Advanced Perl Programming

Using Subroutine References

Let’s look at some common examples of using subroutine references: callback functions and higher-order procedures.

A callback function is an ordinary subroutine whose reference is passed around. The caller (who uses that reference) doesn’t necessarily have an idea of which subroutine is getting invoked. Let’s examine three simple examples involving callback functions: dispatch tables, signal handlers, and plotting functions.

Dispatch Table

A typical dispatch table is an array of subroutine references. The following example shows %options as a dispatch table that maps a set of command-line options to different subroutines:

%options = (       # For each option, call appropriate subroutine.
   "-h"         => \&help,
   "-f"         => sub {$askNoQuestions = 1},
   "-r"         => sub {$recursive = 1},
   "_default_"  => \&default,
);

ProcessArgs (\@ARGV, \%options); # Pass both as references

Some of these references in this code are to named subroutines. Others don’t do much, so it is just simpler to code them as inline, anonymous subroutines. ProcessArgs can now be written in a very generic way. It takes two arguments: a reference to an array that it parses and a mapping of options that it refers to while processing the array. For each option, it calls the appropriate “mapped” function, and if an invalid flag is supplied in @ARGV, it calls the function corresponding to the string _default_.

ProcessArgs is shown in Example 4.1.

Example 4-1. ProcessArgs

ProcessArgs (\@ARGV, \%options); # Pass both as ...