Two Way Merge

merge.go

package main

import (
	"fmt"
	"reflect"
)

func mergeDataStructures(a, b interface{}) (interface{}, error) {
	var retVal interface{}

	switch a.(type) {
	case string, int:
		switch b.(type) {
		case string, int:
			retVal = []interface{}{}
			retVal = append(retVal.([]interface{}), b)
			retVal = append(retVal.([]interface{}), a)
		case []interface{}:
			retVal = append(retVal.([]interface{}), a)
		case map[interface{}]interface{}:
			return nil, fmt.Errorf("unable to merge string with map")
		default:
			return nil, fmt.Errorf("unknown value type %q", reflect.TypeOf(b))
		}
	case []interface{}:
		switch b.(type) {
		case string, int:
			retVal = []interface{}{}
			retVal = append(retVal.([]interface{}), a.([]interface{})...)
			retVal = append(retVal.([]interface{}), b)
		case []interface{}:
			retVal = []interface{}{}
			retVal = append(retVal.([]interface{}), a.([]interface{})...)
			retVal = append(retVal.([]interface{}), b.([]interface{})...)
		case map[interface{}]interface{}:
			return nil, fmt.Errorf("unable to merge slice with map")
		default:
			return nil, fmt.Errorf("unknown value type %q", reflect.TypeOf(b))
		}
	case map[interface{}]interface{}:
		switch b.(type) {
		case string, int:
			return nil, fmt.Errorf("unable to merge map with string")
		case []interface{}:
			return nil, fmt.Errorf("unable to merge map with slice")
		case map[interface{}]interface{}:
			retVal = make(map[interface{}]interface{})
			for k, v := range a.(map[interface{}]interface{}) {
				retVal.(map[interface{}]interface{})[k] = v
			}
			for k, v := range b.(map[interface{}]interface{}) {
				retVal.(map[interface{}]interface{})[k] = v
			}
		default:
			return nil, fmt.Errorf("unknown value type %q", reflect.TypeOf(b))
		}
	default:
		return nil, fmt.Errorf("unknown value type %q", reflect.TypeOf(a))
	}

	return retVal, nil
}

func main() {
  var a, b interface{}

  a = []interface{}{1,2,3}
  b = 4
  c, err := mergeDataStructures(a, b)
  if err != nil {
    panic(err)
  }
  fmt.Printf("%#v\n", c)
}