Emacs ruby-mode and RVM propmt

On emacs ruby-mode, if you do M-x run-ruby (or C-c C-s), the inferior ruby interpreter is provided in the "*ruby*" buffer so that you can evaluate lots of ruby statements or expressions.

When you press M-p (M-x comint-previous-input), Emacs will cycle to the previous input histrory saving input, so that you can easily re-evaulate of your previous input.

It works like other inferior interpreter provided by python-mode, slime, and so on.

What makes me frustrated is, it works for the ruby 1.8 installed in my gentoo system, but it does not work for the ruby 1.9.3 installed via RVM. The minibuffer shows sulpurous error:

Search failed: "^irb(.*)[0-9:]+0> *"

Looking at the ruby-mode's source, I found that string is the value of 'inferior-ruby-first-prompt-pattern' in inf-ruby.el. It looks like that ruby-mode uses inferior-ruby-first-prompt-pattern and inferior-ruby-prompt-pattern for the ruby's prompt pattern.

Another notable different from the start between two rubies (v1.8 system versus v1.9 RVM) are their prompt patterns:

# ruby 1.8 (system)
irb(main):001:0> _

# ruby 1.9.3 (RVM)
ruby-1.9.2-p180 :001 > _

Now it's clear who is the culprit. Due to the difference of the prompt pattern, ruby-mode could not provide me M-p or M-n working.

So I modified two variables in inf-ruby.el and reports a bug to the Ruby issue tracking system and got a quick response.

It turns out that it's not the fault of ruby-mode, but it's the fault of RVM. (Actually, it's not a defect. It just not right for my purpose.)

By default, RVM uses different prompt from the native ruby. To solve this, you can force RVM ruby (irb) to use the native prompt pattern. It's simple: modify $HOME/.irbrc to include following:

IRB.conf[:PROMPT_MODE] = :DEFAULT

I feel embbarassed that I blamed ruby-mode when I found something is wrong :(