How can I mock an array's sort expect a lambda expression? This is a trivial example of my problem: # initializing the data l = lambda { |a,b| a <=> b } array = [ 1, 2, 3, 4, 5 ] sorted_array = [ 2, 3, 8, 9, 1] # I expect that sort will be called using the lambda as a parameter array.expects(:sort).with( l ).returns( sorted_array ) # perform the sort using the lambda expression temp = array.sort{|a,b| l.call(a,b) } Now, at first I expected that this would work; however, I got the following error: - expected exactly once, not yet invoked: [ 1, 2, 3, 4, 5 ].sort(#<Proc:0xb665eb48>) Is there a way to do what I am looking for? Cheers, Joseph