diff --git a/src/main.rs b/src/main.rs
index 0669bff..e5561a9 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,8 +1,9 @@
 #![windows_subsystem = "windows"]
 
+use bitsy_parser::game::Game;
 use gio::prelude::*;
 use gtk::prelude::*;
-use gtk::{Orientation, Adjustment};
+use gtk::Orientation;
 use std::env::args;
 
 const SPACING: i32 = 16;
@@ -30,7 +31,7 @@ fn error_popup(message: &str) {
     });
 }
 
-fn add_text_box(container: &gtk::Box, label: &str) -> gtk::TextView {
+fn add_text_box(container: &gtk::Box, label: &str) -> gtk::TextBuffer {
     let vertical = gtk::Box::new(Orientation::Vertical, SPACING);
     vertical.set_property_expand(true);
 
@@ -42,12 +43,31 @@ fn add_text_box(container: &gtk::Box, label: &str) -> gtk::TextView {
     let scrolled_window = gtk::ScrolledWindow::new(h, v);
     scrolled_window.set_property_expand(true);
     let text_view = gtk::TextView::new();
+    text_view.set_property_monospace(true);
     scrolled_window.add(&text_view);
     vertical.add(&scrolled_window);
 
     container.add(&vertical);
 
-    text_view
+    text_view.get_buffer().unwrap()
+}
+
+fn parse_or_error(game_data: String, label: &str) -> Result<Game, ()> {
+    let main = Game::from(game_data);
+    if main.is_ok() {
+        Ok(main.unwrap())
+    } else {
+        error_popup(&format!(
+            "Couldn't parse {} game data: {:?}",
+            label,
+            main.unwrap_err()));
+        Err(())
+    }
+}
+
+fn buffer_to_string(buffer: &gtk::TextBuffer) -> String {
+    let (start, end) = &buffer.get_bounds();
+    buffer.get_text(start, end, false).unwrap().to_string()
 }
 
 fn build_ui(application: &gtk::Application) {
@@ -60,13 +80,32 @@ fn build_ui(application: &gtk::Application) {
 
     let vertical = gtk::Box::new(Orientation::Vertical, SPACING);
 
-    let input_main = add_text_box(&vertical, "main game data");
-    let input_additional = add_text_box(&vertical, "additional game data");
+    let buffer_main = add_text_box(&vertical, "main game data");
+    let buffer_additional = add_text_box(&vertical, "additional game data");
 
     let button = gtk::Button::with_label("mix!");
     vertical.add(&button);
 
-    let output = add_text_box(&vertical, "output");
+    let separator = gtk::Separator::new(Orientation::Vertical);
+    vertical.add(&separator);
+
+    let buffer_output = add_text_box(&vertical, "output");
+
+    button.connect_clicked(move |_| {
+        let main = buffer_to_string(&buffer_main);
+        let additional = buffer_to_string(&buffer_additional);
+
+        let main       = parse_or_error(main, "main");
+        let additional = parse_or_error(additional, "additional");
+
+        if main.is_ok() && additional.is_ok() {
+            let mut main = main.unwrap();
+            let additional = additional.unwrap();
+            main.merge(additional);
+
+            buffer_output.set_text(&main.to_string());
+        }
+    });
 
     window.add(&vertical);
     window.show_all();