std/json (original) (raw)
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
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
type Foo = object a1, a2, a0, a3, a4: int doAssert $(%* Foo()) == """{"a1":0,"a2":0,"a0":0,"a3":0,"a4":0}"""
Procs
proc $(node: JsonNode): string {....raises: [], tags: [], forbids: [].}
Converts node to its JSON Representation on one line.Source Edit
proc %(b: bool): JsonNode {....raises: [], tags: [], forbids: [].}
Generic constructor for JSON data. Creates a new JBool JsonNode.Source Edit
proc %(n: BiggestInt): JsonNode {....raises: [], tags: [], forbids: [].}
Generic constructor for JSON data. Creates a new JInt JsonNode.Source Edit
proc %(n: BiggestUInt): JsonNode {....raises: [], tags: [], forbids: [].}
Generic constructor for JSON data. Creates a new JInt JsonNode.Source Edit
proc %(n: float): JsonNode {....raises: [], tags: [], forbids: [].}
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
proc %(n: int): JsonNode {....raises: [], tags: [], forbids: [].}
Generic constructor for JSON data. Creates a new JInt JsonNode.Source Edit
proc %(n: uint): JsonNode {....raises: [], tags: [], forbids: [].}
Generic constructor for JSON data. Creates a new JInt JsonNode.Source Edit
Construct a JsonNode that represents the specified enum value as a string. Creates a new JString JsonNode.Source Edit
proc %(o: ref object): JsonNode
Generic constructor for JSON data. Creates a new JObject JsonNode Source Edit
proc %(s: string): JsonNode {....raises: [], tags: [], forbids: [].}
Generic constructor for JSON data. Creates a new JString 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 ==(a, b: JsonNode): bool {.noSideEffect, ...raises: [], tags: [RootEffect],
forbids: [].}
Check two nodes for equalitySource Edit
proc [](node: JsonNode; index: BackwardsIndex): JsonNode {.inline, ...raises: [],
tags: [], forbids: [].}
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
proc [](node: JsonNode; index: int): JsonNode {.inline, ...raises: [], tags: [],
forbids: [].}
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; name: string): JsonNode {.inline, ...raises: [KeyError],
tags: [], forbids: [].}
Gets a field from a JObject, which must not be nil. If the value at name does not exist, raises KeyError.Source Edit
proc [][U, V](a: JsonNode; x: HSlice[U, V]): JsonNode
Slice operation for JArray.
Returns the inclusive range [a[x.a], a[x.b]]:
Example:
import std/json let arr = %[0,1,2,3,4,5] doAssert arr[2..4] == %[2,3,4] doAssert arr[2..^2] == %[2,3,4] doAssert arr[^4..^2] == %[2,3,4]
proc add(father, child: JsonNode) {....raises: [], tags: [], forbids: [].}
Adds child to a JArray node father.Source Edit
proc escapeJson(s: string): string {....raises: [], tags: [], forbids: [].}
Converts a string s to its JSON representation with quotes.Source Edit
proc escapeJson(s: string; result: var string) {....raises: [], tags: [], forbids: [].}
Converts a string s to its JSON representation with quotes. Appends to result.Source Edit
proc escapeJsonUnquoted(s: string): string {....raises: [], tags: [], forbids: [].}
Converts a string s to its JSON representation without quotes.Source Edit
proc escapeJsonUnquoted(s: string; result: var string) {....raises: [], tags: [], forbids: [].}
Converts a string s to its JSON representation without quotes. Appends to result.Source Edit
proc getBiggestInt(n: JsonNode; default: BiggestInt = 0): BiggestInt {. ...raises: [], tags: [], forbids: [].}
Retrieves the BiggestInt value of a JInt JsonNode.
Returns default if n is not a JInt, or if n is nil.
proc getBool(n: JsonNode; default: bool = false): bool {....raises: [], tags: [], forbids: [].}
Retrieves the bool value of a JBool JsonNode.
Returns default if n is not a JBool, or if n is nil.
proc getElems(n: JsonNode; default: seq[JsonNode] = @[]): seq[JsonNode] {. ...raises: [], tags: [], forbids: [].}
Retrieves the array of a JArray JsonNode.
Returns default if n is not a JArray, or if n is nil.
proc getFloat(n: JsonNode; default: float = 0.0): float {....raises: [], tags: [], forbids: [].}
Retrieves the float value of a JFloat JsonNode.
Returns default if n is not a JFloat or JInt, or if n is nil.
proc getInt(n: JsonNode; default: int = 0): int {....raises: [], tags: [], forbids: [].}
Retrieves the int value of a JInt JsonNode.
