Is Ruby a functional language?

Question

Wikipedia says Ruby is a functional language, but I\'m not convinced. Why or why not?

Answer 1

Recursion is common in functional programming. Almost any language does support recursion, but recursive algorithms are often ineffective if there is no tail call optimization (TCO).

Functional programming languages are capable of optimizing tail recursion and can execute such code in constant space. Some Ruby implementations do optimize tail recursion, the other don't, but in general Ruby implementations are not required to do TCO. See Does Ruby perform Tail Call Optimization?

So, if you write some Ruby functional style and rely on TCO of some particular implementation, your code may be very ineffective in another Ruby interpreter. I think this is why Ruby is not a functional language (neither is Python).