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.