GI.Json.Objects.Path

JsonPath is a simple class implementing the JSONPath syntax for extracting data out of a JSON tree.

While the semantics of the JSONPath expressions are heavily borrowed by the XPath specification for XML, the syntax follows the ECMAScript origins of JSON.

Once a JsonPath instance has been created, it has to compile a JSONPath expression using pathCompile before being able to match it to a JSON tree; the same JsonPath instance can be used to match multiple JSON trees. It it also possible to compile a new JSONPath expression using the same JsonPath instance; the previous expression will be discarded only if the compilation of the new expression is successful.

The simple convenience function [funcjson.Path.query] can be used for one-off matching.

Syntax of the JSONPath expressions

A JSONPath expression is composed by path indices and operators. Each path index can either be a member name or an element index inside a JSON tree. A JSONPath expression must start with the $ operator; each path index is separated using either the dot notation or the bracket notation, e.g.:

// dot notation
$.store.book[0].title

// bracket notation
$['store']['book'][0]['title']

The available operators are:

• The $ character represents the root node of the JSON tree, and matches the entire document.
• Child nodes can either be matched using . or []. For instance, both $.store.book and $['store']['book'] match the contents of the book member of the store object.
• Child nodes can be reached without specifying the whole tree structure through the recursive descent operator, or ... For instance, $..author matches all author member in every object.
• Child nodes can grouped through the wildcard operator, or *. For instance, $.store.book[*].author matches all author members of any object element contained in the book array of the store object.
• Element nodes can be accessed using their index (starting from zero) in the subscript operator []. For instance, $.store.book[0] matches the first element of the book array of the store object.
• Subsets of element nodes can be accessed using the set notation operator [i,j,...]. For instance, $.store.book[0,2] matches the elements 0 and 2 (the first and third) of the book array of the store object.
• Slices of element nodes can be accessed using the slice notation operation [start:end:step]. If start is omitted, the starting index of the slice is implied to be zero; if end is omitted, the ending index of the slice is implied to be the length of the array; if step is omitted, the step of the slice is implied to be 1. For instance, $.store.book[:2] matches the first two elements of the book array of the store object.

More information about JSONPath is available on Stefan Gössner's JSONPath website.

Example of JSONPath matches

The following example shows some of the results of using JsonPath on a JSON tree. We use the following JSON description of a bookstore:

json code

{ "store": {
    "book": [
      { "category": "reference", "author": "Nigel Rees",
        "title": "Sayings of the Century", "price": "8.95"  },
      { "category": "fiction", "author": "Evelyn Waugh",
        "title": "Sword of Honour", "price": "12.99" },
      { "category": "fiction", "author": "Herman Melville",
        "title": "Moby Dick", "isbn": "0-553-21311-3",
        "price": "8.99" },
      { "category": "fiction", "author": "J. R. R. Tolkien",
        "title": "The Lord of the Rings", "isbn": "0-395-19395-8",
        "price": "22.99" }
    ],
    "bicycle": { "color": "red", "price": "19.95" }
  }
}

We can parse the JSON using Parser:

c code

JsonParser *parser = json_parser_new ();
json_parser_load_from_data (parser, json_data, -1, NULL);

If we run the following code:

c code

JsonNode *result;
JsonPath *path = json_path_new ();
json_path_compile (path, "$.store..author", NULL);
result = json_path_match (path, json_parser_get_root (parser));

The result node will contain an array with all values of the author member of the objects in the JSON tree. If we use a Generator to convert the result node to a string and print it:

c code

JsonGenerator *generator = json_generator_new ();
json_generator_set_root (generator, result);
char *str = json_generator_to_data (generator, NULL);
g_print ("Results: %s\n", str);

The output will be:

json code

["Nigel Rees","Evelyn Waugh","Herman Melville","J. R. R. Tolkien"]

Since: 0.14

Exported types

newtype Path

Memory-managed wrapper type.

Constructors

Path (ManagedPtr Path)

Instances

Eq Path

Defined in GI.Json.Objects.Path

Methods

(==) :: Path -> Path -> Bool

(/=) :: Path -> Path -> Bool

GObject Path

Defined in GI.Json.Objects.Path

ManagedPtrNewtype Path

Defined in GI.Json.Objects.Path

Methods

toManagedPtr :: Path -> ManagedPtr Path

TypedObject Path

Defined in GI.Json.Objects.Path

Methods

glibType :: IO GType

HasParentTypes Path

Defined in GI.Json.Objects.Path

IsGValue (Maybe Path)

Convert Path to and from GValue. See toGValue and fromGValue.

Defined in GI.Json.Objects.Path

Methods

gvalueGType_ :: IO GType

gvalueSet_ :: Ptr GValue -> Maybe Path -> IO ()

gvalueGet_ :: Ptr GValue -> IO (Maybe Path)

type ParentTypes Path

Defined in GI.Json.Objects.Path

type ParentTypes Path = '[Object]

class (GObject o, IsDescendantOf Path o) => IsPath o

Type class for types which can be safely cast to Path, for instance with toPath.

Instances

(GObject o, IsDescendantOf Path o) => IsPath o

Defined in GI.Json.Objects.Path

toPath :: (MonadIO m, IsPath o) => o -> m Path

Cast to Path, for types for which this is known to be safe. For general casts, use castTo.

Methods

Click to display all available methods, including inherited ones

Methods

bindProperty, bindPropertyFull, compile, forceFloating, freezeNotify, getv, isFloating, match, notify, notifyByPspec, ref, refSink, runDispose, stealData, stealQdata, thawNotify, unref, watchClosure.

Getters

getData, getProperty, getQdata.

Setters

setData, setDataFull, setProperty.

compile

pathCompile

Arguments

:: (HasCallStack, MonadIO m, IsPath a)

=> a

path: a path

-> Text

expression: a JSONPath expression

-> m ()

(Can throw GError)

Validates and decomposes the given expression.

A JSONPath expression must be compiled before calling pathMatch.

Since: 0.14

match

pathMatch

Arguments

:: (HasCallStack, MonadIO m, IsPath a)

=> a

path: a compiled path

-> Node

root: the root node of the JSON data to match

-> m Node

Returns: a newly-created node of type JSON_NODE_ARRAY containing the array of matching nodes

Matches the JSON tree pointed by root using the expression compiled into the JsonPath.

The nodes matching the expression will be copied into an array.

Since: 0.14

new

pathNew

Arguments

:: (HasCallStack, MonadIO m)

=> m Path

Returns: the newly created path

Creates a new JsonPath instance.

Once created, the JsonPath object should be used with pathCompile and pathMatch.

Since: 0.14

query

pathQuery

Arguments

:: (HasCallStack, MonadIO m)

=> Text

expression: a JSONPath expression

-> Node

root: the root of a JSON tree

-> m Node

Returns: a newly-created node of type JSON_NODE_ARRAY containing the array of matching nodes (Can throw GError)

Queries a JSON tree using a JSONPath expression.

This function is a simple wrapper around pathNew, pathCompile, and pathMatch. It implicitly creates a JsonPath instance, compiles the given expression and matches it against the JSON tree pointed by root.

Since: 0.14