Szathmáry László honlapja @ DEIK | Nim2 / Create a JSON file

With your program, you want to create a JSON file. That is, you want to write some data to a file in JSON format.

Build JSON from a literal

import std/json

let doc: JsonNode = %* {
"last": "Doe",
"first": "John",
"age": 39
}

# v1
writeFile("doe.json", $doc) # write in a single line

# v2
writeFile("doe.json", doc.pretty) # nicely indented output

%* is a macro that constructs a JsonNode from a literal
doc.pretty converts a JsonNode to a nicely formatted string

Build JSON programatically

Let's see how to build a JSON object without a literal.

import std/json

var doc = %* {} # empty JsonNode object
doc["last"] = %* "Doe"
doc["first"] = %* "John"
doc["age"] = %* 39

echo doc # {"last":"Doe","first":"John","age":39}

If you want, you can write it to a file.

Let's see some more examples:

Convert a seq

import std/json

let
primes: seq[int] = @[2, 3, 5, 7, 11]
doc = %* primes

writeFile("primes.json", $doc) # [2,3,5,7,11] (compact representation)

Convert a custom object

import std/json

type
Person = object
last: string
first: string
age: int

let
p = Person(last: "Doe", first: "John", age: 39)
doc = %* p

echo p # (last: "Doe", first: "John", age: 39)

echo doc # {"last":"Doe","first":"John","age":39}

writeFile("person.json", doc.pretty)

$ cat person.json
{
  "last": "Doe",
  "first": "John",
  "age": 39
}