Data-first HO forms, fix plan pages, aser error handling (1080/1080)

Evaluator: data-first higher-order forms — ho-swap-args auto-detects
(map coll fn) vs (map fn coll), both work. Threading + HO: (-> data
(map fn)) dispatches through CEK HO machinery via quoted-value splice.
17 new tests in test-cek-advanced.sx.

Fix plan pages: add mother-language, isolated-evaluator, rust-wasm-host
to page-functions.sx plan() — were in defpage but missing from URL router.

Aser error handling: pages.py now catches EvalError separately, renders
visible error banner instead of silently sending empty content. All
except blocks include traceback in logs.

Scope primitives: register collect!/collected/clear-collected!/emitted/
emit!/context in shared/sx/primitives.py so hand-written _aser can
resolve them (fixes ~cssx/flush expansion failure).

New test file: shared/sx/tests/test_aser_errors.py — 19 pytest tests
for error propagation through all aser control flow forms.

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 = "2026-03-15T16:12:31Z";
+  var SX_VERSION = "2026-03-15T17:07:09Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }
@@ -1628,32 +1628,55 @@ PRIMITIVES["reactive-shift-deref"] = reactiveShiftDeref;
 })(); };
 PRIMITIVES["step-eval-call"] = stepEvalCall;
 
+  // ho-form-name?
+  var hoFormName_p = function(name) { return sxOr((name == "map"), (name == "map-indexed"), (name == "filter"), (name == "reduce"), (name == "some"), (name == "every?"), (name == "for-each")); };
+PRIMITIVES["ho-form-name?"] = hoFormName_p;
+
+  // ho-fn?
+  var hoFn_p = function(v) { return sxOr(isCallable(v), isLambda(v)); };
+PRIMITIVES["ho-fn?"] = hoFn_p;
+
+  // ho-swap-args
+  var hoSwapArgs = function(hoType, evaled) { return (isSxTruthy((hoType == "reduce")) ? (function() {
+  var a = first(evaled);
+  var b = nth(evaled, 1);
+  return (isSxTruthy((isSxTruthy(!isSxTruthy(hoFn_p(a))) && hoFn_p(b))) ? [b, nth(evaled, 2), a] : evaled);
+})() : (function() {
+  var a = first(evaled);
+  var b = nth(evaled, 1);
+  return (isSxTruthy((isSxTruthy(!isSxTruthy(hoFn_p(a))) && hoFn_p(b))) ? [b, a] : evaled);
+})()); };
+PRIMITIVES["ho-swap-args"] = hoSwapArgs;
+
   // ho-setup-dispatch
   var hoSetupDispatch = function(hoType, evaled, env, kont) { return (function() {
-  var f = first(evaled);
+  var ordered = hoSwapArgs(hoType, evaled);
+  return (function() {
+  var f = first(ordered);
   return (isSxTruthy((hoType == "map")) ? (function() {
-  var coll = nth(evaled, 1);
+  var coll = nth(ordered, 1);
   return (isSxTruthy(isEmpty(coll)) ? makeCekValue([], env, kont) : continueWithCall(f, [first(coll)], env, [], kontPush(makeMapFrame(f, rest(coll), [], env), kont)));
 })() : (isSxTruthy((hoType == "map-indexed")) ? (function() {
-  var coll = nth(evaled, 1);
+  var coll = nth(ordered, 1);
   return (isSxTruthy(isEmpty(coll)) ? makeCekValue([], env, kont) : continueWithCall(f, [0, first(coll)], env, [], kontPush(makeMapIndexedFrame(f, rest(coll), [], env), kont)));
 })() : (isSxTruthy((hoType == "filter")) ? (function() {
-  var coll = nth(evaled, 1);
+  var coll = nth(ordered, 1);
   return (isSxTruthy(isEmpty(coll)) ? makeCekValue([], env, kont) : continueWithCall(f, [first(coll)], env, [], kontPush(makeFilterFrame(f, rest(coll), [], first(coll), env), kont)));
 })() : (isSxTruthy((hoType == "reduce")) ? (function() {
-  var init = nth(evaled, 1);
-  var coll = nth(evaled, 2);
+  var init = nth(ordered, 1);
+  var coll = nth(ordered, 2);
   return (isSxTruthy(isEmpty(coll)) ? makeCekValue(init, env, kont) : continueWithCall(f, [init, first(coll)], env, [], kontPush(makeReduceFrame(f, rest(coll), env), kont)));
 })() : (isSxTruthy((hoType == "some")) ? (function() {
-  var coll = nth(evaled, 1);
+  var coll = nth(ordered, 1);
   return (isSxTruthy(isEmpty(coll)) ? makeCekValue(false, env, kont) : continueWithCall(f, [first(coll)], env, [], kontPush(makeSomeFrame(f, rest(coll), env), kont)));
 })() : (isSxTruthy((hoType == "every")) ? (function() {
-  var coll = nth(evaled, 1);
+  var coll = nth(ordered, 1);
   return (isSxTruthy(isEmpty(coll)) ? makeCekValue(true, env, kont) : continueWithCall(f, [first(coll)], env, [], kontPush(makeEveryFrame(f, rest(coll), env), kont)));
 })() : (isSxTruthy((hoType == "for-each")) ? (function() {
-  var coll = nth(evaled, 1);
+  var coll = nth(ordered, 1);
   return (isSxTruthy(isEmpty(coll)) ? makeCekValue(NIL, env, kont) : continueWithCall(f, [first(coll)], env, [], kontPush(makeForEachFrame(f, rest(coll), env), kont)));
 })() : error((String("Unknown HO type: ") + String(hoType))))))))));
+})();
 })(); };
 PRIMITIVES["ho-setup-dispatch"] = hoSetupDispatch;
 
