Add live CEK stepper island — interactive stepping debugger

A defisland that lets users type an SX expression, step through CEK
evaluation one transition at a time, and see C/E/K registers update
live. Demonstrates that cek-step is pure data->data.

- cek.sx geography: add ~geography/cek/demo-stepper island with
  source input, step/run/reset buttons, state display, step history
- platform_js.py: register CEK stepping primitives (make-cek-state,
  cek-step, cek-terminal?, cek-value, make-env, sx-serialize) so
  island code can access them

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/static/scripts/sx-browser.js

@@ -14,7 +14,7 @@
   // =========================================================================
 
   var NIL = Object.freeze({ _nil: true, toString: function() { return "nil"; } });
-  var SX_VERSION = "2026-03-14T10:44:25Z";
+  var SX_VERSION = "2026-03-14T13:46:35Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }
@@ -6547,6 +6547,16 @@ return (function() {
     return cekValue(state);
   };
 
+  // CEK stepping primitives — for debugger islands
+  PRIMITIVES["make-cek-state"] = makeCekState;
+  PRIMITIVES["cek-step"] = cekStep;
+  PRIMITIVES["cek-terminal?"] = cekTerminal_p;
+  PRIMITIVES["cek-value"] = cekValue;
+  PRIMITIVES["make-env"] = function() { return merge(PRIMITIVES); };
+  PRIMITIVES["sx-serialize"] = sxSerialize;
+  PRIMITIVES["lambda-name"] = lambdaName;
+  PRIMITIVES["callable?"] = isCallable;
+
 
   // =========================================================================
   // Async IO: Promise-aware rendering for client-side IO primitives

shared/sx/ref/platform_js.py

@@ -1508,6 +1508,16 @@ CEK_FIXUPS_JS = '''
     while (!cekTerminal_p(state)) { state = cekStep(state); }
     return cekValue(state);
   };
+
+  // CEK stepping primitives — for debugger islands
+  PRIMITIVES["make-cek-state"] = makeCekState;
+  PRIMITIVES["cek-step"] = cekStep;
+  PRIMITIVES["cek-terminal?"] = cekTerminal_p;
+  PRIMITIVES["cek-value"] = cekValue;
+  PRIMITIVES["make-env"] = function() { return merge(PRIMITIVES); };
+  PRIMITIVES["sx-serialize"] = sxSerialize;
+  PRIMITIVES["lambda-name"] = lambdaName;
+  PRIMITIVES["callable?"] = isCallable;
 '''
 
 

sx/sx/geography/cek.sx

@@ -148,6 +148,141 @@
         "lisp")))))
 
