Go does not have a built-in `in` operator for slices

contains.go

package main

import "fmt"

// Tempting, but this will NOT compile
//
//func (arr []interface{}) Has(x interface{}) bool {
//	for _, val := range arr {
//		if val == x {
//			return true
//		}
//	}
//	return false
//}

// IsIn will work, sort of, if you're prepared to put up with a lot of nonsense
func IsIn(x interface{}, arr []interface{}) bool {
	for _, val := range arr {
		if val == x {
			return true
		}
	}
	return false
}

func main() {

	// When I said "nonsense", I wasn't kidding
	var x []interface{}

	x = make([]interface{}, 0, 10)
	x = append(x, 3)
	x = append(x, 4)
	x = append(x, 5)
	fmt.Println(IsIn(4, x))
}