TCL Dict to JSON - json

Question

I am trying to convert a dict into JSON format and not seeing any easy method using TclLib Json Package. Say, I have defined a dict as follows :
set countryDict [dict create USA {population 300 capital DC} Canada {population 30 capital Ottawa}]
I want to convert this to json format as shown below:
{
"USA": {
"population": 300,
"captial": "DC"
},
"Canada": {
"population": 30,
"captial": "Ottawa"
}
}
("population" is number and capital is string). I am using TclLib json package (https://wiki.tcl-lang.org/page/Tcllib+JSON) . Any help would be much appreciated.

Answer 1

There's two problems with the “go straight there” approach that you appear to be hoping for:
Tcl's type system is extremely different to JSON's; in Tcl, every value is a (subtype of) string, but JSON expects objects, arrays, numbers and strings to wholly different things.
The capital becomes captial. For bonus fun. (Hopefully that's just a typo on your part, but we'll cope.)
I'd advise using rl_json for this; it's a much more capable package that treats JSON as a fundamental type. (It's even better at it when it comes to querying into the JSON structure.)
package require rl_json
set result {{}}; # Literal empty JSON object
dict for {countryID data} $countryDict {
rl_json::json set result $countryID [rl_json::json template {{
"population": "~N:population",
"captial": "~S:capital"
}} $data]
# Yes, that was {{ … }}, the outer ones are for Tcl & the inner ones for a JSON object
}
puts [rl_json::json pretty $result]
That produces almost exactly the output you asked for, except with different indentation. $result is the “production” version of the output that you can work with for further processing, but which has no excess whitespace at all (which is a great choice when you're dealing with documents over 100MB long).
Notes:
The initial JSON object could have been done like this:
set result "{}"
that would have worked just as well (and been the same Tcl bytecode).
json set puts an item into an object or array; that's exactly what we want here (in a dict for to go over the input data).
json template takes an optional dictionary for mapping substitution names in the template to values; that's perfect for your use case. Otherwise we'd have had to do dict with data {} to map the contents of the dictionary into variables, and that's less than perfect when the input data isn't strictly controlled.
The contents of template argument to json template is itself JSON. The ~N: prefix in a leaf string value says “replace this with a number from the substitution called…”, and ~S: says “replace this with a string from the substitution called…”. There are others.