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This package adds a command [json] to the interpreter, and defines a new Tcl_Obj type to store the parsed JSON document. The [json] command directly manipulates values whose string representation is valid JSON, in a similar way to how the [dict] command directly manipulates values whose string representation is a valid dictionary. It is similar to [dict] in performance.

Also provided is a command [json template] which generates JSON documents by interpolating values into a template from a supplied dictionary or variables in the current call frame. The templates are valid JSON documents containing string values which match the regex "^~[SNBJTL]:.+$". The second character determines what the resulting type of the substituted value will be:

  • S: A string.
  • N: A number.
  • B: A boolean.
  • J: A JSON fragment.
  • T: A JSON template (substitutions are performed on the inserted fragment).
  • L: A literal - the resulting string is simply everything from the forth character onwards (this allows literal strings to be included in the template that would otherwise be interpreted as the substitutions above).

None of the first three characters for a template may be escaped.

The value inserted is determined by the characters following the substitution type prefix. When interpolating values from a dictionary they name keys in the dictionary which hold the values to interpolate. When interpolating from variables in the current scope, they name scalar or array variables which hold the values to interpolate. In either case if the named key or variable doesn't exist, a JSON null is interpolated in its place.

Quick Reference

  • [json get ?-default defaultValue? json_val ?key ...?] - Extract the value of a portion of the json_val, returns the closest native Tcl type (other than JSON) for the extracted portion.
  • [json extract ?-default defaultValue? json_val ?key ...?] - Extract the value of a portion of the json_val, returns the JSON fragment.
  • [json exists json_val ?key ...?] - Tests whether the supplied key path resolve to something that exists in json_val
  • [json set json_variable_name ?key ...? value] - Updates the JSON value stored in the variable json_variable_name, replacing the value referenced by key ... with the JSON value value.
  • [json unset json_variable_name ?key ...?] - Updates the JSON value stored in the variable json_variable_name, removing the value referenced by key ...
  • [json normalize json_val] - Return a "normalized" version of the input json_val - all optional whitespace trimmed.
  • [json template json_val ?dictionary?] - Return a JSON value by interpolating the values from dictionary into the template, or from variables in the current scope if dictionary is not supplied, in the manner described above.
  • [json string value] - Return a JSON string with the value value.
  • [json number value] - Return a JSON number with the value value.
  • [json boolean value] - Return a JSON boolean with the value value. Any of the forms accepted by Tcl_GetBooleanFromObj are accepted and normalized.
  • [json object ?key value ?key value ...??] - Return a JSON object with the keys and values specified. value is a list of two elements, the first being the type {string, number, boolean, null, object, array, json}, and the second being the value.
  • [json object packed_value] - An alternate syntax that takes the list of keys and values as a single arg instead of a list of args, but is otherwise the same.
  • [json array ?elem ...?] - Return a JSON array containing each of the elements given. elem is a list of two elements, the first being the type {string, number, boolean, null, object, array, json}, and the second being the value.
  • [json foreach varlist1 json_val1 ?varlist2 json_val2 ...? script] - Evaluate script in a loop in a similar way to the [foreach] command. In each iteration, the values stored in the iterator variables in varlist are the JSON fragments from json_val. Supports iterating over JSON arrays and JSON objects. In the JSON object case, varlist must be a two element list, with the first specifiying the variable to hold the key and the second the value. In the JSON array case, the rules are the same as the [foreach] command.
  • [json lmap varlist1 json_val1 ?varlist2 json_val2 ...? script] - As for [json foreach], except that it is collecting - the result from each evaluation of script is added to a list and returned as the result of the [json lmap] command. If the script results in a TCL_CONTINUE code, that iteration is skipped and no element is added to the result list. If it results in TCL_BREAK the iterations are stopped and the results accumulated so far are returned.
  • [json amap varlist1 json_val1 ?varlist2 json_val2 ...? script] - As for [json lmap], but the result is a JSON array rather than a list. If the result of each iteration is a JSON value it is added to the array as-is, otherwise it is converted to a JSON string.
  • [json omap varlist1 json_val1 ?varlist2 json_val2 ...? script] - As for [json lmap], but the result is a JSON object rather than a list. The result of each iteration must be a dictionary (or a list of 2n elements, including n = 0). Tcl_ObjType snooping is done to ensure that the iteration over the result is efficient for both dict and list cases. Each entry in the dictionary will be added to the result object. If the value for each key in the iteration result is a JSON value it is added to the array as-is, otherwise it is converted to a JSON string.
  • [json isnull json_val ?key ...?] - Return a boolean indicating whether the named JSON value is null.
  • [json type json_val ?key ...?] - Return the type of the named JSON value, one of "object", "array", "string", "number", "boolean" or "null".
  • [json length json_val ?key ...?] - Return the length of the of the named JSON array, number of entries in the named JSON object, or number of characters in the named JSON string. Other JSON value types are not supported.
  • [json keys json_val ?key ...?] - Return the keys in the of the named JSON object, found by following the path of keys.
  • [json pretty ?-indent indent? json_val ?key ...?] - Returns a pretty-printed string representation of json_val. Useful for debugging or inspecting the structure of JSON data.
  • [json decode bytes ?encoding?] - Decode the binary bytes into a character string according to the JSON standards. The optional encoding arg can be one of utf-8, utf-16le, utf-16be, utf-32le, utf-32be. The encoding is guessed from the BOM (byte order mark) if one is present and encoding isn't specified.
  • [json valid ?-extensions extensionlist? ?-details detailsvar? json_val] - Return true if json_val conforms to the JSON grammar with the extensions in extensionlist. Currently only one extension is supported: comments, and is the default. To reject comments, use -extensions {}. If -details detailsvar is supplied and the validation fails, the variable detailsvar is set to a dictionary with the keys errmsg, doc and char_ofs. errmsg contains the reason for the failure, doc contains the failing json value, and char_ofs is the character index into doc of the first invalid character.


