Fibonacci number calculator implemented using Go generics.

generic-fib.go

// Live code - https://go.dev/play/p/EqW-c0gbRqr

package main

import (
	"errors"
	"fmt"
)

func main() {
	for _, i := range []int{-1, 0, 1, 93, 94} {
		n, err := Fib(i)
		if err != nil {
			Print(i, err)
		} else {
			Print(i, n)
		}
	}
}

// Integer a generic type to represent all supported integer values
type Integer interface {
	~int | ~int8 | ~int32 | ~int64 | ~uint | ~uint8 | ~uint32 | ~uint64
}

// Print pretty prints result for Fibonacci calculation
func Print[T Integer](n T, v any) {
	fmt.Printf("Fib(%d) = %v\n", n, v)
}

// Fib returns the n-th fibonacci number
func Fib[T Integer](_n T) (uint64, error) {
	// Sentinel condition
	if _n < 0 {
		return 0, errors.New("bad input")
	}

	// Up-cast provided input
	n := uint64(_n)
	if n <= 1 {
		return n, nil
	}

	var n1, n2 uint64 = 0, 1
	for i := uint64(1); i < n; i++ {
		n1, n2 = n2, n1+n2
		if n2 < n1 {
			return 0, errors.New("overflow")
		}
	}

	return n2, nil
}

Output:

Fib(-1) = bad input
Fib(0) = 0
Fib(1) = 1
Fib(93) = 12200160415121876738
Fib(94) = overflow