80.go

package main

import (
	"fmt"
	"os"
	"strconv"
)

func main() {
	for _, v := range os.Args[1:] {
		IfPanics(
			PrintErrorMessage,
			ValidInteger(v, func(i int) {
				fmt.Printf("%v!: %v\n", i, Factorial(i))
			}))
	}
}

type Integer interface {
	int | int8 | int16 | int32 | int64 |
		uint | uint8 | uint16 | uint32 | uint64
}

func ValidInteger[T Integer](v string, f func(i T)) func() {
	return func() {
		if x, e := strconv.Atoi(v); e == nil {
			f(T(x))
		} else {
			panic(v)
		}
	}
}

func IfPanics(e, f func()) {
	defer e()
	f()
}

func PrintErrorMessage() {
	if x := recover(); x != nil {
		fmt.Printf("no defined value for %v\n", x)
	}
}

func Factorial[T Integer](n T) (r T) {
	if n < 0 {
		panic(n)
	}
	for r = 1; n > 0; n-- {
		r *= n
	}
	return
}