如何在Go中为标量派生的类型实现UnmarshalJSON？

Question

I have a simple type that implements conversion of subtyped integer consts to strings and vice versa in Go. I want to be able to automatically unmarshal strings in JSON to values of this type. I can't, because UnmarshalJSON doesn't give me a way to return or modify the scalar value. It's expecting a struct, whose members are set by UnmarshalJSON. The ",string" method doesn't work either for other than builtin scalar types. Is there a way to implement UnmarshalJSON correctly for a derived scalar type?

Here's an example of what I'm after. I want it to print "Hello Ralph" four times, but it prints "Hello Bob" four times because the PersonID isn't being changed.

package main

import (
    "encoding/json"
    "fmt"
)

type PersonID int

const (
    Bob PersonID = iota
    Jane
    Ralph
    Nobody = -1
)

var nameMap = map[string]PersonID{
    "Bob":    Bob,
    "Jane":   Jane,
    "Ralph":  Ralph,
    "Nobody": Nobody,
}

var idMap = map[PersonID]string{
    Bob:    "Bob",
    Jane:   "Jane",
    Ralph:  "Ralph",
    Nobody: "Nobody",
}

func (intValue PersonID) Name() string {
    return idMap[intValue]
}

func Lookup(name string) PersonID {
    return nameMap[name]
}

func (intValue PersonID) UnmarshalJSON(data []byte) error {
    // The following line is not correct
    intValue = Lookup(string(data))
    return nil
}

type MyType struct {
    Person   PersonID `json: "person"`
    Count    int      `json: "count"`
    Greeting string   `json: "greeting"`
}

func main() {
    var m MyType
    if err := json.Unmarshal([]byte(`{"person": "Ralph", "count": 4, "greeting": "Hello"}`), &m); err != nil {
        fmt.Println(err)
    } else {
        for i := 0; i < m.Count; i++ {
            fmt.Println(m.Greeting, m.Person.Name())
        }
    }
}

Answer 1

Use a pointer receiver for the unmarshal method. If a value receiver is used, changes to the receiver are lost when the method returns.

The argument to the unmarshal method is JSON text. Unmarshal the JSON text to get a plain string with all JSON quoting removed.

func (intValue *PersonID) UnmarshalJSON(data []byte) error {
  var s string
  if err := json.Unmarshal(data, &s); err != nil {
    return err
  }
  *intValue = Lookup(s)
  return nil
}

There's a mismatch between the JSON tags an the example JSON. I changed the JSON to match the tag, but you can change it the other way.

if err := json.Unmarshal([]byte(`{"person": "Ralph", "count": 4, "greeting": "Hello"}`), &m); err != nil {

<kbd>playground example</kbd>