std/json

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This module implements a simple high performance JSON parser. JSON (JavaScript Object Notation) is a lightweight data-interchange format that is easy for humans to read and write (unlike XML). It is easy for machines to parse and generate. JSON is based on a subset of the JavaScript Programming Language, Standard ECMA-262 3rd Edition - December 1999.

See also

Overview

Parsing JSON

JSON often arrives into your program (via an API or a file) as a string. The first step is to change it from its serialized form into a nested object structure called a JsonNode.

The parseJson procedure takes a string containing JSON and returns a JsonNode object. This is an object variant and it is either a JObject, JArray, JString, JInt, JFloat, JBool or JNull. You check the kind of this object variant by using the kind accessor.

For a JsonNode who's kind is JObject, you can access its fields using the [] operator. The following example shows how to do this:

import std/json

let jsonNode = parseJson("""{"key": 3.14}""")

doAssert jsonNode.kind == JObject
doAssert jsonNode["key"].kind == JFloat

Reading values

Once you have a JsonNode, retrieving the values can then be achieved by using one of the helper procedures, which include:

  • getInt
  • getFloat
  • getStr
  • getBool

To retrieve the value of "key" you can do the following:

import std/json

let jsonNode = parseJson("""{"key": 3.14}""")

doAssert jsonNode["key"].getFloat() == 3.14

Important: The [] operator will raise an exception when the specified field does not exist.

Handling optional keys

By using the {} operator instead of [], it will return nil when the field is not found. The get-family of procedures will return a type's default value when called on nil.

import std/json

let jsonNode = parseJson("{}")

doAssert jsonNode{"nope"}.getInt() == 0
doAssert jsonNode{"nope"}.getFloat() == 0
doAssert jsonNode{"nope"}.getStr() == ""
doAssert jsonNode{"nope"}.getBool() == false

Using default values

The get-family helpers also accept an additional parameter which allow you to fallback to a default value should the key's values be null:

import std/json

let jsonNode = parseJson("""{"key": 3.14, "key2": null}""")

doAssert jsonNode["key"].getFloat(6.28) == 3.14
doAssert jsonNode["key2"].getFloat(3.14) == 3.14
doAssert jsonNode{"nope"}.getFloat(3.14) == 3.14 # note the {}

Unmarshalling

In addition to reading dynamic data, Nim can also unmarshal JSON directly into a type with the to macro.

Note: Use Option for keys sometimes missing in json responses, and backticks around keys with a reserved keyword as name.

import std/json
import std/options

type
  User = object
    name: string
    age: int
    `type`: Option[string]

let userJson = parseJson("""{ "name": "Nim", "age": 12 }""")
let user = to(userJson, User)
if user.`type`.isSome():
  assert user.`type`.get() != "robot"

Creating JSON

This module can also be used to comfortably create JSON using the %* operator:

import std/json

var hisName = "John"
let herAge = 31
var j = %*
  [
    { "name": hisName, "age": 30 },
    { "name": "Susan", "age": herAge }
  ]

var j2 = %* {"name": "Isaac", "books": ["Robot Dreams"]}
j2["details"] = %* {"age":35, "pi":3.1415}
echo j2

See also: std/jsonutils for hookable json serialization/deserialization of arbitrary types.

Example:

import std/json
## Note: for JObject, key ordering is preserved, unlike in some languages,
## this is convenient for some use cases. Example:
type Foo = object
  a1, a2, a0, a3, a4: int
doAssert $(%* Foo()) == """{"a1":0,"a2":0,"a0":0,"a3":0,"a4":0}"""

Types

JsonNodeKind = enum
  JNull, JBool, JInt, JFloat, JString, JObject, JArray
possible JSON node types   Source   Edit
JsonNode = ref JsonNodeObj
JSON node   Source   Edit
JsonNodeObj {.acyclic.} = object
  isUnquoted: bool
  case kind*: JsonNodeKind
  of JString:
      str*: string

  of JInt:
      num*: BiggestInt

  of JFloat:
      fnum*: float

  of JBool:
      bval*: bool

  of JNull:
      nil

  of JObject:
      fields*: OrderedTable[string, JsonNode]

  of JArray:
      elems*: seq[JsonNode]

  
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Procs

proc newJString(s: string): JsonNode {....raises: [], tags: [].}
Creates a new JString JsonNode.   Source   Edit
proc newJInt(n: BiggestInt): JsonNode {....raises: [], tags: [].}
Creates a new JInt JsonNode.   Source   Edit
proc newJFloat(n: float): JsonNode {....raises: [], tags: [].}
Creates a new JFloat JsonNode.   Source   Edit
proc newJBool(b: bool): JsonNode {....raises: [], tags: [].}
Creates a new JBool JsonNode.   Source   Edit
proc newJNull(): JsonNode {....raises: [], tags: [].}
Creates a new JNull JsonNode.   Source   Edit
proc newJObject(): JsonNode {....raises: [], tags: [].}
Creates a new JObject JsonNode   Source   Edit
proc newJArray(): JsonNode {....raises: [], tags: [].}
Creates a new JArray JsonNode   Source   Edit
proc getStr(n: JsonNode; default: string = ""): string {....raises: [], tags: [].}

Retrieves the string value of a JString JsonNode.