Returns default if n is not a JInt, or if n is nil.
proc getOrDefault(node: JsonNode; key: string): JsonNode {....raises: [], tags: [], forbids: [].}
Gets a field from a node. If node is nil or not an object or value at key does not exist, returns nilSource Edit
proc getStr(n: JsonNode; default: string = ""): string {....raises: [], tags: [], forbids: [].}
Retrieves the string value of a JString JsonNode.
Returns default if n is not a JString, or if n is nil.
proc hash(n: JsonNode): Hash {.noSideEffect, ...raises: [Exception], tags: [RootEffect], forbids: [].}
Compute the hash for a JSON nodeSource Edit
proc len(n: JsonNode): int {....raises: [], tags: [], forbids: [].}
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 newJArray(): JsonNode {....raises: [], tags: [], forbids: [].}
Creates a new JArray JsonNode Source Edit
proc newJNull(): JsonNode {....raises: [], tags: [], forbids: [].}
Creates a new JNull JsonNode.Source Edit
proc newJObject(): JsonNode {....raises: [], tags: [], forbids: [].}
Creates a new JObject JsonNode Source Edit
proc parseFile(filename: string): JsonNode {. ...raises: [IOError, OSError, JsonParsingError, ValueError], tags: [ReadIOEffect, WriteIOEffect], forbids: [].}
Parses file into a JsonNode. If file contains extra data, it will raise JsonParsingError.Source Edit
proc parseJson(buffer: string; rawIntegers = false; rawFloats = false): JsonNode {. ...raises: [IOError, OSError, JsonParsingError, ValueError], tags: [ReadIOEffect, WriteIOEffect], forbids: [].}
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 parseJson(s: Stream; filename: string = ""; rawIntegers = false; rawFloats = false): JsonNode {. ...raises: [IOError, OSError, IOError, OSError, JsonParsingError, ValueError], tags: [ReadIOEffect, WriteIOEffect], forbids: [].}
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 pretty(node: JsonNode; indent = 2): string {....raises: [], tags: [], forbids: [].}
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 } }"""
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]
proc toUgly(result: var string; node: JsonNode) {....raises: [], tags: [], forbids: [].}
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.
proc {}(node: JsonNode; index: varargs[int]): JsonNode {....raises: [], tags: [],
forbids: [].}
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 {}(node: JsonNode; key: string): JsonNode {....raises: [], tags: [],
forbids: [].}
Gets a field from a node. If node is nil or not an object or value at key does not exist, returns nilSource Edit
proc {}(node: JsonNode; keys: varargs[string]): JsonNode {....raises: [],
tags: [], forbids: [].}
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)
proc {}=(node: JsonNode; keys: varargs[string]; value: JsonNode) {.
...raises: [KeyError], tags: [], forbids: [].}
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
Iterators
iterator items(node: JsonNode): JsonNode {....raises: [], tags: [], forbids: [].}
Iterator for the items of node. node has to be a JArray.Source Edit
iterator keys(node: JsonNode): string {....raises: [], tags: [], forbids: [].}
Iterator for the keys in node. node has to be a JObject.Source Edit
iterator mitems(node: var JsonNode): var JsonNode {....raises: [], tags: [], forbids: [].}
Iterator for the items of node. node has to be a JArray. Items can be modified.Source Edit
iterator mpairs(node: var JsonNode): tuple[key: string, val: var JsonNode] {. ...raises: [], tags: [], forbids: [].}
Iterator for the child elements of node. node has to be a JObject. Values can be modifiedSource Edit
iterator pairs(node: JsonNode): tuple[key: string, val: JsonNode] {....raises: [], tags: [], forbids: [].}
Iterator for the child elements of node. node has to be a JObject.Source Edit
iterator parseJsonFragments(s: Stream; filename: string = ""; rawIntegers = false; rawFloats = false): JsonNode {. ...raises: [IOError, OSError, IOError, OSError, JsonParsingError, ValueError], tags: [ReadIOEffect, WriteIOEffect], forbids: [].}
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
Convert an expression to a JsonNode directly, without having to specify % for every element.Source Edit
Exports
$, $, $, $, $, $, JsonEventKind, JsonError, JsonParser, JsonKindError, open, open, open, open, open, open, close, close, close, close, close, str, getInt, getFloat, kind, kind, getColumn, getLine, getFilename, errorMsg, errorMsgExpected, next, JsonParsingError, raiseParseErr, nimIdentNormalize