\.\d*$/;
}
There are few more minor issues:
Perl will automatically break out from the given()
statement after the execution of block. If you would like to
force checking the additional when() statements use the
continue keyword.
On the other hand if you would like to break out from a given()
statement before reaching the end of the when() block, you can use the
break statement yourself.
The given() actually assigns the $value to $_ so you can use that
as well to write when clauses such as this:
when($_ < 18) { say "Less than 18"; }
when() can be used outside the given() block as well.
