using the module file

src/example_3/source/app.d

@@ -1,10 +1,5 @@
 import std.stdio;
-
-void fun(ref uint x, double y)
-{
-    x = 42;
-    y = 3.14;
-}
+import fun_;
 
 void main()
 {

src/example_3/source/fun.d

@@ -0,0 +1,7 @@
+module fun_;
+
+void fun(ref uint x, double y)
+{
+    x = 42;
+    y = 3.14;
+}