 
+;; ---------------------------------------------------------------------------
+;; CEK stepper: interactive stepping debugger
+;; ---------------------------------------------------------------------------
+
+(defisland ~geography/cek/demo-stepper (&key initial-expr)
+  (let ((source (signal (or initial-expr "(+ 1 (* 2 3))")))
+        (state (signal nil))
+        (steps (signal 0))
+        (history (signal (list)))
+        (error-msg (signal nil)))
+
+    ;; Parse and create initial CEK state
+    (define start-eval
+      (fn ()
+        (reset! error-msg nil)
+        (reset! history (list))
+        (reset! steps 0)
+        (let ((parsed (sx-parse (deref source))))
+          (if (empty? parsed)
+            (reset! error-msg "Parse error: empty expression")
+            (reset! state (make-cek-state (first parsed) (make-env) (list)))))))
+
+    ;; Single step
+    (define do-step
+      (fn ()
+        (when (and (deref state) (not (cek-terminal? (deref state))))
+          (let ((prev (deref state)))
+            (swap! history (fn (h) (append h (list prev))))
+            (swap! steps inc)
+            (reset! state (cek-step prev))))))
+
+    ;; Run to completion
+    (define do-run
+      (fn ()
+        (when (deref state)
+          (let run-loop ((n 0))
+            (when (and (not (cek-terminal? (deref state))) (< n 200))
+              (do-step)
+              (run-loop (+ n 1)))))))
+
+    ;; Reset
+    (define do-reset
+      (fn ()
+        (reset! state nil)
+        (reset! steps 0)
+        (reset! history (list))
+        (reset! error-msg nil)))
+
+    ;; Format control for display
+    (define fmt-control
+      (fn (s)
+        (if (nil? s) "\u2014"
+          (let ((c (get s "control")))
+            (if (nil? c) "\u2014"
+              (sx-serialize c))))))
+
+    ;; Format value
+    (define fmt-value
+      (fn (s)
+        (if (nil? s) "\u2014"
+          (let ((v (get s "value")))
+            (cond
+              (nil? v) "nil"
+              (callable? v) (str "\u03bb:" (or (lambda-name v) "fn"))
+              :else (sx-serialize v))))))
+
+    ;; Format kont
+    (define fmt-kont
+      (fn (s)
+        (if (nil? s) "\u2014"
+          (let ((k (get s "kont")))
+            (if (empty? k) "[]"
+              (str "[" (join " " (map (fn (f) (get f "type")) k)) "]"))))))
+
+    ;; Initialize on first render
+    (start-eval)
+
+    (div :class "space-y-4"
+      ;; Input
+      (div :class "flex gap-2 items-end"
+        (div :class "flex-1"
+          (label :class "text-xs text-stone-400 block mb-1" "Expression")
+          (input :type "text" :bind source
+                 :class "w-full px-3 py-1.5 rounded border border-stone-300 font-mono text-sm focus:outline-none focus:border-violet-400"
+                 :on-change (fn (e) (start-eval))))
+        (div :class "flex gap-1"
+          (button :on-click (fn (e) (start-eval))
+                  :class "px-3 py-1.5 rounded bg-stone-200 text-stone-700 text-sm hover:bg-stone-300" "Reset")
+          (button :on-click (fn (e) (do-step))
+                  :class "px-3 py-1.5 rounded bg-violet-500 text-white text-sm hover:bg-violet-600" "Step")
+          (button :on-click (fn (e) (do-run))
+                  :class "px-3 py-1.5 rounded bg-violet-700 text-white text-sm hover:bg-violet-800" "Run")))
+
+      ;; Error
+      (when (deref error-msg)
+        (div :class "text-red-600 text-sm" (deref error-msg)))
+
+      ;; Current state
+      (when (deref state)
+        (div :class "rounded border border-stone-200 bg-white p-3 font-mono text-sm space-y-1"
+          (div :class "flex gap-4"
+            (span :class "text-stone-400 w-16" "Step")
+            (span :class "font-bold" (deref steps)))
+          (div :class "flex gap-4"
+            (span :class "text-stone-400 w-16" "Phase")
+            (span :class (str "font-bold " (if (= (get (deref state) "phase") "eval") "text-blue-600" "text-green-600"))
+              (get (deref state) "phase")))
+          (div :class "flex gap-4"
+            (span :class "text-violet-500 w-16" "C")
+            (span (fmt-control (deref state))))
+          (div :class "flex gap-4"
+            (span :class "text-amber-600 w-16" "V")
+            (span (fmt-value (deref state))))
+          (div :class "flex gap-4"
+            (span :class "text-emerald-600 w-16" "K")
+            (span (fmt-kont (deref state))))
+          (when (cek-terminal? (deref state))
+            (div :class "mt-2 pt-2 border-t border-stone-200 text-stone-800 font-bold"
+              (str "Result: " (sx-serialize (cek-value (deref state))))))))
+
+      ;; Step history
+      (when (not (empty? (deref history)))
+        (div :class "rounded border border-stone-100 bg-stone-50 p-2"
+          (div :class "text-xs text-stone-400 mb-1" "History")
+          (div :class "space-y-0.5 font-mono text-xs max-h-48 overflow-y-auto"
+            (map-indexed (fn (i s)
+              (div :class "flex gap-2 text-stone-500"
+                (span :class "text-stone-300 w-6 text-right" (+ i 1))
+                (span :class (if (= (get s "phase") "eval") "text-blue-400" "text-green-400") (get s "phase"))
+                (span :class "text-violet-400 truncate" (fmt-control s))
+                (span :class "text-amber-400" (fmt-value s))
+                (span :class "text-emerald-400" (fmt-kont s))))
+              (deref history))))))))
+
+
 ;; ---------------------------------------------------------------------------
 ;; Demo page content
 ;; ---------------------------------------------------------------------------
@@ -157,7 +292,15 @@
 
     (~docs/section :title "What this demonstrates" :id "what"
       (p "These are " (strong "live islands") " evaluated by the CEK machine. Every " (code "eval-expr") " goes through " (code "cek-run") ". Every " (code "(deref sig)") " in an island creates a reactive DOM binding via continuation frames.")
-      (p "The CEK machine is defined in " (code "cek.sx") " (160 lines) and " (code "frames.sx") " (100 lines) — pure s-expressions, bootstrapped to both JavaScript and Python."))
+      (p "The CEK machine is defined in " (code "cek.sx") " and " (code "frames.sx") " — pure s-expressions, bootstrapped to both JavaScript and Python."))
+
+    (~docs/section :title "Stepper" :id "stepper"
+      (p "The CEK machine is pure data\u2192data. Each step takes a state dict and returns a new one. "
+         "Type an expression, click Step to advance one CEK transition.")
+      (~geography/cek/demo-stepper :initial-expr "(let ((x 10)) (+ x (* 2 3)))")
+      (~docs/code :code (highlight
+        "(defisland ~demo-stepper (&key initial-expr)\n  (let ((source (signal (or initial-expr \"(+ 1 2)\")))\n        (state (signal nil))\n        (steps (signal 0))\n        (history (signal (list))))\n    ;; Parse and create initial CEK state\n    (define start-eval (fn ()\n      (let ((parsed (sx-parse (deref source))))\n        (reset! state (make-cek-state (first parsed) (make-env) (list))))))\n    ;; Single step\n    (define do-step (fn ()\n      (when (and (deref state) (not (cek-terminal? (deref state))))\n        (swap! history (fn (h) (append h (list (deref state)))))\n        (swap! steps inc)\n        (reset! state (cek-step (deref state))))))\n    (start-eval)\n    (div\n      (input :type \"text\" :bind source :on-change (fn (e) (start-eval)))\n      (button :on-click (fn (e) (do-step)) \"Step\")\n      ;; Display C, V, K registers from (deref state)\n      (pre (str \"C: \" (sx-serialize (get (deref state) \"control\")))))))"
+        "lisp")))
 
     (~docs/section :title "1. Counter" :id "demo-counter"
       (p (code "(deref count)") " in text position creates a reactive text node. " (code "(deref doubled)") " is a computed that updates when count changes.")