@@ -1771,7 +1794,8 @@ PRIMITIVES["step-ho-for-each"] = stepHoForEach;
   return (isSxTruthy(isEmpty(remaining)) ? makeCekValue(value, fenv, restK) : (function() {
   var form = first(remaining);
   var restForms = rest(remaining);
-  return (function() {
+  var newKont = (isSxTruthy(isEmpty(rest(remaining))) ? restK : kontPush(makeThreadFrame(rest(remaining), fenv), restK));
+  return (isSxTruthy((isSxTruthy((typeOf(form) == "list")) && isSxTruthy(!isSxTruthy(isEmpty(form))) && isSxTruthy((typeOf(first(form)) == "symbol")) && hoFormName_p(symbolName(first(form))))) ? makeCekState(cons(first(form), cons([new Symbol("quote"), value], rest(form))), fenv, newKont) : (function() {
   var result = (isSxTruthy((typeOf(form) == "list")) ? (function() {
   var f = trampoline(evalExpr(first(form), fenv));
   var rargs = map(function(a) { return trampoline(evalExpr(a, fenv)); }, rest(form));
@@ -1782,7 +1806,7 @@ PRIMITIVES["step-ho-for-each"] = stepHoForEach;
   return (isSxTruthy((isSxTruthy(isCallable(f)) && !isSxTruthy(isLambda(f)))) ? f(value) : (isSxTruthy(isLambda(f)) ? trampoline(callLambda(f, [value], fenv)) : error((String("-> form not callable: ") + String(inspect(f))))));
 })());
   return (isSxTruthy(isEmpty(restForms)) ? makeCekValue(result, fenv, restK) : makeCekValue(result, fenv, kontPush(makeThreadFrame(restForms, fenv), restK)));
-})();
+})());
 })());
 })() : (isSxTruthy((ft == "arg")) ? (function() {
   var f = get(frame, "f");

shared/sx/pages.py

@@ -23,11 +23,28 @@ import logging
 import os
 from typing import Any
 
-from .types import PageDef
+import traceback
+
+from .types import EvalError, PageDef
 
 logger = logging.getLogger("sx.pages")
 
 
+def _eval_error_sx(e: EvalError, context: str) -> str:
+    """Render an EvalError as SX content that's visible to the developer."""
+    from .ref.sx_ref import escape_html as _esc
+    msg = _esc(str(e))
+    ctx = _esc(context)
+    return (
+        f'(div :class "sx-eval-error" :style '
+        f'"background:#fef2f2;border:1px solid #fca5a5;'
+        f'color:#991b1b;padding:1rem;margin:1rem 0;'
+        f'border-radius:0.5rem;font-family:monospace;white-space:pre-wrap"'
+        f' (p :style "font-weight:700;margin:0 0 0.5rem" "SX EvalError in {ctx}")'
+        f' (p :style "margin:0" "{msg}"))'
+    )
+
+
 # ---------------------------------------------------------------------------
 # Registry — service → page-name → PageDef
 # ---------------------------------------------------------------------------
@@ -511,8 +528,12 @@ async def execute_page_streaming(
             aside_sx = await _eval_slot(page_def.aside_expr, data_env, ctx) if page_def.aside_expr else ""
             menu_sx = await _eval_slot(page_def.menu_expr, data_env, ctx) if page_def.menu_expr else ""
             await _stream_queue.put(("data-single", content_sx, filter_sx, aside_sx, menu_sx))
+        except EvalError as e:
+            logger.error("Streaming data task failed (EvalError): %s\n%s", e, traceback.format_exc())
+            error_sx = _eval_error_sx(e, "page content")
+            await _stream_queue.put(("data-single", error_sx, "", "", ""))
         except Exception as e:
-            logger.error("Streaming data task failed: %s", e)
+            logger.error("Streaming data task failed: %s\n%s", e, traceback.format_exc())
             await _stream_queue.put(("data-done",))
 
