[Smaug123]

Instead of recursing twice, recurse once, by carrying around not fib(n-1) but the pair (fib(n-2), fib(n-1)).

[dotancohen]

True, but then the "worker" fib() method should be called fib_calculate() and should be wrapped by fib() which then returns a single integer. I believe that breaks the spirit of the question.

[Taek]

Really it's about this type of discussion. Already with your 'but then' you are showing that you know your way around a program.

I'd still have you write the full thing out, but I'm guessing you'd do just fine.

[Smaug123]

I'd use `go`, by ancient Haskell law and custom ;)