Does Ruby have a 'which' like method

[Scott G Smith]

Most UNIX shells have a which command.

It tells me the file which corresponds to a shell command.

Demo:
which date
/bin/date

In ruby,
Suppose I'm dealing with a String.

This String has had some methods mixed-in.

For example,

s = 'hello'
p(s.length)

Is there a way I can find the module or string.rb which defined the
length() method?

I'd like it to work like this:

p(s.which('length'))
/my/rubyfiles/string_module.rb

Thanks,
Scott