Mac OS X 10.5 (Leopard) provides a build of MRI (Matz’s Ruby Interpreter), version 1.8.6. This is the current de facto standard for Ruby interpreters; it is stable, well documented, tested, and understood, etc. If you need to run a legacy Ruby script, with a minimum of hassle, the default ruby(1) command is probably the right choice. Similarly, if you have a legacy RubyCocoa application which you simply wish to run, RubyCocoa is certainly the right choice.
However, if you have needs that aren’t well met by these offerings, MacRuby is certainly worthy of your consideration. MacRuby began as an attempt to work around many problems inherent in RubyCocoa. In the course of solving these problems, MacRuby has also solved numerous problems in Ruby 1.8. Consequently, there are a number of reasons (e.g., convenience, efficiency, flexibility, performance) why one might wish to use MacRuby for new (and ongoing) Ruby applications:
MacRuby is based on Ruby 1.9, so it is powered by the YARV bytecode interpreter. This greatly reduces the execution time of Ruby programs.
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