Add config substitution example

dub.json

@@ -25,6 +25,17 @@
       "copyFiles": ["examples/json/config.json"],
       "targetPath": "bin/examples/json",
       "workingDirectory": "bin/examples/json"
+    },
+    {
+      "name": "valueSubstitutionExample",
+      "targetType": "executable",
+      "description": "Example of how to use environment and config substitution.",
+      "targetName": "valueSubstitutionExample",
+      "sourcePaths": ["examples/valueSubstitution"],
+      "importPaths": ["source"],
+      "copyFiles": ["examples/valueSubstitution/config.json"],
+      "targetPath": "bin/examples/valueSubstitution",
+      "workingDirectory": "bin/examples/valueSubstitution"
     }
   ]
 }

examples/valueSubstitution/app.d

@@ -0,0 +1,23 @@
+/**
+ * Authors:
+ *  Mike Bierlee, m.bierlee@lostmoment.com
+ * Copyright: 2022 Mike Bierlee
+ * License:
+ *  This software is licensed under the terms of the MIT license.
+ *  The full terms of the license can be found in the LICENSE file.
+ */
+
+import mirage.json : loadJsonConfig, parseJsonConfig;
+
+import std.stdio : writeln;
+import std.process : environment;
+
+void main() {
+    // This example shows how values in configuration can be substituted from
+    // environment variables or other configuration paths.
+
+    environment["subject"] = "world";
+    auto config = loadJsonConfig("config.json");
+
+    writeln(config.get("greeting"));
+}

examples/valueSubstitution/config.json

@@ -0,0 +1,8 @@
+{
+  "parameters": {
+    "greeting": "Hello",
+    "subject": "$CONFIG_EXAMPLE_SUBJECT",
+    "followUp": "${CONFIG_EXAMPLE_FOLLOW_UP:Enjoy your day!}"
+  },
+  "greeting": "${parameters.greeting} ${parameters.subject}! ${parameters.followUp}"
+}

makefile

@@ -14,4 +14,7 @@ clean:
 run-examples: run-jsonExample
 
 run-jsonExample:
-	dub run --build=release --config=jsonExample
+	dub run --build=release --config=jsonExample
+
+run-valueSubstitutionExample:
+	dub run --build=release --config=valueSubstitutionExample