Returns default if n is not a JString, or if n is nil.

  Source   Edit
proc getInt(n: JsonNode; default: int = 0): int {....raises: [], tags: [].}

Retrieves the int value of a JInt JsonNode.

Returns default if n is not a JInt, or if n is nil.

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proc getBiggestInt(n: JsonNode; default: BiggestInt = 0): BiggestInt {.
    ...raises: [], tags: [].}

Retrieves the BiggestInt value of a JInt JsonNode.

Returns default if n is not a JInt, or if n is nil.

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proc getFloat(n: JsonNode; default: float = 0.0): float {....raises: [], tags: [].}

Retrieves the float value of a JFloat JsonNode.

Returns default if n is not a JFloat or JInt, or if n is nil.

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proc getBool(n: JsonNode; default: bool = false): bool {....raises: [], tags: [].}

Retrieves the bool value of a JBool JsonNode.

Returns default if n is not a JBool, or if n is nil.

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proc getFields(n: JsonNode; default = initOrderedTable(2)): OrderedTable[string,
    JsonNode] {....raises: [], tags: [].}

Retrieves the key, value pairs of a JObject JsonNode.

Returns default if n is not a JObject, or if n is nil.

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proc getElems(n: JsonNode; default: seq[JsonNode] = @[]): seq[JsonNode] {.
    ...raises: [], tags: [].}

Retrieves the array of a JArray JsonNode.

Returns default if n is not a JArray, or if n is nil.

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proc add(father, child: JsonNode) {....raises: [], tags: [].}
Adds child to a JArray node father.   Source   Edit
proc add(obj: JsonNode; key: string; val: JsonNode) {....raises: [], tags: [].}
Sets a field from a JObject.   Source   Edit
proc `%`(s: string): JsonNode {....raises: [], tags: [].}
Generic constructor for JSON data. Creates a new JString JsonNode.   Source   Edit
proc `%`(n: uint): JsonNode {....raises: [], tags: [].}
Generic constructor for JSON data. Creates a new JInt JsonNode.   Source   Edit
proc `%`(n: int): JsonNode {....raises: [], tags: [].}
Generic constructor for JSON data. Creates a new JInt JsonNode.   Source   Edit
proc `%`(n: BiggestUInt): JsonNode {....raises: [], tags: [].}
Generic constructor for JSON data. Creates a new JInt JsonNode.   Source   Edit
proc `%`(n: BiggestInt): JsonNode {....raises: [], tags: [].}
Generic constructor for JSON data. Creates a new JInt JsonNode.   Source   Edit
proc `%`(n: float): JsonNode {....raises: [], tags: [].}
Generic constructor for JSON data. Creates a new JFloat JsonNode.

Example:

assert $(%[NaN, Inf, -Inf, 0.0, -0.0, 1.0, 1e-2]) == """["nan","inf","-inf",0.0,-0.0,1.0,0.01]"""
assert (%NaN).kind == JString
assert (%0.0).kind == JFloat
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proc `%`(b: bool): JsonNode {....raises: [], tags: [].}
Generic constructor for JSON data. Creates a new JBool JsonNode.   Source   Edit
proc `%`(keyVals: openArray[tuple[key: string, val: JsonNode]]): JsonNode {.
    ...raises: [], tags: [].}
Generic constructor for JSON data. Creates a new JObject JsonNode   Source   Edit
proc `%`[T](elements: openArray[T]): JsonNode
Generic constructor for JSON data. Creates a new JArray JsonNode   Source   Edit
proc `%`[T](table: Table[string, T] | OrderedTable[string, T]): JsonNode
Generic constructor for JSON data. Creates a new JObject JsonNode.   Source   Edit
proc `%`[T](opt: Option[T]): JsonNode
Generic constructor for JSON data. Creates a new JNull JsonNode if opt is empty, otherwise it delegates to the underlying value.   Source   Edit
proc `[]=`(obj: JsonNode; key: string; val: JsonNode) {.inline, ...raises: [],
    tags: [].}
Sets a field from a JObject.   Source   Edit
proc `%`[T: object](o: T): JsonNode
Construct JsonNode from tuples and objects.   Source   Edit
proc `%`(o: ref object): JsonNode
Generic constructor for JSON data. Creates a new JObject JsonNode   Source   Edit
proc `%`(o: enum): JsonNode
Construct a JsonNode that represents the specified enum value as a string. Creates a new JString JsonNode.   Source   Edit
proc `==`(a, b: JsonNode): bool {....raises: [KeyError], tags: [].}
Check two nodes for equality   Source   Edit
proc hash(n: JsonNode): Hash {....raises: [Exception], tags: [RootEffect].}
Compute the hash for a JSON node   Source   Edit
proc hash(n: OrderedTable[string, JsonNode]): Hash {.noSideEffect,
    ...raises: [Exception], tags: [RootEffect].}
  Source   Edit
proc len(n: JsonNode): int {....raises: [], tags: [].}
If n is a JArray, it returns the number of elements. If n is a JObject, it returns the number of pairs. Else it returns 0.   Source   Edit
proc `[]`(node: JsonNode; name: string): JsonNode {.inline, ...raises: [KeyError],
    tags: [].}
Gets a field from a JObject, which must not be nil. If the value at name does not exist, raises KeyError.   Source   Edit
proc `[]`(node: JsonNode; index: int): JsonNode {.inline, ...raises: [], tags: [].}
Gets the node at index in an Array. Result is undefined if index is out of bounds, but as long as array bound checks are enabled it will result in an exception.   Source   Edit
proc `[]`(node: JsonNode; index: BackwardsIndex): JsonNode {.inline, ...raises: [],
    tags: [].}

