spec: promises — delay/force/delay-force/make-promise/promise?

25 tests pass on OCaml (4357 total) and JS. Promise represented as
mutable dict {:_promise true :forced :thunk :value}; delay-force
adds :_iterative for chain-following semantics.

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

spec/evaluator.sx

@@ -2037,8 +2037,10 @@
         nil))))
 
 (define
-  values
-  (fn (&rest vs) (if (= (len vs) 1) (first vs) {:_values true :_list vs})))
+  sf-delay
+  (fn
+    (args env)
+    (let ((thunk (make-lambda (list) (first args) env))) {:forced false :value nil :thunk thunk :_promise true})))
 
 ;; ═══════════════════════════════════════════════════════════════
 ;; Part 11: Entry Points
@@ -2047,6 +2049,42 @@
 ;; eval-expr / trampoline: top-level bindings that override the
 ;; forward declarations from Part 5.
 ;; ═══════════════════════════════════════════════════════════════
+(define
+  sf-delay-force
+  (fn
+    (args env)
+    (let ((thunk (make-lambda (list) (first args) env))) {:_iterative true :forced false :value nil :thunk thunk :_promise true})))
+
+(define promise? (fn (v) (and (dict? v) (get v :_promise false))))
+
+(define make-promise (fn (v) {:forced true :value v :_promise true}))
+
+(define
+  force
+  (fn
+    (p)
+    (if
+      (not (promise? p))
+      p
+      (if
+        (get p :forced false)
+        (get p :value nil)
+        (let
+          ((result (apply (get p :thunk nil) (list))))
+          (let
+            ((final (if (and (get p :_iterative false) (promise? result)) (force result) result)))
+            (dict-set! p :forced true)
+            (dict-set! p :value final)
+            final))))))
+
+(register-special-form! "delay" sf-delay)
+
+(register-special-form! "delay-force" sf-delay-force)
+
+(define
+  values
+  (fn (&rest vs) (if (= (len vs) 1) (first vs) {:_values true :_list vs})))
+
 (register-special-form! "define-type" sf-define-type)
 
 (define

spec/tests/test-promises.sx

@@ -0,0 +1,150 @@
+(defsuite
+  "promises"
+  (deftest
+    "delay creates a promise"
+    (do (assert (promise? (delay 42)))))
+  (deftest
+    "delay does not evaluate immediately"
+    (do
+      (let
+        ((count 0))
+        (let
+          ((p (delay (do (set! count (+ count 1)) count))))
+          (assert= 0 count)))))
+  (deftest
+    "force evaluates the expression"
+    (do (assert= 42 (force (delay 42)))))
+  (deftest
+    "force with arithmetic"
+    (do (assert= 10 (force (delay (+ 3 7))))))
+  (deftest
+    "force memoises result"
+    (do
+      (let
+        ((count 0))
+        (let
+          ((p (delay (do (set! count (+ count 1)) count))))
+          (force p)
+          (force p)
+          (assert= 1 count)))))
+  (deftest
+    "force returns same value on repeated calls"
+    (do
+      (let
+        ((p (delay (+ 1 2))))
+        (assert= 3 (force p))
+        (assert= 3 (force p)))))
+  (deftest
+    "make-promise creates an already-forced promise"
+    (do
+      (let
+        ((p (make-promise 99)))
+        (assert (promise? p))
+        (assert= 99 (force p)))))
+  (deftest
+    "make-promise memoises without evaluating"
+    (do
+      (let
+        ((count 0))
+        (let
+          ((p (make-promise 42)))
+          (force p)
+          (force p)
+          (assert= 0 count)))))
+  (deftest
+    "promise? returns true for delay"
+    (do (assert (promise? (delay 1)))))
+  (deftest
+    "promise? returns true for make-promise"
+    (do (assert (promise? (make-promise 1)))))
+  (deftest
+    "promise? returns false for non-promise"
+    (do
+      (assert= false (promise? 42))
+      (assert= false (promise? "hello"))
+      (assert= false (promise? nil))
+      (assert= false (promise? (list 1 2)))))
+  (deftest
+    "force non-promise returns value unchanged"
+    (do
+      (assert= 42 (force 42))
+      (assert= "hi" (force "hi"))
+      (assert= nil (force nil))))
+  (deftest
+    "delay captures environment"
+    (do
+      (let
+        ((x 10))
+        (let
+          ((p (delay (+ x 5))))
+          (assert= 15 (force p))))))
+  (deftest
+    "delay-force basic"
+    (do (assert= 42 (force (delay-force (delay 42))))))
+  (deftest
+    "delay-force chains"
+    (do
+      (assert=
+        5
+        (force (delay-force (delay-force (delay 5)))))))
+  (deftest
+    "delay with string"
+    (do (assert= "hello" (force (delay "hello")))))
+  (deftest
+    "delay with list"
+    (do
+      (assert-equal
+        (list 1 2 3)
+        (force (delay (list 1 2 3))))))
+  (deftest
+    "delay with function call"
+    (do (assert= 6 (force (delay (* 2 3))))))
+  (deftest
+    "nested delay"
+    (do
+      (let
+        ((p (delay (delay 99))))
+        (assert (promise? (force p))))))
+  (deftest
+    "force already forced promise"
+    (do
+      (let
+        ((p (make-promise 7)))
+        (assert= 7 (force p))
+        (assert= 7 (force p)))))
+  (deftest
+    "lazy stream first element"
+    (do
+      (define (stream-cons x s) (delay (list x s)))
+      (define (stream-car s) (first (force s)))
+      (define (stream-cdr s) (nth (force s) 1))
+      (let
+        ((s (stream-cons 1 (stream-cons 2 (stream-cons 3 nil)))))
+        (assert= 1 (stream-car s))
+        (assert= 2 (stream-car (stream-cdr s))))))
+  (deftest
+    "delay-force is a promise"
+    (do (assert (promise? (delay-force (delay 1))))))
+  (deftest
+    "force with side effects runs once"
+    (do
+      (let
+        ((log (list)))
+        (let
+          ((p (delay (do (set! log (cons 42 log)) 42))))
+          (force p)
+          (force p)
+          (assert= 1 (len log))))))
+  (deftest
+    "make-promise with nil"
+    (do
+      (let
+        ((p (make-promise nil)))
+        (assert (promise? p))
+        (assert= nil (force p)))))
+  (deftest
+    "delay in let binding"
+    (do
+      (let
+        ((p (delay (+ 10 20))))
+        (assert= 30 (force p))))))