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Posts Tagged ‘repl’

More on REPLs

Most people who think they want a REPL don’t actually want a REPL. It is what a REPL can give them – a Perl (or a Python or a Ruby) machine to which you can add functionality a bit at a time.

For an emacser like me, a sufficiently flexible command-line REPL can give me that incrementally adaptable machine. For most people, a graphical interface to that machine would probably be better.

And thanks, folks, for the suggestions of REPLs to take a look at. No doubt I’ll get around to evaluating a few of them at some point.

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The Problem with Scripting Language REPLs

I figured out what the problem is with Devel::REPL and the command line REPLs provided by Python and Ruby – evaluation is not a separate step. When I press enter, I’m forced to evaluate the current line.

Yes, you say, that is what REPL means – Read, Evalute, Print, Loop. Evaluate comes after Read.

In reality, usable REPLs, such as Emacs1, let you control when the read evaluate print sequence happens. I can craft the most beautiful function I can think of. Better still, if I change my mind, I can easily modify the function and redefine it with a keystroke. Well, a key chord at least.

In contrast, with Devel::REPL, once I have pressed enter, changing my mind is painful. Integrating it with Emacs comint will probably alleviate a lot of that pain.

Or better yet, as Anonymous recommends, I should take a look at Sepia or PDE which already have emacs integration. Having said that, basic integration is, what, 20 lines of emacs-lisp?


1. Yes, Emacs is a REPL. Kinda.

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I’m somewhat amused at one of the more recent comments here – Nathan L. Walls defends his "choice" of Ruby with some very woolly justifications (emphasis mine):

"Ruby’s community feels more vibrant. No, not something you can measure. It is a feeling."

"Yes, there are equivalents in Perl, but they are far rougher. Again, not really measurable, but a feeling."

Of course, his day job is still writing Perl. Moving swiftly on…

Devel::Repl

The main thing I got out of the comment apart from a chuckle, was it motivated me to look at Devel::REPL.

One of the other main tools in my toolbox is emacs and when writing emacs lisp, I make full use of the REPL. But I’ve never even wanted an equivalent in Perl.

One cpanm invocation later and I’m ready.

Wait, no I’m not. I copied Caleb’s repl.rc config to make it more usable. I added MultiLine::PPI which resulted in a bunch of errors at start-up. It turns out I need to add File::Next and B::Keywords separately.

$ cpanm File::Next
$ cpanm B::Keywords

Okay, now I’m good to go.

First REPL session

$ jared@localhost $ re.pl
$ sub f
$ {
> say 'h';
> say 'hello';
> }
h
hello
1 $ f();
Runtime error: Undefined subroutine &Devel::REPL::Plugin::Packages::DefaultScratchpad::f called at (eval 290) line 5.
$ sub f {
> say 'h';
> say 'hello';
> }
$ f();
h
hello
1 $

It still isn’t quite perfect. But to be honest, I find it (and the Python and irb REPLs) pretty useless. I probably need to look into integrating it with emacs comint.

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