[NTG-context] Json module and lua->context communication question

[NTG-context] Json module and lua->context communication question

[Andreas Hof via ntg-context]

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Hi all here,

I have a question in the area of the json module and in general about the interaction between Lua and ConTeXt.

In m-json.mkxl we have for example:

    interfaces.implement {
        name      = "tablefield",
        actions   = function(namespace,name)
            local data = field(namespace,name)
            if data then
                context(data)
            end
        end,
        arguments = "2 arguments",
        public    = true,
        protected = true,
    }
 
Means, that, if I ask for a table field from json file the value will be typeset through context(data).

Is there any possibility to check the value passed from Lua to ConTeXt on ConTeXt level? 

I like to use the json module for getting addresses for a mass lettering. 
Prominent examples, where a check of the value from a json-database is needed is the title of a person (typeset it plus a space or do nothing if empty), 
or if you need to have alternatives for an opening sentence depending on the salutation or gender. Or whatever you can think of in a mass letter depending on the dataset ...

The idea is to do the special handlings on ConTeXt level, but I didn’t manage to get it running together with the existing json module. 

Also checks against non empty field values don’t work for me.
I know that context(data) is not really returning a value, like a return value in procedural languages. But I hope there is a solution to do the conditionals on ConTeXt level.    

What I also don’t understand is, why different variants of how the lua code is called, behave different, my expectation was that they simply run under a different catcode regime.

% typsets: equal (as expected)
\edef\compareVal{A}
\edef\temp{\ctxlua{context("A")}}
\ifx\temp\compareVal equal\else not equal\fi

But in the following variants I always branch to the \else path:

\edef\temp{\ctxluacode{context("A")}}
\ifx\temp\compareVal equal\else not equal\fi

\edef\temp{\startluacode context("A") \stopluacode}
\ifx\temp\compareVal equal\else not equal\fi

\edef\temp{\startlua context("A") \stoplua}
\ifx\temp\compareVal equal\else not equal\fi

The following mwe (files are also attached), shows one use case (address field in the letter)

I hope someone here, is able to help. 


\usemodule[json]
\loadjsonfile[mglist][addr.json]
\showmakeup[space]

\starttext
  % read all adresses
  \dorecurse{\jsonlength{mglist}{records}}{%
    % create macros for later use ...
    \edef\Salutaion{\jsonfield{mglist}{records[\recurselevel].salutation}}
    \edef\Title{\jsonfield{mglist}{records[\recurselevel].title}}
    \edef\Name{\jsonfield{mglist}{records[\recurselevel].name}}
    \edef\Firstname{\jsonfield{mglist}{records[\recurselevel].firstname}}
    \edef\Street{\jsonfield{mglist}{records[\recurselevel].street}}
    \edef\ZIP{\jsonfield{mglist}{records[\recurselevel].zip}}
    \edef\Location{\jsonfield{mglist}{records[\recurselevel].location}}
    % print the Adress ... 
    \Salutaion\space\ifx\Title\empty\else\Title\space\fi\Firstname\space\Name \blank[small]
    \Street \blank[small]
    \ZIP\space\Location 
    \blank[line]
  }
  % works ...
  \edef\temp{\ctxlua{context("")}}
  \ifx\temp\empty empty\else not empty\fi
  % works ...
  \edef\compareVal{A}
  \edef\temp{\ctxlua{context("A")}}
  \ifx\temp\compareVal equal\else not equal\fi
\stoptext

With a sample Json file (addr.json):

{
  "records": [
    {
      "salutation": "Mrs.",
      "title": "",
      "name": "von Bingen",
      "firstname": "Hildegard",
      "street": "past road 1",
      "zip": "00000",
      "location": "Bingen am Rhein"
    },
    {
      "salutation": "Mr.",
      "title": "Dr.",
      "name": "von Kos",
      "firstname": "Hippokrates",
      "street": "long ago road 1",
      "zip": "00001",
      "location": "Kos"
    }
  ]
}

Thanks for help …
Andreas




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\usemodule[json]
\loadjsonfile[mglist][addr.json]
\showmakeup[space]

