XLAT Question

[g4-lisz at tonarchiv.ch]

April 14, 2026 at 2:27 PM, "Alan DeKok via Freeradius-Users" <freeradius-users at lists.freeradius.org mailto:freeradius-users at lists.freeradius.org?to=%22Alan%20DeKok%20via%20Freeradius-Users%22%20%3Cfreeradius-users%40lists.freeradius.org%3E > wrote:

> 
> On Apr 13, 2026, at 4:59 PM, g4-lisz at tonarchiv.ch wrote:
> 
> > 
> > I read about the default / alternation syntax for string expansion, like %{%{Foo}:-bar}.
> >  
> >  But what I needed is this: 
> >  if (foo != '' ) ? '[ %{foo} ]' : 'null'.
> >  
> >  This is for a JSON request string in a REST module configuration. The request property in question must be a JSON array ( [ .. ] ) or 'null'.
> >  
> >  I could use Unlang IF and declare the complete JSON request in different versions depending on foo. But I would prefer to do this with expansion inside the JSON string definition, if possible.
> > 
>  You'll have to use 'if'. There is a mods-available/json, and you can do:
> 
>  %{json_encode:&foo}
> 
>  Except that if "foo" doesn't exist, it returns an empty json object: "{}"
> 
Thanks for your reply, Alan.

I've decided that it's best to add our own xlat function. We use a patched Rest module anyway:

if (strcmp(fmt, "serviceIds") == 0) {
    if ((vp = fr_pair_find_by_num(request->config, PW_CUST_SERVICE_ID, 0, T_OP_EQ)) != NULL) {
        len = snprintf(out, freespace, "[ %s ]", vp->data.strvalue);
    }
    else {
        len = snprintf(out, freespace, "null");
    }
    out += len;
    freespace -= len;
 }
 *out = '\0';

Now we can simply use:

authenticate {
....
    data: {\
        ....
        "serviceIds": %{vendor:serviceIds},\
....
}

Cheers,
Till