Gets the node at array.len-i in an array through the ^ operator.

i.e. j[^i] is a shortcut for j[j.len-i].

Example:

let
  j = parseJson("[1,2,3,4,5]")

doAssert j[^1].getInt == 5
doAssert j[^2].getInt == 4
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proc hasKey(node: JsonNode; key: string): bool {....raises: [], tags: [].}
Checks if key exists in node.   Source   Edit
proc contains(node: JsonNode; key: string): bool {....raises: [], tags: [].}
Checks if key exists in node.   Source   Edit
proc contains(node: JsonNode; val: JsonNode): bool {....raises: [KeyError],
    tags: [].}
Checks if val exists in array node.   Source   Edit
proc `{}`(node: JsonNode; keys: varargs[string]): JsonNode {....raises: [],
    tags: [].}

Traverses the node and gets the given value. If any of the keys do not exist, returns nil. Also returns nil if one of the intermediate data structures is not an object.

This proc can be used to create tree structures on the fly (sometimes called autovivification):

Example:

var myjson = %* {"parent": {"child": {"grandchild": 1}}}
doAssert myjson{"parent", "child", "grandchild"} == newJInt(1)
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proc `{}`(node: JsonNode; index: varargs[int]): JsonNode {....raises: [], tags: [].}
Traverses the node and gets the given value. If any of the indexes do not exist, returns nil. Also returns nil if one of the intermediate data structures is not an array.   Source   Edit
proc getOrDefault(node: JsonNode; key: string): JsonNode {....raises: [], tags: [].}
Gets a field from a node. If node is nil or not an object or value at key does not exist, returns nil   Source   Edit
proc `{}`(node: JsonNode; key: string): JsonNode {....raises: [], tags: [].}
Gets a field from a node. If node is nil or not an object or value at key does not exist, returns nil   Source   Edit
proc `{}=`(node: JsonNode; keys: varargs[string]; value: JsonNode) {.
    ...raises: [KeyError], tags: [].}
Traverses the node and tries to set the value at the given location to value. If any of the keys are missing, they are added.   Source   Edit
proc delete(obj: JsonNode; key: string) {....raises: [KeyError], tags: [].}
Deletes obj[key].   Source   Edit
proc copy(p: JsonNode): JsonNode {....raises: [], tags: [].}
Performs a deep copy of a.   Source   Edit
proc escapeJsonUnquoted(s: string; result: var string) {....raises: [], tags: [].}
Converts a string s to its JSON representation without quotes. Appends to result.   Source   Edit
proc escapeJsonUnquoted(s: string): string {....raises: [], tags: [].}
Converts a string s to its JSON representation without quotes.   Source   Edit
proc escapeJson(s: string; result: var string) {....raises: [], tags: [].}
Converts a string s to its JSON representation with quotes. Appends to result.   Source   Edit
proc escapeJson(s: string): string {....raises: [], tags: [].}
Converts a string s to its JSON representation with quotes.   Source   Edit
proc toUgly(result: var string; node: JsonNode) {....raises: [], tags: [].}

Converts node to its JSON Representation, without regard for human readability. Meant to improve $ string conversion performance.

JSON representation is stored in the passed result

This provides higher efficiency than the pretty procedure as it does not attempt to format the resulting JSON to make it human readable.

