src/lang.zig

@@ -116,7 +116,7 @@ pub const NewCommand = struct {
         };
     }
 
-    pub fn cast(base: *@This(), comptime T: type) ?*T {
+    pub fn cast(base: *const @This(), comptime T: type) ?*const T {
         if (base.tag != T.base_tag)
             return null;
 

src/runner.zig

@@ -374,7 +374,23 @@ pub const Runner = struct {
         try image.runPlugin("http://calf.sourceforge.net/plugins/VintageDelay", pos, params);
     }
 
-    fn newRunCommand(self: *@This(), cmd: lang.NewCommand) !void {}
+    fn newRunCommandSingle(
+        self: *@This(),
+        cmd: lang.NewCommand,
+        comptime tag: lang.NewCommand.Tag,
+    ) !void {
+        comptime const typ = lang.NewCommand.tagToType(tag);
+        const command = cmd.cast(typ).?;
+
+        std.debug.warn("{}\n", .{command});
+    }
+
+    fn newRunCommand(self: *@This(), cmd: lang.NewCommand) !void {
+        switch (cmd.tag) {
+            .load => try self.newRunCommandSingle(cmd, .load),
+            else => @panic("TODO"),
+        }
+    }
 
     fn runCommand(self: *Runner, cmd: *lang.Command) !void {
         var params = ParamList.init(self.allocator);