tcl: replace eager coroutine pre-execution with true suspension via fibers

Rewrote the coroutine implementation to use lib/fiber.sx (make-fiber,
fiber-resume, fiber-done?) instead of eagerly running the proc body and
collecting all yields into a list. Each coroutine is now a live fiber —
calls to the coro command invoke fiber-resume, yield suspends via call/cc.

- make-tcl-interp: remove :coroutines/:in-coro/:coro-yields, add :coro-yield-fn nil
- tcl-cmd-yield: calls :coro-yield-fn (fiber's yield fn) to truly suspend
- tcl-cmd-yieldto: same pattern, yields "" to resumer
- make-coro-cmd: takes fiber (not coro-name), calls fiber-resume on each invoke
- tcl-cmd-coroutine: creates a fiber whose body runs the proc with :coro-yield-fn set
- tcl-call-proc result merge: drop :coro-yields/:coroutines propagation
- test.sh: load lib/fiber.sx before lib/tcl/runtime.sx in epoch 4

All 337/337 tests pass including all 20 coro tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/tcl/runtime.sx

@@ -1,5 +1,6 @@
 ; Tcl-on-SX runtime evaluator
 ; State: {:frame frame :commands cmd-table :result last-result :output accumulated-output}
+; Requires lib/fiber.sx to be loaded first (provides make-fiber, fiber-resume, fiber-done?)
 
 (define make-frame (fn (level parent) {:level level :locals {} :parent parent}))
 
@@ -20,7 +21,7 @@
     (frame name val)
     (assoc frame :locals (assoc (get frame :locals) name val))))
 
-(define make-tcl-interp (fn () {:result "" :output "" :code 0 :errorinfo "" :errorcode "" :frame (make-frame 0 nil) :frame-stack (list) :procs {} :commands {} :current-ns "::" :coroutines {} :in-coro false :coro-yields (list)}))
+(define make-tcl-interp (fn () {:result "" :output "" :code 0 :errorinfo "" :errorcode "" :frame (make-frame 0 nil) :frame-stack (list) :procs {} :commands {} :current-ns "::" :coro-yield-fn nil}))
 
 (define
   tcl-register
@@ -297,8 +298,6 @@
                         :result result-val
                         :output (str caller-output proc-output)
                         :code (if (= code 2) 0 code)
-                        :coro-yields (get result-interp :coro-yields)
-                        :coroutines (get result-interp :coroutines)
                         :commands (get result-interp :commands))))))))))))))
 
 (define
@@ -2769,65 +2768,49 @@
           ((equal? sub "tclversion") (assoc interp :result "8.6"))
           (else (error (str "info: unknown subcommand \"" sub "\""))))))))
 
-; make-coro-cmd: returns a command function that pops values from the coroutine's yields list
+; tcl-cmd-yield: suspend the current coroutine fiber, returning val to the resumer
 (define
   tcl-cmd-yield
   (fn
     (interp args)
     (let
       ((val (if (> (len args) 0) (first args) "")))
-      (if
-        (get interp :in-coro)
-        (assoc
-          (assoc
-            interp
-            :coro-yields (append (get interp :coro-yields) (list val)))
-          :result "")
-        (error "yield called outside coroutine")))))
+      (let
+        ((yield-fn (get interp :coro-yield-fn)))
+        (if
+          (nil? yield-fn)
+          (error "yield called outside coroutine")
+          (let
+            ((resume-val (yield-fn val)))
+            (assoc interp :result (if (nil? resume-val) "" resume-val))))))))
 
-; coroutine: execute proc eagerly in a coroutine context, collecting all yields
+; tcl-cmd-yieldto: suspend the current coroutine fiber (simplified: yields "" to resumer)
 (define
   tcl-cmd-yieldto
   (fn
     (interp args)
-    (if
-      (get interp :in-coro)
-      (assoc
-        (assoc
-          interp
-          :coro-yields (append (get interp :coro-yields) (list "")))
-        :result "")
-      (error "yieldto called outside coroutine"))))
+    (let
+      ((yield-fn (get interp :coro-yield-fn)))
+      (if
+        (nil? yield-fn)
+        (error "yieldto called outside coroutine")
+        (let
+          ((resume-val (yield-fn "")))
+          (assoc interp :result (if (nil? resume-val) "" resume-val)))))))
 
