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Checking 1.8 vs 1.9 compatibility with Multiruby

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Developers working on Ruby libraries or applications have a new task on their hands: making sure their code works across Ruby versions. Alternative Ruby implementations are one reason, but they generally aim to work exactly like MRI. With the release of Ruby 1.9, this has changed: Ruby 1.9 has some changes that aren't compatible or at least change the behavior of some code. 

For instance, pre-1.9, Ruby's had a problem with local variables in blocks. For instance:
 a = 1
 foo.do_something {|a|
 a = 42
 puts a
In Ruby 1.8, this code prints "42", because the block variable a is actually the same a of the containing scope. Ruby 1.9 fixes this to make blocks behave as expected, i.e. print "1". Of course, if a library's or application's code relied on the old behaviour, it will behave differently on Ruby 1.9.

A part of the Zentest package helps to avoid this: Multiruby. Ryan Davis, creator of Zentest, explains how to handle Ruby 1.8 and 1.9 with Multiruby:
Now that 1.9 is out, it is time to talk about multi-version testing using multiruby. multiruby is a lesser known tool in the zentest family. It automatically builds and privately installs multiple versions of ruby and multiplexes commands to all of them, allowing you to run your tests across multiple versions of ruby all at once.
Installing Zentest is simple:
gem install ZenTest 
Multiruby works as such:
  • It will download and compile Ruby versions, eg. 1.8 and 1.9
  • Then it executes it's arguments with each one of the available Ruby versions
The multiruby command simply passes all given arguments through to the Ruby binary. Running the code example from above will have this output:
VERSION = 1.8.6-p111

VERSION = 1.9.0-0

 failures out of 2

Passed: 1.8.6-p111, 1.9.0-0
As can be seen: the output in 1.8.x is "42", but in 1.9 becomes "1". (The "RESULT" here is the exit code of the Ruby interpreter).

Naturally, Multiruby will only help to expose problems if a large enough percentage of the code is covered by tests. With ever more Ruby versions, JRuby 1.0 and 1.1, IronRuby and Rubinius becoming available, a tool that simplifies testing across runtimes is quite useful.

Have you started porting or at least testing your applications or libraries with Ruby 1.9?

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