sample code of mapstructure's DecodeHook

decode_hook.go

package main

import (
	"fmt"
	"log"
	"reflect"

	"github.com/mitchellh/mapstructure"
	"github.com/suzuki-shunsuke/go-set"
)

func main() {
	a := []string{}
	s := set.NewStrSet()
	input := []interface{}{"foo"}
	if err := mapstructure.Decode(input, &a); err != nil {
		log.Fatal("failed to decode to []string: ", err)
	}
	fmt.Println("a:", a)
	decoder, err := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
		DecodeHook: DecodeListToStrSet,
		Result:     s,
	})
	if err != nil {
		log.Fatal("failed to get a new decoder: ", err)
	}
	if err := decoder.Decode(input); err != nil {
		log.Fatal(err)
	}
	fmt.Println("s.ToList(): ", s.ToList())
}

func DecodeListToStrSet(fromType reflect.Type, toType reflect.Type, from interface{}) (interface{}, error) {
	if reflect.TypeOf(set.StrSet{}) != toType {
		return from, nil
	}
	if arr, ok := from.([]interface{}); ok {
		s := set.NewStrSet()
		for _, a := range arr {
			if k, ok := a.(string); ok {
				s.Add(k)
				continue
			}
			return from, fmt.Errorf("type assertion to string is failure")
		}
		return s, nil
	}
	return from, nil
}

sample.go

package main

import (
	"fmt"
	"log"

	"github.com/mitchellh/mapstructure"
	"github.com/suzuki-shunsuke/go-set"
)

func main() {
	a := []string{}
	s := set.NewStrSet()
	input := []interface{}{"foo"}
	if err := mapstructure.Decode(input, &a); err != nil {
		log.Fatal("failed to decode to []string: ", err)
	}
	fmt.Println("a:", a)

	if err := mapstructure.Decode(input, s); err != nil {
		// '' expected a map, got 'slice'
		// https://github.com/mitchellh/mapstructure/blob/60249754210871616548ae28fdadeecbd04303b3/mapstructure.go#L871
		log.Fatal("failed to decode to set.StrSet: ", err)
	}
	fmt.Println("s:", s)
}