 ; --- clock command (stubs) ---
 
 (define
   make-coro-cmd
   (fn
-    (coro-name)
+    (fiber)
     (fn
       (interp args)
       (let
-        ((coros (get interp :coroutines)))
+        ((resume-val (if (> (len args) 0) (first args) "")))
         (let
-          ((coro (get coros coro-name)))
-          (if
-            (nil? coro)
-            (error (str "coroutine \"" coro-name "\" not found"))
-            (let
-              ((yields (get coro :yields)) (pos (get coro :pos)))
-              (if
-                (>= pos (len yields))
-                (assoc interp :result "")
-                (let
-                  ((val (nth yields pos)))
-                  (let
-                    ((new-coro (assoc coro :pos (+ pos 1))))
-                    (assoc
-                      (assoc
-                        interp
-                        :coroutines (assoc coros coro-name new-coro))
-                      :result val)))))))))))
+          ((yielded (fiber-resume fiber resume-val)))
+          (assoc interp :result (if (nil? yielded) "" yielded)))))))
 
 ; --- file I/O stubs ---
 
@@ -2843,24 +2826,34 @@
           (cmd-name (nth args 1))
           (call-args (rest (rest args))))
         (let
-          ((coro-interp (assoc interp :in-coro true :coro-yields (list) :result "" :code 0)))
+          ((base-interp (assoc interp :result "" :code 0 :coro-yield-fn nil)))
           (let
-            ((cmd-fn (get (get coro-interp :commands) cmd-name)))
+            ((fiber
+                (make-fiber
+                  (fn
+                    (fiber-yield _)
+                    (let
+                      ((coro-interp (assoc base-interp :coro-yield-fn fiber-yield)))
+                      (let
+                        ((cmd-fn (get (get coro-interp :commands) cmd-name)))
+                        (if
+                          (nil? cmd-fn)
+                          (let
+                            ((proc-entry (tcl-proc-lookup coro-interp cmd-name)))
+                            (if
+                              (nil? proc-entry)
+                              (error
+                                (str "coroutine: unknown command \"" cmd-name "\""))
+                              (tcl-call-proc
+                                coro-interp
+                                (get proc-entry :name)
+                                (get proc-entry :def)
+                                call-args)))
+                          (cmd-fn coro-interp call-args))))))))
             (let
-              ((exec-result (if (nil? cmd-fn) (let ((proc-entry (tcl-proc-lookup coro-interp cmd-name))) (if (nil? proc-entry) (error (str "coroutine: unknown command \"" cmd-name "\"")) (tcl-call-proc coro-interp (get proc-entry :name) (get proc-entry :def) call-args))) (cmd-fn coro-interp call-args))))
-              (let
-                ((yields (get exec-result :coro-yields)))
-                (let
-                  ((new-coros (assoc (get exec-result :coroutines) coro-name {:yields yields :pos 0})))
-                  (let
-                    ((new-commands (assoc (get exec-result :commands) coro-name (make-coro-cmd coro-name))))
-                    (assoc
-                      exec-result
-                      :coroutines new-coros
-                      :commands new-commands
-                      :in-coro false
-                      :coro-yields (list)
-                      :result "")))))))))))
+              ((new-commands
+                  (assoc (get interp :commands) coro-name (make-coro-cmd fiber))))
+              (assoc interp :commands new-commands :result ""))))))))
 
 (define
   tcl-cmd-clock

lib/tcl/test.sh

@@ -39,6 +39,7 @@ cat > "$TMPFILE" << EPOCHS
 (epoch 3)
 (load "lib/tcl/tests/parse.sx")
 (epoch 4)
+(load "lib/fiber.sx")
 (load "lib/tcl/runtime.sx")
 (epoch 5)
 (load "lib/tcl/tests/eval.sx")