package main

import (
    "encoding/json"
    "fmt"
    ms "github.com/mitchellh/mapstructure"
)

type s1 struct {
    Type  string `"json:type"`
    Value int    `"json:value"`
}

type s2 struct {
    Type  string `"json:type"`
    Value string `"json:value"`
}

func main() {
    raw := []byte(`[{"type": "number", "value": 15}, {"type": "string", "value": "Hello!"}]`)

    var input []map[string]interface{}
    if err := json.Unmarshal(raw, &input); err != nil {
        fmt.Println(err)
        return
    }

    fmt.Println(input)

    for _, r := range input {
        switch t := r["type"]; t {
        case "number":
            var v1 s1
            config := &ms.DecoderConfig{TagName: "json", Result: &v1}
            decoder, _ := ms.NewDecoder(config)
            decoder.Decode(r)
            fmt.Printf("%+v\n", v1)
        case "string":
            var v2 s2
            config := &ms.DecoderConfig{TagName: "json", Result: &v2}
            decoder, _ := ms.NewDecoder(config)
            decoder.Decode(r)
            fmt.Printf("%+v\n", v2)
        }
    }
}
// Output:
// [map[type:number value:15] map[value:Hello! type:string]]
// {Type:number Value:15}
// {Type:string Value:Hello!}