How do I convert an EK9 object to JSON?

← JSON and Data Processing · Ref: Q189

EK9 uses the $$ (to-JSON) operator to convert values to JSON. Records with 'default operator $$' automatically serialize all fields to a JSON object.

TO-JSON ON BUILT-IN TYPES

All built-in types support $$:

  jsonInt <- $$ 42
  jsonStr <- $$ "hello"
  jsonBool <- $$ true

TO-JSON ON RECORDS

Records with 'default operator $$' serialize all fields:

  defines record
    Person
      name as String: String()
      age as Integer: Integer()
      default operator $$

Then: jsonPerson <- $$ person
The result is a JSON object with properties for each field.

UNSET FIELDS

Unset fields in a record become JSON null values. EK9's tri-state model maps naturally to JSON:

  set value maps to JSON value
  unset value maps to JSON null
  absent value maps to JSON null

See Q188 for JSON as a first-class type. See Q97 for records. See Q29 for unset variables.

Example

defines module qa.jsondata.tojson

  defines record

    Person
      name as String: String()
      age as Integer: Integer()

      Person()
        ->
          n as String
          a as Integer
        name: n
        age: a

      default operator ?
      default operator $$

  defines program

    JsonToJsonDemo()
      stdout <- Stdout()

      // === $$ ON BUILT-IN TYPES ===

      jsonInt <- $$ 42
      stdout.println(`Integer as JSON: ${jsonInt}`)

      jsonStr <- $$ "hello"
      stdout.println(`String as JSON: ${jsonStr}`)

      jsonBool <- $$ true
      stdout.println(`Boolean as JSON: ${jsonBool}`)

      // === $$ ON RECORDS ===

      person <- Person("Alice", 30)
      jsonPerson <- $$ person
      stdout.println(`Person as JSON: ${jsonPerson}`)

      // === UNSET FIELDS BECOME NULL ===

      partial <- Person()
      jsonPartial <- $$ partial
      stdout.println(`Partial person JSON: ${jsonPartial}`)

Common mistakes

E08180 — Record fields must be initialized inline with a default value. Declaring a field without an initializer triggers E08180. Use 'String()' for an unset default or provide a literal default value. See ek9 -h E08180 for details.

Incorrect:

name as String

Correct:

name as String: String()

E50060 — EK9 types do not have a 'toJSON()' method. Use the $$ operator to convert any value to its JSON representation. For records, 'default operator $$' auto-generates JSON serialization of all fields. See ek9 -h E50060 for details.

Incorrect:

jsonPerson <- person.toJSON()

Correct:

jsonPerson <- $$ person

Coming from another language?

Java: Jackson @JsonSerialize or Gson.toJson() for serialization. Python: json.dumps() with custom encoder. Rust: serde_json::to_string(). Go: json.Marshal(). Kotlin: kotlinx.serialization @Serializable. EK9: 'default operator $$' on records for automatic JSON serialization, $$ operator on any value.

Keywords: unset, field, class, convert, null, serialize, operator, record, json, data