\starttext
  % read all adresses
  \dorecurse{\jsonlength{mglist}{records}}{%
    % create macros for later use ...
    \edef\Salutaion{\jsonfield{mglist}{records[\recurselevel].salutation}}
    \edef\Title{\jsonfield{mglist}{records[\recurselevel].title}}
    \edef\Name{\jsonfield{mglist}{records[\recurselevel].name}}
    \edef\Firstname{\jsonfield{mglist}{records[\recurselevel].firstname}}
    \edef\Street{\jsonfield{mglist}{records[\recurselevel].street}}
    \edef\ZIP{\jsonfield{mglist}{records[\recurselevel].zip}}
    \edef\Location{\jsonfield{mglist}{records[\recurselevel].location}}
    % print the Adress ... 
    \Salutaion\space\ifx\Title\empty\else\Title\space\fi\Firstname\space\Name \blank[small]
    \Street \blank[small]
    \ZIP\space\Location 
    \blank[line]
  }
  % works ...
  \edef\temp{\ctxlua{context("")}}
  \ifx\temp\empty empty\else not empty\fi
  % works ...
  \edef\compareVal{A}
  \edef\temp{\ctxlua{context("A")}}
  \ifx\temp\compareVal equal\else not equal\fi
\stoptext

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[NTG-context] Re: Json module and lua->context communication question

[Hans Hagen via ntg-context]

On 2/13/2026 11:20 AM, Andreas Hof via ntg-context wrote:
> \edef\compareVal{A}
> \edef\temp{\ctxlua{context("A")}}
> \ifx\temp\compareVal equal\else not equal\fi
> 
> But in the following variants I always branch to the \else path:
> 
> \edef\temp{\ctxluacode{context("A")}}
> \ifx\temp\compareVal equal\else not equal\fi

\meaningful\ctxluacode

shows that it is (1) protected and (2) does more

> \edef\temp{\startluacode context("A") \stopluacode}
> \ifx\temp\compareVal equal\else not equal\fi

idem

> \edef\temp{\startlua context("A") \stoplua}
> \ifx\temp\compareVal equal\else not equal\fi

idem

> The following mwe (files are also attached), shows one use case (address 
> field in the letter)

so just use \ctxlua which expands


> I hope someone here, is able to help.

also, there is:

   - helper:

   \doifelsesomething{\temp}{not empty}{empty}

   - primitive \ifempty  (also takes {})
  
\Salutaion\space\ifempty\Title\else\Title\space\fi\Firstname\space\Name 
\blank[small]

   - primitive \iftok

   \iftok{\ctxlua{context("A")}}{A}\else not \fi equal

Hans



-----------------------------------------------------------------
                                           Hans Hagen | PRAGMA ADE
               Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
        tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl
-----------------------------------------------------------------
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___________________________________________________________________________________

[NTG-context] Re: Json module and lua->context communication question

[Andreas Hof via ntg-context]

Hello Hans,

thanks for the quick response. 

However, the answer to just use \ctxlua as it expands, doesn’t help with the usage of the module m-json.mkxl (from the context distribution) which has all the interfaces in:

\startluacode
...
\stopluacode

So the simple question is: Is there a possibility to check the value passed to context by the  \tablefield interface? 
Or is it better to extend the module like this for example?

% first quick shot:

\startluacode
    local field      = utilities.json.field
    interfaces.implement{
      name      = "tablefieldset",
      actions   = function(namespace, path, csname)
        local data = field(namespace, path)
        tex.sprint(string.format("\\gdef\\%s{%s}", csname, data))
      end, 
      arguments = "3 arguments",
      public    = true,
      protected = true,
    }
\stopluacode

And then:

% in the loop: instead of \edef\Title{\jasonfield{}{}}
...
\tablefieldset{mglist}{records[\recurselevel].title}{Title} % creates a macro \Titel
...		
 
\Salutaion\space\ifempty\Title\else\Title~\fi\Firstname\space\Name


Btw. Nice and very responsive group … 

Thanks. 
Andreas




Am 13.02.2026 um 13:24 schrieb Hans Hagen via ntg-context <ntg-context@ntg.nl>:

On 2/13/2026 11:20 AM, Andreas Hof via ntg-context wrote:
> \edef\compareVal{A}
> \edef\temp{\ctxlua{context("A")}}
> \ifx\temp\compareVal equal\else not equal\fi
> But in the following variants I always branch to the \else path:
> \edef\temp{\ctxluacode{context("A")}}
> \ifx\temp\compareVal equal\else not equal\fi

\meaningful\ctxluacode

shows that it is (1) protected and (2) does more

> \edef\temp{\startluacode context("A") \stopluacode}
> \ifx\temp\compareVal equal\else not equal\fi

idem

> \edef\temp{\startlua context("A") \stoplua}
> \ifx\temp\compareVal equal\else not equal\fi

idem

> The following mwe (files are also attached), shows one use case (address field in the letter)

so just use \ctxlua which expands


> I hope someone here, is able to help.

