September 21, 2007

Devel::REPL: Now with Multiline Support

I've been using Devel::REPL for a while now. Like all good modules (Perl::Critic, POE, Plagger, etc), it's very extensible. Devel::REPL's design is worth studying: keep a simple core and ship all the fancy behavior as plugins. Moose amplifies the power and convenience of this design with roles, method modifiers, and general awesomeness.

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. However, 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

in lua:

% lua
Lua 5.1.2  Copyright (C) 1994-2007 Lua.org, PUC-Rio
> function fact(n)
>> if n < 2 then
>> return 1
>> else
>> return n * fact(n-1)
>> end
>> end
> =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

D'oh. OK, let's try again..

% 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! :)

You can change the default prompts with my FancyPrompt plugin. Here's how I actually have my prompt (code is slightly different to show off nesting). It may look like "ugh", but it's actually really nice when you're in the driver's seat.

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

> fact 10
3628800

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

I'll continue stealing good features from other REPLs. :)