How to Implement UnmarshalJSON for Derived Scalar Types in Go?

UnmarshalJSON Implementation for Derived Scalars in Go

Problem:
A custom type that converts subtyped integer constants to strings and vice versa requires automatic unmarshalling of JSON strings. This is challenging because UnmarshalJSON does not provide a way to modify the scalar value without using a struct.

Solution:
To implement UnmarshalJSON for a derived scalar type, consider the following steps:

Use a Pointer Receiver:
Use a pointer receiver for the UnmarshalJSON method to modify the value of the receiver.

Unmarshal to a String:
Unmarshal the JSON text into a plain string, removing all JSON quoting.

Lookup and Set Value:
Use the Lookup function to retrieve the corresponding PersonID based on the string value. Assign the result to the receiver.

Example:

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
}

Code Example:

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 {
  var s string
  if err := json.Unmarshal(data, &s); err != nil {
    return err
  }
  *intValue = Lookup(s)
  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())
        }
    }
}

Output:

Hello Ralph
Hello Ralph
Hello Ralph
Hello Ralph

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