Go cannot range over (type interface {})

Question

I\'m in the infant stages of trying to wrap my mind around Go. At the moment, I\'m simulating an API request that returns a JSON-formatted string containing an array of objects.

Answer 1

The defaults that the json package will decode into when the type isn't declared are:

bool, for JSON booleans
float64, for JSON numbers
string, for JSON strings
[]interface{}, for JSON arrays
map[string]interface{}, for JSON objects
nil for JSON null

Since each record is (in your example) a json object, you can assert each one as a map[string]interface{} like so:

for _, record := range view {
    log.Printf(" [===>] Record: %s", record)

    if rec, ok := record.(map[string]interface{}); ok {
        for key, val := range rec {
            log.Printf(" [========>] %s = %s", key, val)
        }
    } else {
        fmt.Printf("record not a map[string]interface{}: %v\n", record)
    }
}

Answer 2

I prefer to declare my own Slice and Map, then I can add methods onto them to aid in the type assertions. I only included two methods below, but you can add more as needed:

package main
type Map map[string]interface{}
type Slice []interface{}

func (a Slice) M(n int) Map {
   return a[n].(map[string]interface{})
}

func (m Map) N(s string) float64 {
   return m[s].(float64)
}

Result:

package main

import (
   "encoding/json"
   "fmt"
   "strings"
)

func main() {
   response := strings.NewReader(`[
      {"month": 12, "day": 31},
      {"month": 11, "day": 30}
   ]`)

   view := Slice{}
   json.NewDecoder(response).Decode(&view)

   for index := range view {
      record := view.M(index)
      for key := range record {
         val := record.N(key)
         fmt.Println(key, val)
      }
   }
}

https://golang.org/ref/spec#Type_declarations