Hi Zak,

> I've gotten a bit spoiled using Emacs with SLIME while
> learning Lisp.  It can evaluate individual expressions
> within the file being edited, making it easy to test
> things in very small increments.

The Emacs mode that comes with Ruby does this.

See `ruby-send-region', bound to C-c C-r by default.
There's also `ruby-send-definition', bound to C-c C-x,
and `ruby-send-block', bound to C-c C-b.  To go to the
Ruby REPL (an IRB session), type C-c C-s.

There's also the handy C-c M-r, C-c M-x and C-c M-b, which
do the same thing as the aforementioned `ruby-send-*'
commands but also switch to the REPL buffer for you.

> [...]

> In Lisp, a function definition, class definition,
> etc... is a single expression, with subexpressions.
> I'm not quite sure how the terminology translates to Ruby

No translation necessary. :-)

Just about everything in Ruby is an expression.

-- 
Daniel Brockman <daniel / brockman.se>

    So really, we all have to ask ourselves:
    Am I waiting for RMS to do this?   --TTN.