The commands [json get], [json extract], [json set], [json unset] and [json exists] accept a path specification that names some subset of the supplied json_val. The rules are similar to the equivalent concept in the [dict] command, except that the paths used by [json] allow indexing into JSON arrays by the integer key (or a string matching the regex "^end(-[0-9]+)?$"). If a path to [json set] includes a key within an object that doesn't exist, it and all later elements of the path are created as nested keys into (new) objects. If a path element into an array is outside the current bounds of the array, it resolves to a JSON null (for [json get], [json extract], [json exists]), or appends or prepends null elements to resolve the path (for [json set], or does nothing ([json unset]).

json get {
        "foo": [
            { "name": "first" },
            { "name": "second" },
            { "name": "third" }
} foo end-1 name

Returns "second"

Properly Interpreting JSON from Other Systems

Rl_json operates on characters, not bytes, and so considerations of encoding are strictly out of scope. However, interoperating with other systems properly in a way that conforms to the standards is a bit tricky, and requires support for encodings Tcl currently doesn't natively support, like utf-32be. To ease this burden and take care of things like replacing broken encoding sequences, the [json decode] subcommand is provided. Using it in an application would look something like:

proc readjson file {
    set h [open $file rb]    ;# Note that the file is opened in binary mode
    try {
        json decode [read $h]
    } finally {
        close $h

If the encoding is known via some out-of-band channel (like headers in an HTTP response), it can be supplied to override the BOM-based detection. The supported encodings are those listed in the JSON standards: utf-8 (the default), utf-16le, utf-16be, utf-32le and utf-32be.


Creating a document from a template

Produce a JSON value from a template: ~~~tcl json template { { "thing1": "~S:val1", "thing2": ["a", "~N:val2", "~S:val2", "~B:val2", "~S:val3", "~L:~S:val1"], "subdoc1": "~J:subdoc", "subdoc2": "~T:subdoc" } } { val1 hello val2 1e6 subdoc { { "thing3": "~S:val1" } } } ~~~ Result: ~~~json {"thing1":"hello","thing2":["a",1000000.0,"1e6",true,null,"~S:val1"],"subdoc1":{"thing3":"~S:val1"},"subdoc2":{"thing3":"hello"}} ~~~

Construct a JSON array from a SQL result set

# Given:
# sqlite> select * from languages;
# 'Tcl',1,''
# 'Node.js',1,''
# 'Python',1,''
# 'INTERCAL',0,''
# 'Unlambda',0,NULL

set langs {[]}
sqlite3 db languages.sqlite3
db eval {
} {
    if {$url eq ""} {unset url}

    json set langs end+1 [json template {
            "id":       "~N:rowid",
            "name":     "~S:name",
            "details": {
                "active":   "~B:active",  // Template values can be nested anywhere
                "url":      "~S:url"      /* Both types of comments are
                                             allowed but stripped at parse-time */

puts [json pretty $langs]

Result: ~~~json [ { "id": 1, "name": "Tcl", "details": { "active": true, "url": "" } }, { "id": 2, "name": "Node.js", "details": { "active": true, "url": "" } }, { "id": 3, "name": "Python", "details": { "active": true, "url": "" } }, { "id": 4, "name": "INTERCAL", "details": { "active": false, "url": "" } }, { "id": 5, "name": "Unlambda", "details": { "active": false, "url": null } } ] ~~~


Good performance was a requirement for rl_json, because it is used to handle large volumes of data flowing to and from various JSON based REST apis. It's generally the fastest option for working with JSON values in Tcl from the options I've tried, with the next closest being yajltcl. These benchmarks report the median times in microseconds, and produce quite stable results between runs. Benchmarking was done on a MacBook Air running Ubuntu 14.04 64bit, Tcl 8.6.3 built with -O3 optimization turned on, and using an Intel i5 3427U CPU.


