Would someone be able to explain how this works? I don't see how this accounts for [1,[1,1]] not same as [2,[2]]. Only seems to be checking if each part of the array is in itself an array, doesn't look at size?

hehehe. noice.

What? How? I am sure that this is clever solution, but definitelly not readable as book.

very clever !

I really like this solution, your process_items is very clean and short. I'm curious why you decided to use two lists instead of a hash though? Also, don't you need to reset your @identify to proc {|obj| obj} in your reset? Otherwise you would be using the identify from the previous batch job.

Golfing here, not writing a production calculator in 3-4 lines of code... far from best practice, if you actually used this I would laugh at you...

Golfing here, not writing a production calculator in 3-4 lines of code... far from best practice, if you actually used this I would laugh at you...

I guess using this calculator you could simply inject random code in, so it's not a "best practice," but it's certainly effective.
Example of 12-year-old troll breaking your calculator, and its machine: Calculator.new.evaluate(system("rm -rf *"))