Fast Tcl only JSON parser (


Title: ton and TON Subtitle: Tcl Object Notation and some code to manipulate it Author: Georg Lehner Copyright: Georg Lehner, 2018

What is it

ton was born as a pure Tcl JSON parser. It parses JSON from right to left and it is faster than other comparable tools, at least on Tcl8.6 and in March 2018.

How it works

ton provides a function to convert a JSON string into TON - a data representation equivalent to JSON. TON stands for Tcl Object Notation, every TON representation is a Tcl script.

TON is decoded by defining six functions: o, a, s, i, d and l, which decode their argument(s) as object, array, string, number (i .. integer, d .. double float) and literal respectively.

ton provides namespaces with these function sets for decoding TON into:

a nested Tcl lists
a Tcl dictionary in the same format as the jimhttp JSON parser.
a Tcl dictionary in the same format as the Tcllib JSON parser.

ton::2json: :__ an unformatted JSON string.

The function ton::json2ton converts a JSON string into TON. In order to convert a JSON dict in the variable json to the Tcllib dictionary format you would run the following:

namespace eval ton::2dict [ton::json2ton $json]]

How to choose a Tcl decoder

ton::a2dict is the most user friendly decoder, and the fastest one for general workload. All data is extracted with a single:

dict get $data key key ...

ton::2list allows for type checking on access and is faster for huge arrays. Data is extracted best with a provided access function

ton::2list::get $data key key ...

ton::2dict seems fastest when processing small JSON strings. Data extraction for mixed array and object data is cumbersome. Suppose we have an array of objects and want to get the email of the 43rd object:

dict get [lindex $data 43] email

Design Goals

ton provides a small parser implementation which can be included directly in source code into a Tcl script and parses JSON correctly, without overstretching correctness.


Other then taking care of backslash escaped quotes \" on parsing, no processing for backslash escapes is done.

Numbers are only validated with Tcl's string is function. Hex or octal numbers in the JSON string are therefore admissible.

Security: TON should be executed in a save slave interpreter to avoid arbitrary code execution with malicious crafted JSON or TON strings. I believe, that ton::json2ton does not generate dangerous TON, but this has not been scrutinized.


Craft test cases for each error in the code and for corner cases like empty arrays.

Review the decoded data with respect to empty arrays and objects and guarantee, that JSON -> TON -> JSON is the identity function.


Original article:

Tcler's Wiki:


License Terms

In order to comply with general usage in the Tcl/Tk community ton is released under a BSD style license, copied directly from the Tcllib source tree and available in the file license.terms

Testing offers a small test suite on the page which we use to test ton. goes wild about (non)-compliance and missing clarity of specifications. It's test suite is here:

Bugs fixed in Version 0.3

An invalid literal like [0e] would not be recognized as such, but rather emit a string exhausted error.

Empty objects or arrays must not have an empty list as argument in TON, e.g.: o {} -> o

Bugs fixed in Version 0.2

Thanks to the test suite, we could identify a list of bugs:

The following two valid JSON strings produce an error in ton::json2ton:


The following invalid JSON string (and also any extra ]) sends ton into an infinite loop:


A stray string gives an error (the double quote must be the first character):


An empty JSON string sends ton into an infinite loop.

White space in keys result in wrong or invalid TON.

ton::json2ton will parse the rightmost valid JSON construct in a string, and terminate. No check is done for extra characters.