This benchmark compares the relative performance of extracting the field containing the string "obj" from the JSON doc:

	"foo": "bar",
	"baz": ["str", 123, 123.4, true, false, null, {"inner": "obj"}]

The compared methods are:

Name Notes Code
old_json_parse Pure Tcl parser dict get [lindex [dict get [json_old parse [string trim $json]] baz] end] inner
rl_json_parse dict get [lindex [dict get [json parse [string trim $json]] baz] end] inner
rl_json_get Using the built-in accessor method json get [string trim $json] baz end inner
yajltcl dict get [lindex [dict get [yajl::json2dict [string trim $json]] baz] end] inner
rl_json_get_native json get $json baz end inner

The use of [string trim $json] is to defeat the caching of the parsed representation, forcing it to reparse the string each time since we're measuring the parse performance here. The exception is the rl_json_get_native test which demonstrates the performance of the cached case.

-- parse-1.1: "Parse a small JSON doc and extract a field" --------------------
                   | This run
    old_json_parse |  241.595
     rl_json_parse |    5.540
       rl_json_get |    4.950
           yajltcl |    8.800
rl_json_get_native |    0.800


If the requirement is to validate a JSON value, the [json valid] command is a light-weight version of the parsing engine that skips allocating values from the document and only returns whether the parsing succeeded or failed, and optionally a description of the failure. It takes about a third of the time to validate a document as parsing it, so the performance win is substantial. On a relatively modern CPU validation takes about 11 cycles per byte, or around 200MB of JSON per second on a 2.3 GHz Intel i7.


This benchmark compares the relative performance of various ways of dynamically generating a JSON document. Although all the methods produce the same string, only the "template" and "template_dict" variants handle nulls in the general case - the others manually test for null only for the one field that is known to be null, so the performance of these variants would be worse in a real-world scenario where all fields would need to be tested for null.

The JSON doc generated in each case is the one produced by the following JSON template (where a(not_defined) does not exist and results in a null value in the produced document):

	"foo": "~S:bar",
	"baz": [
		"~L:~S:not a subst",

The produced JSON doc is:

{"foo":"Bar","baz":["str\"foo\nbar",123,123.4,true,false,null,"~S:not a subst",{"inner":"Bar"},{"inner2":"Bar"}]}

The code for these variants are too long to include in this table, refer to bench/new.bench for the details.

Name Notes
old_json_fmt Pure Tcl implementation, builds JSON from type-annotated Tcl values
rl_json_new rl_json's [json new], API compatible with the pure Tcl version used in old_json_fmt
template rl_json's [json template]
yajltcl yajltcl's type-annotated Tcl value approach
template_dict As for template, but using a dict containing the values to substitute
yajltcl_dict As for yajltcl, but extracting the values from the same dict used by template_dict
-- new-1.1: "Various ways of dynamically assembling a JSON doc" ---------------
                 | This run
    old_json_fmt |   49.450
     rl_json_new |   10.240
        template |    4.520
         yajltcl |    7.700
   template_dict |    2.500
    yajltcl_dict |    7.530


Version 0.10.0 deprecates various subcommands and features, which will be removed in a near future version:

  • [json get_type json_val ?key ...?] - Removed
    • lassign [json get_type json_val ?key ...?] val type -> set val [json get json_val ?key ...?]; set type [json type json_val ?key ...?]
  • [json parse json_val] - A deprecated synonym for [json get json_val].
  • [json fmt type value] - A deprecated synonym for [json new type value], which is itself deprecated, see below.
  • [json new type value] - Use direct subcommands of [json]:
    • [json new string value] -> [json string value]
    • [json new number value] -> [json number value]
    • [json new boolean value] -> [json boolean value]
    • [json new true] -> true
    • [json new false] -> false
    • [json new null] -> null
    • [json new json value] -> value
    • [json new object ...] -> json object ...
    • [json new array ...] -> json array ...
  • modifiers at the end of a path - Modifiers like [json get json_val foo ?type] are deprecated. Replacements are:
    • ?type - use [json type json_val ?key ...?]
    • ?length - use [json length json_val ?key ...?]
    • ?size - use [json length json_val ?key ...?]
    • ?keys - use [json keys json_val ?key ...?]

Under the Hood

Older versions used the yajl c library to parse the JSON string and properly quote generated strings when serializing JSON values, but currently a custom built parser and string quoter is used, removing the libyajl dependency. JSON values are parsed to an internal format using Tcl_Objs and stored as the internal representation for a new type of Tcl_Obj. Subsequent manipulation of that value use the internal representation directly.


Copyright 2015-2023 Ruby Lane. Licensed under the same terms as the Tcl core.