Smalltalk exception handling for Ruby 1.9.x

proc.rb

class Proc
  def on(exception_class)
    begin
      self.call
    rescue exception_class => e
      yield e
    end
  end
end

proc_spec.rb

describe Proc do

  describe "smalltalk exception handling style" do

    it "should execute a block normally" do
      evaluated = false
      -> { evaluated = true }.on StandardError do
        fail 'no exception occurred, so this block does not have to evaluated'
      end
      evaluated.should be_true
    end

    it "should catch an exception and evaluate the corresponding block" do
      evaluated = false
      -> { raise StandardError }.on StandardError do
        evaluated = true
      end
      evaluated.should be_true
    end

    it "should not catch an exception for a different handler" do
      class MyCustomError < StandardError; end
      expect { 
        -> { raise StandardError }.on MyCustomError { }
      }.to raise_error(StandardError)
    end

    it "should pass as argument the exception object" do
      -> {
        raise StandardError, 'the message'
      }.on StandardError do |exception_object|
        exception_object.message.should == 'the message'
      end
    end
  end
end