use Perl

There are plugins to dump output with Data::Dump::Streamer, enable tab completion of the current lexical environment and loaded modules, save input history across sessions, and more. If you dabble in other P-languages such as Python and Ruby (know thy enemy.. honest!) you'll find yourself wanting more out of Devel::REPL. Let's take the example of writing a factorial function in python:

% python
Python 2.3.5 (#1, Dec  7 2006, 14:50:51)
[GCC 4.0.1 (Apple Computer, Inc. build 5363) (+4864187)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def fact(n):
...     if n < 2:
...         return 1
...     return n * fact(n - 1)
...
>>> fact(10)
3628800

in irb:

% irb
irb(main):001:0> def fact(n)
irb(main):002:1> if n < 2
irb(main):003:2> 1
irb(main):004:2> else
irb(main):005:2* n * fact(n-1)
irb(main):006:2> end
irb(main):007:1> end
=> nil
irb(main):008:0> fact 10
=> 3628800

and in Devel::REPL:

% re.pl
$ sub fact {
Compile error: Missing right curly or square bracket at (eval 60) line
8, at end of line
syntax error at (eval 60) line 8, at EOF

% re.pl
$ sub fact { my $n = shift; return 1 if $n < 2; $n * fact($n - 1) }

$ fact 10
3628800

Well, that works in this case, but one big line of code quickly becomes unmanageable.

Recently I had the idea to use PPI to figure out if the current line of code is complete. PPI::Dumper quickly confirmed that I can detect the most important case: a PPI::Structure that doesn't have both a ->start and ->finish. Structures encompass { {nested { blocks } } }, (parentheses), [array indexing], {hash indexing}, and so on. Hopefully future versions of PPI will be able to figure out that, say, an s/// or quoted string is incomplete.

Here's what the factorial example looks like with the MultiLine::PPI plugin.

% re.pl
$ load_plugin 'MultiLine::PPI'
1
$ sub fact {
> my $n = shift;
> return 1 if $n < 2;
> $n * fact($n - 1);
> }

$ fact 10
3628800

I believe Devel::REPL is the only Perl REPL that can do this. Hooray! :)

MultiLine::PPI hasn't been CPANed yet, but you can get it (and other new plugins) from the Devel::REPL Subversion repository.

I'll continue stealing good features from irb and python.