Question

Basically I want to get user input and decide what to do, and I need to have the same outcome even for different user inputs sometimes. Example code:

given ($command) {
      when ('a' || 'add') {add}
      when ('s' || 'subtract') {subtract}
      when ('m' || 'multiply') {multiply}
      when ('d' || 'divide') {divide}
      default { print "try again, usage: add, subtract, multiply, divide (a, s, m, d)\n" }

Now the trouble with this is that it only works if I type the single letter commands at the beginning of the or statements. If I type something like “add” into the prompt, I get the default message telling me to try again.

I could make separate cases for these, but it would just have the same body as the single letters, which is just redundant.

Answer 1

The given/when construct uses the smart match operator, which doesn’t have any special-case behavior for the || operator. The expression

'a' || 'add'

evaluates to 'a', so it just matches $command against the string 'a'.

If you want to match $command against any of a list of values, you can use an array value:

given ($command) {
    when ([qw(a add)]) { add }
...

(As usual, there are several ways to express this.)

References:

perldoc -f given

perldoc perlsyn; search for “Switch statements” and “Smart matching in detail”.

EDIT :

Another approach for this problem, if you want to allow $command to be any prefix of the command name (e.g., "s", "su", "sub", …, "subtract") is something like this:

sub match {
    my($s, $target) = @_;
    return index($target, $s) == 0;
}

...

if (match($command, 'add') { ... }
elsif (match ($command, 'subtract') { ... }
...