A simple example of writing a formatted string out to a file or to json in Dlang

gistfile1.d

import std.stdio;
import std.string;
import std.file;
import std.json;

static const auto HK_DB = "hk_db.json"; // this is the filename

//
  // Use a formatting string to fill in values for this config file
  //
  static const auto HAP_SERVICE_DEF_FORMAT = "%s\n" ~
												"_hap._tcp.\n" ~
												"8008\n" ~ // XXX PORTNO should be configured
												"c#=%s\n" ~
												"ff=%s\n" ~
												"id=%s\n" ~
												"md=%s\n" ~
												"pv=1.0\n" ~
												"s#=%s\n" ~
												"sf=%s\n" ~
												"ci=%s\n" ~
												"Priority=0\n" ~
												"Weight=0\n" ~
												"Port=8008\n" ~
												"Target=%s\n";

//
// Ignoring that we don't have all of these variables defined, focus on the idea
// We fill the format string with values and write to a file
std.file.write(HAP_SERVICE_DEF_TXT, format(HAP_SERVICE_DEF_FORMAT, mdns_name, params["HK_CNUM"], params["HK_FF"], chomp(params["HK_ID"]), params["HK_MD"], params["HK_SNUM"], params["HK_SF"], params["HK_CI"], mdns_hostname)); // XXX Add exception handling
                        
                        
// Here's another example with json
static void write_hk_db(JSONValue jsondata) { // XXX If this is all we do, we don't need a special function
 		immutable jsonstr = jsondata.toString();
 		std.file.write(HK_DB, jsonstr);
 	}
  
static JSONValue load_hk_db() {
 		auto datastr = `{ "cnum" : 1, "snum": 1}`;
 		if (!exists(HK_DB.dup)) {
 			std.file.write(HK_DB, datastr); // XXX Add exception handling
 			writeln("HK_DB file does not exist, created, and wrote ", datastr);
 		} else {
 			datastr = std.file.readText(HK_DB);
 			writeln("HK_DB read ", datastr);
 		}

 		JSONValue jsondata = parseJSON(datastr);

 		return jsondata;
 	}

Note, the above won't compile, however, you can borrow the ideas, which are just really basic example of writing data out to a file. A little more robustness is needed. Also, writes are synchronous.