Why do people keep representing this as bizarre when C libraries parallel install just fine and you probably all have multiple versions of the same library installed right now? Accept it's a legitimate situation please. What it needs is radical change in languages like ruby/python/etc to resolve dependencies explicitly at runtime. Currently they hide the problem with import path priorities.

(late, but replying anyway)

Multiple versions of a library are certainly possible with C, but are generally avoided on Debian where possible. Where that isn't possible -- for instance, for libraries which completely changed their API like gtk1.4 -> gtk2, they simply create completely separate packages for the two versions.

With gems, this isn't quite so easy, as there's no way to distinguish between a minor bugfix and a huge API-breaking change without either guessing based on the version number or having a human read the changelog. Even then, there's no guarantee that dependent packages will depend on distinct sets of versions.

Rubygems adds the gem() method, which allows you to specify at runtime which version of a dependency you want, and does not rely on path munging.

How do you specify which version you want in your program?

The naive way will just load the highest-numbered version you have installed:

  require 'rack'

When you have rubygems loaded, this is equivalent to:

  gem 'rack', '>=0'

You can specify exact and relative dependencies easily. There are a fuckton of frameworks to handle this for you, many of them oriented around copying the depended-upon gems into your app when it's deployed, as is common in the Java world.

Bundler[1] is one way.

[1] http://gembundler.com/rationale.html