Commands and subscriptions, and gui-update

gui-update is a plain function, so it cannot wait. It returns the work to do
as values. A command is an (Event %m), synced in a fiber of its own, whose
answer is the next message. A subscription is a source of messages that the
model asks for: after each update the app starts the new ones and cancels the
ones that are gone, by key. every sends a message on a timer, and
subscription takes a source of one's own.

The fibers belong to the app's scope and post their messages to the UI
thread, so gui-update and view only run there. Closing the window cancels
them. The prelude's path macros already use the name update, hence
gui-update.

🤖 Generated with [ECA](https://eca.dev) (anthropic/claude-opus-4-8)

Co-Authored-By: eca-agent <git@eca.dev>

examples/counter.bjo

@@ -13,12 +13,13 @@
 (type (: Msg (Union Inc Dec (: Typed string))))
 (type (: Model (Record (: count int) (: name string))))
 
-(: update (-> Msg Model Model))
-(defun (update msg m)
-  (match msg
-    (Inc (record-set m (count (+ (.count m) 1))))
-    (Dec (record-set m (count (- (.count m) 1))))
-    ((Typed s) (record-set m (name s)))))
+(: gui-update (-> Msg Model (Tuple Model (List (Event Msg)))))
+(defun (gui-update msg m)
+  (next
+    (match msg
+      (Inc (record-set m (count (+ (.count m) 1))))
+      (Dec (record-set m (count (- (.count m) 1))))
+      ((Typed s) (record-set m (name s))))))
 
 (: view (-> Model (View Msg)))
 (defun (view m)
@@ -31,4 +32,4 @@
       (button "+" Inc #:width 40.0))))
 
 (defun (main)
-  (run-app (Model (count 0) (name "")) update view #:title "Counter" #:width 320.0 #:height 200.0))
+  (run-app (Model (count 0) (name "")) gui-update view #:title "Counter" #:width 320.0 #:height 200.0))

examples/stopwatch.bjo

@@ -0,0 +1,55 @@
+;; This Source Code Form is subject to the terms of the Mozilla Public
+;; License, v. 2.0. If a copy of the MPL was not distributed with this
+;; file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+;; bjogui/examples/stopwatch.bjo — a subscription and a command.
+;;
+;;     bjo run examples/stopwatch.bjo
+;;
+;; The stopwatch ticks while it runs: `subscriptions` asks for a tick only
+;; then, so stopping it stops the fiber. "Lap" saves the time after a pause,
+;; to show a command: the pause runs in a fiber, and the window does not wait
+;; for it.
+
+(import (std prelude))
+(import (bjogui view))
+(import (bjogui app))
+
+(type (: Msg (Union Toggle Tick Reset Lap (: Saved int))))
+(type (: Model (Record (: tenths int) (: running bool) (: saving bool) (: laps (List int)))))
+
+(: gui-update (-> Msg Model (Tuple Model (List (Event Msg)))))
+(defun (gui-update msg m)
+  (match msg
+    (Toggle (next (record-set m (running (not (.running m))))))
+    (Tick (next (record-set m (tenths (+ (.tenths m) 1)))))
+    (Reset (next (record-set m (tenths 0) (laps '()))))
+    ;; The time is taken now, and saved half a second later.
+    (Lap (next (record-set m (saving #t))
+               (wrap (timeout (milliseconds 500)) (fun (_) (Saved (.tenths m))))))
+    ((Saved t) (next (record-set m (saving #f) (laps (cons t (.laps m))))))))
+
+(: subscriptions (-> Model (List (Sub Msg))))
+(defun (subscriptions m)
+  (if (.running m) (list (every (milliseconds 100) Tick)) '()))
+
+(: (-> int string))
+(defun (show tenths)
+  #"${(/ tenths 10)}.${(% tenths 10)} s")
+
+(: view (-> Model (View Msg)))
+(defun (view m)
+  (column #:spacing 8.0 #:margin 16.0
+    (text (show (.tenths m)) #:size 32.0)
+    (row #:spacing 8.0
+      (button (if (.running m) "Stop" "Start") Toggle #:width 80.0)
+      (button "Lap" Lap #:enabled (not (.saving m)) #:width 80.0)
+      (button "Reset" Reset #:width 80.0))
+    (apply column (vec->array (vec-map (fun (t) (text #"lap: ${(show t)}"))
+                                       (list->vec (.laps m)))))))
+
+(defun (main)
+  (run-app (Model (tenths 0) (running #f) (saving #f) (laps '()))
+           gui-update view
+           #:subscriptions subscriptions
+           #:title "Stopwatch" #:width 320.0 #:height 300.0))