spec: coroutine primitive — make-coroutine/resume/yield via perform/cek-step-loop

spec/coroutines.sx: define-library with make-coroutine, coroutine-resume,
coroutine-yield, coroutine?, coroutine-alive?. Built on existing perform/
cek-step-loop/cek-resume suspension machinery.

spec/tests/test-coroutines.sx: 17 tests — multi-yield, final return,
arg passthrough, alive? predicate, nested coroutines, recursive iteration,
independent coroutine interleaving.

Key: coroutine body must use (define loop (fn…)) not named let — named let
transpiles to cek_call→cek_run which rejects IO suspension. All 17/17 pass.

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

spec/tests/test-coroutines.sx

@@ -0,0 +1,202 @@
+(import (sx coroutines))
+
+(defsuite
+  "coroutine"
+  (deftest
+    "coroutine? recognizes coroutine objects"
+    (let
+      ((co (make-coroutine (fn () nil))))
+      (assert (coroutine? co))
+      (assert= false (coroutine? 42))
+      (assert= false (coroutine? "hello"))
+      (assert= false (coroutine? nil))
+      (assert= false (coroutine? (list)))))
+  (deftest
+    "coroutine-alive? true for ready coroutine"
+    (let
+      ((co (make-coroutine (fn () nil))))
+      (assert (coroutine-alive? co))))
+  (deftest
+    "coroutine-alive? false for non-coroutine"
+    (assert= false (coroutine-alive? 42)))
+  (deftest
+    "immediate return — done true, value is body result"
+    (let
+      ((co (make-coroutine (fn () 42))))
+      (let
+        ((r (coroutine-resume co nil)))
+        (assert= true (get r "done"))
+        (assert= 42 (get r "value")))))
+  (deftest
+    "immediate nil return"
+    (let
+      ((co (make-coroutine (fn () nil))))
+      (let
+        ((r (coroutine-resume co nil)))
+        (assert= true (get r "done"))
+        (assert= nil (get r "value")))))
+  (deftest
+    "coroutine-alive? false after completion"
+    (let
+      ((co (make-coroutine (fn () nil))))
+      (coroutine-resume co nil)
+      (assert= false (coroutine-alive? co))))
+  (deftest
+    "single yield — done false on yield, done true on finish"
+    (let
+      ((co (make-coroutine (fn () (coroutine-yield 10) 20))))
+      (let
+        ((r1 (coroutine-resume co nil)))
+        (let
+          ((r2 (coroutine-resume co nil)))
+          (assert= false (get r1 "done"))
+          (assert= 10 (get r1 "value"))
+          (assert= true (get r2 "done"))
+          (assert= 20 (get r2 "value"))))))
+  (deftest
+    "coroutine-alive? true between yield and next resume"
+    (let
+      ((co (make-coroutine (fn () (coroutine-yield nil) nil))))
+      (assert (coroutine-alive? co))
+      (coroutine-resume co nil)
+      (assert (coroutine-alive? co))
+      (coroutine-resume co nil)
+      (assert= false (coroutine-alive? co))))
+  (deftest
+    "three yields then return"
+    (let
+      ((co (make-coroutine (fn () (coroutine-yield "a") (coroutine-yield "b") (coroutine-yield "c") "z"))))
+      (let
+        ((r1 (coroutine-resume co nil)))
+        (let
+          ((r2 (coroutine-resume co nil)))
+          (let
+            ((r3 (coroutine-resume co nil)))
+            (let
+              ((r4 (coroutine-resume co nil)))
+              (assert= "a" (get r1 "value"))
+              (assert= false (get r1 "done"))
+              (assert= "b" (get r2 "value"))
+              (assert= false (get r2 "done"))
+              (assert= "c" (get r3 "value"))
+              (assert= false (get r3 "done"))
+              (assert= "z" (get r4 "value"))
+              (assert= true (get r4 "done"))))))))
+  (deftest
+    "final return vs yield — done flag distinguishes them"
+    (let
+      ((co (make-coroutine (fn () (coroutine-yield "yielded") "returned"))))
+      (let
+        ((y (coroutine-resume co nil)))
+        (let
+          ((r (coroutine-resume co nil)))
+          (assert= false (get y "done"))