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proc pretty(node: JsonNode; indent = 2): string {....raises: [], tags: [].}

Returns a JSON Representation of node, with indentation and on multiple lines.

Similar to prettyprint in Python.

Example:

let j = %* {"name": "Isaac", "books": ["Robot Dreams"],
            "details": {"age": 35, "pi": 3.1415}}
doAssert pretty(j) == """
{
  "name": "Isaac",
  "books": [
    "Robot Dreams"
  ],
  "details": {
    "age": 35,
    "pi": 3.1415
  }
}"""
  Source   Edit
proc `$`(node: JsonNode): string {....raises: [], tags: [].}
Converts node to its JSON Representation on one line.   Source   Edit
proc parseJson(s: Stream; filename: string = ""; rawIntegers = false;
               rawFloats = false): JsonNode {....raises: [IOError, OSError,
    IOError, OSError, JsonParsingError, ValueError, Exception],
    tags: [ReadIOEffect, WriteIOEffect].}
Parses from a stream s into a JsonNode. filename is only needed for nice error messages. If s contains extra data, it will raise JsonParsingError. This closes the stream s after it's done. If rawIntegers is true, integer literals will not be converted to a JInt field but kept as raw numbers via JString. If rawFloats is true, floating point literals will not be converted to a JFloat field but kept as raw numbers via JString.   Source   Edit
proc parseJson(buffer: string; rawIntegers = false; rawFloats = false): JsonNode {.
    ...raises: [IOError, OSError, JsonParsingError, ValueError, Exception],
    tags: [ReadIOEffect, WriteIOEffect].}
Parses JSON from buffer. If buffer contains extra data, it will raise JsonParsingError. If rawIntegers is true, integer literals will not be converted to a JInt field but kept as raw numbers via JString. If rawFloats is true, floating point literals will not be converted to a JFloat field but kept as raw numbers via JString.   Source   Edit
proc parseFile(filename: string): JsonNode {.
    ...raises: [IOError, OSError, JsonParsingError, ValueError, Exception],
    tags: [ReadIOEffect, WriteIOEffect].}
Parses file into a JsonNode. If file contains extra data, it will raise JsonParsingError.   Source   Edit
proc to[T](node: JsonNode; t: typedesc[T]): T

Unmarshals the specified node into the object type specified.

Known limitations:

  • Heterogeneous arrays are not supported.
  • Sets in object variants are not supported.
  • Not nil annotations are not supported.

Example:

let jsonNode = parseJson("""
        {
          "person": {
            "name": "Nimmer",
            "age": 21
          },
          "list": [1, 2, 3, 4]
        }
      """)

type
  Person = object
    name: string
    age: int

  Data = object
    person: Person
    list: seq[int]

var data = to(jsonNode, Data)
doAssert data.person.name == "Nimmer"
doAssert data.person.age == 21
doAssert data.list == @[1, 2, 3, 4]
  Source   Edit

Iterators

iterator items(node: JsonNode): JsonNode {....raises: [], tags: [].}
Iterator for the items of node. node has to be a JArray.   Source   Edit
iterator mitems(node: var JsonNode): var JsonNode {....raises: [], tags: [].}
Iterator for the items of node. node has to be a JArray. Items can be modified.   Source   Edit
iterator pairs(node: JsonNode): tuple[key: string, val: JsonNode] {....raises: [],
    tags: [].}
Iterator for the child elements of node. node has to be a JObject.   Source   Edit
iterator keys(node: JsonNode): string {....raises: [], tags: [].}
Iterator for the keys in node. node has to be a JObject.   Source   Edit
iterator mpairs(node: var JsonNode): tuple[key: string, val: var JsonNode] {.
    ...raises: [], tags: [].}
Iterator for the child elements of node. node has to be a JObject. Values can be modified   Source   Edit
iterator parseJsonFragments(s: Stream; filename: string = "";
                            rawIntegers = false; rawFloats = false): JsonNode {....raises: [
    IOError, OSError, IOError, OSError, JsonParsingError, ValueError, Exception],
    tags: [ReadIOEffect, WriteIOEffect].}
Parses from a stream s into JsonNodes. filename is only needed for nice error messages. The JSON fragments are separated by whitespace. This can be substantially faster than the comparable loop for x in splitWhitespace(s): yield parseJson(x). This closes the stream s after it's done. If rawIntegers is true, integer literals will not be converted to a JInt field but kept as raw numbers via JString. If rawFloats is true, floating point literals will not be converted to a JFloat field but kept as raw numbers via JString.   Source   Edit

Macros

macro `%*`(x: untyped): untyped
Convert an expression to a JsonNode directly, without having to specify % for every element.   Source   Edit
macro isRefSkipDistinct(arg: typed): untyped
internal only, do not use   Source   Edit

Templates

template `%`(j: JsonNode): JsonNode
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