How to convert object to json in Nim

The marshal module includes a generic object-to-json serialisation algorithm that works for any type (currently, it uses run-time type introspection).

import marshal

type
  Person = object
    age: int
    name: string

var p = Person(age: 38, name: "Torbjørn")

echo($$p)

The output will be:

{"age": 38, "name": "Torbj\u00F8rn"}

In Nim you use the json module to create JsonNode objects which are object variants. These can be constructed with the individual procs like newJObject() and then populate the fields sequence. Another quicker way is to use the %() proc which accepts a sequence of tuples where one value is the string with the json field and the other the individual JsonNode.

Here's an example showing both ways:

import json

type
  Person = object ## Our generic person record.
    age: int ## The age of the person.
    name: string ## The name of the person.

proc `%`(p: Person): JsonNode =
  ## Quick wrapper around the generic JObject constructor.
  result = %[("age", %p.age), ("name", %p.name)]

proc myCustomJson(p: Person): JsonNode =
  ## Custom method where we replicate manual construction.
  result = newJObject()
  # Initialize empty sequence with expected field tuples.
  var s: seq[tuple[key: string, val: JsonNode]] = @[]
  # Add the integer field tuple to the sequence of values.
  s.add(("age", newJInt(p.age)))
  # Add the string field tuple to the sequence of values.
  s.add(("name", newJString(p.name)))
  result.fields = s

proc test() =
  # Tests making some jsons.
  var p: Person
  p.age = 24
  p.name = "Minah"
  echo(%p) # { "age": 24,  "name": "Minah"}
  p.age = 33
  p.name = "Sojin"
  echo(%p) # { "age": 33,  "name": "Sojin"}
  p.age = 40
  p.name = "Britney"
  echo p.myCustomJson # { "age": 40,  "name": "Britney"}

when isMainModule: test()