What is a console? How it can assist you?

Most modern languages have consoles. The console is an interactive tool that evaluates your input while you type it. It gives you several advantages:

• Quickly test some thought or tricky expression
• Run some code bigger than one line without a temporary file
• Play around with libraries and modules
• You can even call a console in your script and play around in script's context

For Ruby it would be irb, for Python is... python by itself and for perl... and there was nothing for perl (except that ugly perl -d -e "" and several failed projects) until Devel::REPL was written by Matt S Trout (a.k.a. mst) from ShadowCatSystems <http://www.shadowcatsystems.co.uk>.

Devel::REPL - the Perl console

REPL stands for Read, Evaluate, Print, Loop. Lets install and try it.

       $ cpan Devel::REPL

After installation you have a lot of new modules, but the most interesting things are:

• Devel::REPL
  A top level module.
• re.pl
  Wrapper script, running console.

And a bunch of plugins (I'll describe them later). In command line type:

      $ re.pl

If everything is ok you'll see a prompt (underlined $). That's it. You can start typing expressions.

An example session:

  $ sub factorial {
  > my $number = shift;
  > return $number > 1 ? $number * factorial($number-1) : $number;
  > }
  $ factorial 1 # by the way, comments are allowed
  1 # our return value
  $ factorial 5
  120
  $ [1,2,3,4,5,6,7]
  $ARRAY1 = [
              1,
              2,
              3, # return values are printed with Data::Dumper::Streamer.
              4, # See Plugins section
              5,
              6,
              7
            ];
  $ {apple=>1,fruit=>'apple',cart=>['apple','banana']}
  $HASH1 = {
            apple => 1,
            cart  => [
                      'apple',
                      'banana'
            ],
            fruit => 'apple'
  };
  $ package MyPackage; # create a package
  $ sub say_hi { # define a sub
  > print "Hi!\n";
  > } # statement is evaluated only after we've finished typing block.
      # See Plugins section.
  > __PACKAGE__
  MyPackage
  > package main;
  > __PACKAGE_
  main
  > MyPackage->say_hi
  Hi!
  1
  $

Control files a.k.a. I don't want to type it every time

Devel::REPL has a control files feature. Control files are evaluated on session start in the same way as you would type them manually in the console.

The default control file is located at $HOME/.re.pl/repl.rc.

You can store there any statements you would normally type in.

I.e. my $HOME/.re.pl/repl.rc has next lines:

      use feature 'say'; # to don't write \n all the time
      use Data::Dumper;
      # pretty print data structures
      sub pp { print Data::Dumper->Dump([@_]) }

You can have multiple control files and they can be anywhere in the file system. To make re.pl use some rc-file other than repl.rc, call it like this:

      $ re.pl --rcfile /path/to/your/rc.file

If your rc-file is in $HOME/.re.pl directory, you can omit the path:

      $ re.pl --rcfile rc.file

If you have rc-file with the same name in current directory and you don't want to type path, you can:

      $ re.pl --rcfile ./rc.file

I want it to bark, fly, jump and swim! or Plugins

Plugins extend functionality and change behavior of Devel::REPL. Bundled plugins are:

• Devel::REPL::Plugin::History
  No comments. Simply history.
• Devel::REPL::Plugin::!LexEnv
  Provides a lexical environment for the Devel::REPL.
• Devel::REPL::Plugin::DDS
  Formats return values with Data::Dump::Streamer module.
• Devel::REPL::Plugin::Packages
  Keeps track of which package your're in.
• Devel::REPL::Plugin::Commands
  Generic command creation plugin using injected functions.
• Devel::REPL::Plugin::MultiLine::PPI
  Makes Devel::REPL read your input until your block
  is finished. What does this means: you can type a part of a block
  on one line and second part on another:

         $ sub mysub {
         > print "Hello, World!\n"; ## notice prompt change
         > }
         $ mysub
         Hello, World!
         1
         $
    but this *doesn't* mean you can print sub name or identifier
    on several lines. Don't do that! It won't work.
      

There are lots of contributed plugins you can find at CPAN.