also, there is:

 - helper:

 \doifelsesomething{\temp}{not empty}{empty}

 - primitive \ifempty  (also takes {})
\Salutaion\space\ifempty\Title\else\Title\space\fi\Firstname\space\Name \blank[small]

 - primitive \iftok

 \iftok{\ctxlua{context("A")}}{A}\else not \fi equal

Hans



-----------------------------------------------------------------
                                         Hans Hagen | PRAGMA ADE
             Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
      tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl
-----------------------------------------------------------------
___________________________________________________________________________________
If your question is of interest to others as well, please add an entry to the Wiki!

maillist : ntg-context@ntg.nl / https://mailman.ntg.nl/mailman3/lists/ntg-context.ntg.nl
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archive  : https://github.com/contextgarden/context
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___________________________________________________________________________________

___________________________________________________________________________________
If your question is of interest to others as well, please add an entry to the Wiki!

maillist : ntg-context@ntg.nl / https://mailman.ntg.nl/mailman3/lists/ntg-context.ntg.nl
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___________________________________________________________________________________

[NTG-context] Re: Json module and lua->context communication question

[Hans Hagen via ntg-context]

On 2/13/2026 3:19 PM, Andreas Hof via ntg-context wrote:
> Hello Hans,
> 
> thanks for the quick response.
> 
> However, the answer to just use \ctxlua as it expands, doesn’t help with the usage of the module m-json.mkxl (from the context distribution) which has all the interfaces in:
> 
> \startluacode
> ...
> \stopluacode
> 
> So the simple question is: Is there a possibility to check the value passed to context by the  \tablefield interface?
> Or is it better to extend the module like this for example?

at the tex end

\doifelsetext{...}

Hans

-----------------------------------------------------------------
                                           Hans Hagen | PRAGMA ADE
               Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
        tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl
-----------------------------------------------------------------
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___________________________________________________________________________________

[NTG-context] Re: Json module and lua->context communication question

[Andreas Hof via ntg-context]

Great, this one works!
Thanks a lot!

Andreas.
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[NTG-context] Re: Json module and lua->context communication question

[Aditya Mahajan]

On Fri, 13 Feb 2026, Andreas Hof via ntg-context wrote:

> Hi all here,
> 
> I have a question in the area of the json module and in general about the interaction between Lua and ConTeXt.
> 
> In m-json.mkxl we have for example:
> 
>     interfaces.implement {
>         name      = "tablefield",
>         actions   = function(namespace,name)
>             local data = field(namespace,name)
>             if data then
>                 context(data)
>             end
>         end,
>         arguments = "2 arguments",
>         public    = true,
>         protected = true,
>     }
>  
> Means, that, if I ask for a table field from json file the value will be typeset through context(data).
> 
> Is there any possibility to check the value passed from Lua to ConTeXt on ConTeXt level? 
> 
> I like to use the json module for getting addresses for a mass lettering. 
> Prominent examples, where a check of the value from a json-database is needed is the title of a person (typeset it plus a space or do nothing if empty), 
> or if you need to have alternatives for an opening sentence depending on the salutation or gender. Or whatever you can think of in a mass letter depending on the dataset ...

For such tasks, one option is convert the data to a lua table, and then typeset using CLD. For example, 

  addresses = utilities.json.load("addr.json")
  inspect(addresses)

gives

table={
 ["records"]={
  {
   ["firstname"]="Hildegard",
   ["location"]="Bingen am Rhein",
   ["name"]="von Bingen",
   ["salutation"]="Mrs.",
   ["street"]="past road 1",
   ["title"]="",
   ["zip"]="00000",
  },
  {
   ["firstname"]="Hippokrates",
   ["location"]="Kos",
   ["name"]="von Kos",
   ["salutation"]="Mr.",
   ["street"]="long ago road 1",
   ["title"]="Dr.",
   ["zip"]="00001",
  },
 },
}

So, you can loop over the entries, check for title/salutation and write the logic in lua (no need to worry about expansion and stuff). 

Aditya

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[NTG-context] Re: Json module and lua->context communication question

[Andreas Hof via ntg-context]

Hello Aditya,
thanks for pointing to that option. 
Until now I never tried it ...

Andreas
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