Ha ha, nysus just reminded me of an old interview question I used to ask. Implement a simple dispatch table.

Let's start with a specification:

• The key of the dispatch table is a string \w+
• The name of the callback function is the key name with _callback appended
• Each callback function takes a single string parameter and returns a positive number

You must write the invoker function, which takes two arguments (the name and the string argument to be passed to the callback):

• If the name is invalid (e.g. "fred" below), invoker must return a negative number
• Otherwise, invoker must pass its second argument to the callback function and return what the callback function returns

To clarify, here is a sample implementation.

use strict;
use warnings;

# Callback functions ---------------------------------------

sub first_callback {
   my $z = shift;
   print "in first_callback, z=$z\n";
   return 1;
}

sub last_callback {
   my $z = shift;
   print "in last_callback, z=$z\n";
   return 2;
}

# Implementation of dispatch table -------------------------
# (You need to write this code)

my %op_table = ( first  => \&first_callback,
                 last   => \&last_callback,
               );

sub invoker
{
   my ($name, $z) = @_;
   exists($op_table{$name}) or return -1;
   $op_table{$name}->($z);
}

# Main program for testing ---------------------------------

for my $name ( "first", "last", "fred" ) {
   my $rc = invoker( $name, $name . '-arg' );
   print "$name: rc=$rc\n";
}
[download]

Running the above test program produces:

in first_callback, z=first-arg
first: rc=1
in last_callback, z=last-arg
last: rc=2
fred: rc=-1
[download]

Points to consider:

• Is a hash the recommended way to implement a dispatch table in Perl?
• How many other ways can you think of to implement it in Perl? (working demonstration code would be good)