Advanced Perl Programming

Pretty-Printing

In building complicated data structures, it is always nice to have a pretty-printer handy for debugging. There are at least two options for pretty-printing data structures. The first is the Perl debugger itself. It uses a function called dumpValue in a file called dumpvar.pl, which can be found in the standard library directory. We can help ourselves to it, with the caveat that it is an unadvertised function and could change someday. To pretty-print this structure, for example:

@sample = (11.233,{3 => 4, "hello" => [6,7]});

we write the following:

require 'dumpvar.pl';
dumpValue(\@sample); # always pass by reference

This prints something like this:

0  11.233
1  HASH(0xb75dc0)
   3 => 4
   'hello' => ARRAY(0xc70858)
      0  6
      1  7

We will cover the require statement in Chapter 6. Meanwhile, just think of it as a fancy #include (which doesn’t load the file if it is already loaded).

The Data::Dumper module available from CPAN is another viable alternative for pretty-printing. Chapter 10, covers this module in some detail, so we will not say any more about it here. Both modules detect circular references and handle subroutine and glob references.

It is fun and instructive to write a pretty-printer ourselves. Example 2.5 illustrates a simple effort, which accounts for circular references but doesn’t follow typeglobs or subroutine references. This example is used as follows:

pretty_print(@sample); # Doesn't need a reference

This prints

11.233 { # HASH(0xb78b00) : 3 => 4 : hello => : : [ # ...