Golang - detecting duplicate keys in JSON input

I recently completed a project where I used the "object hook" in Python to detect whether a JSON key was a duplicate of another key. Normal JSON decoders seem to just give the last value encountered, but I would like to be able to detect and return an error. My new project (at new company) is to write this in golang, so wondering if there is a similar method to Python's object hook. I had also used a different object hook to get an "ordered dict" in Python; essentially a list form of the JSON input, with the ordering of the original JSON preserved. Haven't been tasked with that in golang yet, but I bet it's coming.... anyway, input on either of these JSON capabilities as relate to golang appreciated!

What you need is a json library which works like a bufio.Scanner or SAX. One is implemented here: github.com/garyburd/json. It will generate events as it scans which you can use to discover duplicate keys.

Here's an example of how to use it:

package main
import (
"fmt"
"github.com/garyburd/json"
"io"
"strings"
)
type Nothing struct{}
type Context struct {
Kind json.Kind
Keys map[string]Nothing
}
func Validate(rdr io.Reader) error {
scanner := json.NewScanner(rdr)
stack := []Context{}
for scanner.Scan() {
if scanner.Kind() == json.Object || scanner.Kind() == json.Array {
stack = append(stack, Context{
Kind: scanner.Kind(),
Keys: map[string]Nothing{},
})
} else if scanner.Kind() == json.End {
if len(stack) == 0 {
return fmt.Errorf("expected start object or array")
}
stack = stack[:len(stack)-1]
} else if len(stack) > 0 {
current := stack[len(stack)-1]
if current.Kind == json.Object {
key := string(scanner.Name())
_, exists := current.Keys[key]
if exists {
return fmt.Errorf("found duplicate key: %v", key)
}
current.Keys[key] = Nothing{}
}
}
}
return nil
}
func main() {
rdr := strings.NewReader(`
{
"x": 10,
"y": {
"z": 1,
"z": 2
},
"z": [1,2,3,4,5]
}
`)
err := Validate(rdr)
if err == nil {
fmt.Println("valid json!")
} else {
fmt.Println("invalid json:", err)
}
}

As it walks through the JSON object it builds a stack of hash tables. (for nested objects / arrays) Any duplicate keys in one of those hash tables results in an error. If you need more detail you could easily add a Name property to the Context and walk the stack backwards to generate a json path. (like a.b.c.d is a duplicate key)