> By the way, it appears rather difficult to make a Ruby IDE
> with tab-completion

The Zeus for Windows programmer's editor has a generic form
of code completion that derives it's information from the
tags generated by Exuberant Ctags.

The Exuberant Ctags lists Ruby as one of the supported
languages:

  http://ctags.sourceforge.net/languages.html

so in theory this means the Zeus, Ruby code completion
should work in some limited fashion.

> because you can't know in advance the type of any object,
> and methods may be added and changed on the fly.

This form of dynamic code completion is definitely very
difficult to implement as it requires the editor/IDE to
have built-in knowledge of the language.

But for the more static type of coding, where some form
of ctags information is generated, a limited form of code
completion should be available.

Jussi Jumppanen
Author: Zeus for Windows
Note: Zeus is shareware (45 day trial).