Hash of Functions

Hats off!! to larry wall.. a code snippet from his book for me to recall his creative brilliance.

When writing a complex application or network service in Perl, you might want to make a large number of commands available to your users. Such a program might have code like this to examine the user's selection and take appropriate action:

 if    ($cmd =~ /^exit$/i)     { exit }
 elsif ($cmd =~ /^help$/i)     { show_help() }
 elsif ($cmd =~ /^watch$/i)    { $watch = 1 }
 elsif ($cmd =~ /^mail$/i)     { mail_msg($msg) }
 elsif ($cmd =~ /^edit$/i)     { $edited++; editmsg($msg); }
 elsif ($cmd =~ /^delete$/i)   { confirm_kill() }
 else {
   warn "Unknown command: `$cmd'; Try `help' next time\n";
 }

You can also store references to functions in your data structures, just as you can store references to arrays or hashes:

 %HoF = (                           # Compose a hash of functions
   exit    =>  sub { exit },
   help    =>  \&show_help,
   watch   =>  sub { $watch = 1 },
   mail    =>  sub { mail_msg($msg) },
   edit    =>  sub { $edited++; editmsg($msg); },
   delete  =>  \&confirm_kill,
 );

 if   ($HoF{lc $cmd}) { $HoF{lc $cmd}->() }   # Call function
 else { warn "Unknown command: `$cmd'; Try `help' next time\n" }

In the second to last line, we check whether the specified command name (in lowercase) exists in our "dispatch table", %HoF. If so, we invoke the appropriate command by dereferencing the hash value as a function and pass that function an empty argument list. We could also have dereferenced it as &{ $HoF{lc $cmd} }(), or simply $HoF{lc $cmd}().

For more complex datastructures and references refer to :

http://docstore.mik.ua/orelly/perl/prog3/ch09_06.htm