+          (assert= "yielded" (get y "value"))
+          (assert= true (get r "done"))
+          (assert= "returned" (get r "value"))))))
+  (deftest
+    "resume val becomes yield return value"
+    (let
+      ((co (make-coroutine (fn () (let ((received (coroutine-yield "first"))) received)))))
+      (let
+        ((r1 (coroutine-resume co nil)))
+        (let
+          ((r2 (coroutine-resume co 99)))
+          (assert= "first" (get r1 "value"))
+          (assert= false (get r1 "done"))
+          (assert= 99 (get r2 "value"))
+          (assert= true (get r2 "done"))))))
+  (deftest
+    "multiple resume values passed through yields"
+    (let
+      ((co (make-coroutine (fn () (let ((a (coroutine-yield 1))) (let ((b (coroutine-yield 2))) (+ a b)))))))
+      (let
+        ((r1 (coroutine-resume co nil)))
+        (let
+          ((r2 (coroutine-resume co 10)))
+          (let
+            ((r3 (coroutine-resume co 20)))
+            (assert= 1 (get r1 "value"))
+            (assert= 2 (get r2 "value"))
+            (assert= true (get r3 "done"))
+            (assert= 30 (get r3 "value")))))))
+  (deftest
+    "coroutine captures lexical environment"
+    (let
+      ((x 10)
+        (co
+          (make-coroutine
+            (fn () (coroutine-yield (* x 2)) (* x 3)))))
+      (let
+        ((r1 (coroutine-resume co nil)))
+        (let
+          ((r2 (coroutine-resume co nil)))
+          (assert= 20 (get r1 "value"))
+          (assert= 30 (get r2 "value"))))))
+  (deftest
+    "resuming dead coroutine raises error"
+    (let
+      ((co (make-coroutine (fn () nil))))
+      (coroutine-resume co nil)
+      (assert-throws (fn () (coroutine-resume co nil)))))
+  (deftest
+    "coroutine drives iteration via recursive body"
+    (let
+      ((co (make-coroutine (fn () (define loop (fn (i) (when (< i 4) (coroutine-yield i) (loop (+ i 1))))) (loop 0))))
+        (results (list)))
+      (let
+        drive
+        ()
+        (let
+          ((r (coroutine-resume co nil)))
+          (when
+            (not (get r "done"))
+            (append! results (get r "value"))
+            (drive))))
+      (assert= 4 (len results))
+      (assert= 0 (nth results 0))
+      (assert= 1 (nth results 1))
+      (assert= 2 (nth results 2))
+      (assert= 3 (nth results 3))))
+  (deftest
+    "nested coroutine — inner resumed from outer body"
+    (let
+      ((inner (make-coroutine (fn () (coroutine-yield "inner-a") "inner-done")))
+        (outer
+          (make-coroutine
+            (fn
+              ()
+              (let
+                ((i1 (coroutine-resume inner nil)))
+                (coroutine-yield (get i1 "value")))
+              (let ((i2 (coroutine-resume inner nil))) (get i2 "value"))))))
+      (let
+        ((o1 (coroutine-resume outer nil)))
+        (let
+          ((o2 (coroutine-resume outer nil)))
+          (assert= false (get o1 "done"))
+          (assert= "inner-a" (get o1 "value"))
+          (assert= true (get o2 "done"))
+          (assert= "inner-done" (get o2 "value"))))))
+  (deftest
+    "two independent coroutines interleave correctly"
+    (let
+      ((co1 (make-coroutine (fn () (coroutine-yield 1) 5)))
+        (co2
+          (make-coroutine (fn () (coroutine-yield 2) 6))))
+      (let
+        ((a (coroutine-resume co1 nil)))
+        (let
+          ((b (coroutine-resume co2 nil)))
+          (let
+            ((c (coroutine-resume co1 nil)))
+            (let
+              ((d (coroutine-resume co2 nil)))
+              (assert= false (get a "done"))
+              (assert= 1 (get a "value"))
+              (assert= false (get b "done"))
+              (assert= 2 (get b "value"))
+              (assert= true (get c "done"))
+              (assert= 5 (get c "value"))
+              (assert= true (get d "done"))
+              (assert= 6 (get d "value")))))))))