Spec M32 — examples/desktop: composing windows with GTK4 from mc (owner's test, 2026-09-03)

Goal: prove that an mc program (and then a taught UI language) can drive a desktop toolkit to compose windows. Toolkit: GTK4 via Homebrew (/opt/homebrew/lib/libgtk-4.1.dylib plus GLib/GObject/ GIO), chosen because its C API uses only integers, pointers and callbacks — everything mc has today (extern, #dylib/[libs]+[externs], &fn, callp). AppKit through objc_msgSend follows after M24 (NSRect travels in v0..v3).

Part A — plain mc binding (examples/desktop/main.mc, lib/gtk.mc) #

• lib/gtk.mc: extern declarations for the subset used: g_application_new, g_application_run, g_signal_connect_data, g_object_unref, gtk_application_new, gtk_application_window_new, gtk_window_set_title, gtk_window_set_default_size, gtk_window_present, gtk_window_close, gtk_window_set_transient_for, gtk_box_new, gtk_box_append, gtk_label_new, gtk_label_set_text, gtk_label_get_text, gtk_button_new_with_label, gtk_entry_new, gtk_editable_get_text, gtk_editable_set_text, gtk_widget_set_margin_*, gtk_header_bar_new, gtk_window_set_titlebar, gtk_list_box_new, gtk_list_box_append, gtk_check_button_new_with_label, gtk_check_button_get_active, constants (GTK_ORIENTATION_*, G_APPLICATION_DEFAULT_FLAGS). mc.toml maps prefixes to libraries: gtk_* -> gtk4, g_signal_*/g_object_* -> gobject, g_application_* -> gio (first real use of [externs] prefix mapping with several dylibs).
• main.mc: an app with a main window (header bar, a counter label with +/- buttons, an entry whose text is echoed to a label on Enter, a list box that grows with entries) and a second window opened by a button (transient dialog with a close button). Callbacks are mc functions passed with &fn to g_signal_connect_data; state lives in globals or in a small arena struct passed as user_data.
• --self-test: builds the widget tree without running the main loop, invokes the callbacks directly (on_plus, on_entry_activate with the entry set programmatically), prints the label texts and exits 0 — the CI-safe test (check-desktop skips when GTK4 is not installed).
• Manual/visual acceptance: the architect runs the app and captures the windows with the desktop tooling (computer-use), attaching screenshots to the report. CI never opens a window: the GitHub runners have no GTK4, so check-desktop prints a skip line there; the self-test is the only automated check and it needs no display server (owner, 2026-09-03).

Part B — a UI language taught by the surface (examples/desktop/ui.mc, .ui files) #

Using the lx machinery (M22) or a small dedicated module: declarative composition lowered to Part A:

window "Counter" size 360 200 {
    vbox {
        label count "0";
        hbox { button "-" -> on_minus; button "+" -> on_plus; }
        entry name placeholder "your name" -> on_name;
    }
}
fn on_plus() { count.text = itoa(state.n += 1); }

• Handlers are ordinary functions; the module generates the g_signal_connect_data calls and the widget globals; nested containers by recursion in the syntax_stmt("window") handler.
• Same --self-test idea; the same app expressed in the DSL must produce the same self-test output.

Acceptance #

• mc build examples/desktop (Part A) and mc build examples/desktop/ui (Part B) produce signed --exe binaries linked to gtk4/gobject/gio; otool -L shows the four dylibs; both self-tests print the same expected lines; check-desktop in make check runs the self-tests when pkg-config --exists gtk4, otherwise prints a skip line.
• Visual: screenshots of the main and secondary windows (light and dark if the toolkit follows the system), attached to the milestone report by the architect.
• Linux: the same source builds for linux/arm64 with the Alpine gtk4-dev sysroot in Docker when the [target] is linux (headless run limited to --self-test).

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