This has no business looking this satisfying

Goofy but aesthetically pleasing impl

Dynamic programming /s

I was gonna say you could probably actually have that properly calculated and O(1), if you stuck the result in a lazy list. But then I realized this hard-coded implementation probably already exhausts ulong, so there really is no point to that.

ghci> φ = (1 + sqrt 5) / 2
ghci> ψ =  1 - φ
ghci> fib n = (φ ** n - ψ ** n) / (sqrt 5)

best of both world with Lisp macros :)

(defmacro gen-fib [size]
  (let [values (->> (iterate (fn [[a b]] [b (+ a b)]) [0 1])
                    (map first)
                    (take size)
                    (vec))]
    `(defn ~'fib [~'n] (~values ~'n))))

(macroexpand-1 '(gen-fib 10))
;=> (clojure.core/defn fib [n] ([0 1 1 2 3 5 8 13 21 34] n))

(gen-fib 10)

(fib 9)
;=> 34