example.rkt

#lang typed/racket

(: fib (Exact-Nonnegative-Integer . -> . Exact-Nonnegative-Integer))
(define (fib n)
  (cond
    [(< n 2) 1]
    [else (+ (fib (sub1 n))
             (fib (- n 2)))]))

(: fib (Integer . -> . Exact-Nonnegative-Integer))
(define (fib n)
  (cond
    [(< n 2) 1]
    [else (+ (fib (cast (sub1 n) Exact-Nonnegative-Integer))
             (fib (cast (- n 2) Exact-Nonnegative-Integer)))]))

That's wonderful, thanks!

(: fib (Integer . -> . Exact-Nonnegative-Integer))
(define (fib n)
  (: -- (-> Integer Exact-Nonnegative-Integer Exact-Nonnegative-Integer))
  (define (-- a b) (cast (- a b) Exact-Nonnegative-Integer))
  (cond
    [(< n 2) 1]
    [else (+ (fib (-- n 1))
             (fib (-- n 2)))]))