     async def _eval_headers():
@@ -524,7 +545,7 @@ async def execute_page_streaming(
             menu = await layout.mobile_menu(tctx, **layout_kwargs)
             await _stream_queue.put(("headers", rows, menu))
         except Exception as e:
-            logger.error("Streaming headers task failed: %s", e)
+            logger.error("Streaming headers task failed: %s\n%s", e, traceback.format_exc())
             await _stream_queue.put(("headers", "", ""))
 
     data_task = asyncio.create_task(_eval_data_and_content())
@@ -629,7 +650,7 @@ async def execute_page_streaming(
                 elif kind == "data-done":
                     remaining -= 1
             except Exception as e:
-                logger.error("Streaming resolve failed for %s: %s", kind, e)
+                logger.error("Streaming resolve failed for %s: %s\n%s", kind, e, traceback.format_exc())
 
         yield "\n</body>\n</html>"
 
@@ -733,8 +754,13 @@ async def execute_page_streaming_oob(
                     await _stream_queue.put(("data-done",))
                     return
             await _stream_queue.put(("data-done",))
+        except EvalError as e:
+            logger.error("Streaming OOB data task failed (EvalError): %s\n%s", e, traceback.format_exc())
+            error_sx = _eval_error_sx(e, "page content")
+            await _stream_queue.put(("data", "stream-content", error_sx))
+            await _stream_queue.put(("data-done",))
         except Exception as e:
-            logger.error("Streaming OOB data task failed: %s", e)
+            logger.error("Streaming OOB data task failed: %s\n%s", e, traceback.format_exc())
             await _stream_queue.put(("data-done",))
 
     async def _eval_oob_headers():
@@ -745,7 +771,7 @@ async def execute_page_streaming_oob(
             else:
                 await _stream_queue.put(("headers", ""))
         except Exception as e:
-            logger.error("Streaming OOB headers task failed: %s", e)
+            logger.error("Streaming OOB headers task failed: %s\n%s", e, traceback.format_exc())
             await _stream_queue.put(("headers", ""))
 
     data_task = asyncio.create_task(_eval_data())
@@ -836,7 +862,7 @@ async def execute_page_streaming_oob(
                 elif kind == "data-done":
                     remaining -= 1
             except Exception as e:
-                logger.error("Streaming OOB resolve failed for %s: %s", kind, e)
+                logger.error("Streaming OOB resolve failed for %s: %s\n%s", kind, e, traceback.format_exc())
 
     return _stream_oob_chunks()
 

shared/sx/primitives.py

@@ -573,3 +573,32 @@ def prim_json_encode(value) -> str:
     import json
     return json.dumps(value, indent=2)
 
+
+# ---------------------------------------------------------------------------
+# Scope primitives — delegate to sx_ref.py's scope stack implementation
+# (shared global state between transpiled and hand-written evaluators)
+# ---------------------------------------------------------------------------
+
+def _lazy_scope_primitives():
+    """Register scope/provide/collect primitives from sx_ref.py.
+
+    Called at import time — if sx_ref.py isn't built yet, silently skip.
+    These are needed by the hand-written _aser in async_eval.py when
+    expanding components that use scoped effects (e.g. ~cssx/flush).
+    """
+    try:
+        from .ref.sx_ref import (
+            sx_collect, sx_collected, sx_clear_collected,
+            sx_emitted, sx_emit, sx_context,
+        )
+        _PRIMITIVES["collect!"] = sx_collect
+        _PRIMITIVES["collected"] = sx_collected
+        _PRIMITIVES["clear-collected!"] = sx_clear_collected
+        _PRIMITIVES["emitted"] = sx_emitted
+        _PRIMITIVES["emit!"] = sx_emit
+        _PRIMITIVES["context"] = sx_context
+    except ImportError:
+        pass
+
+_lazy_scope_primitives()
+

shared/sx/tests/test_aser_errors.py

@@ -0,0 +1,245 @@
+"""Tests for aser (SX wire format) error propagation.
+
+Verifies that evaluation errors inside control flow forms (case, cond, if,
+when, let, begin) propagate correctly — they must throw, not silently
+produce wrong output or fall through to :else branches.
+
+This test file targets the production bug where a case body referencing an
+undefined symbol was silently swallowed, causing the case to appear to fall
+through to :else instead of raising an error.
+"""
+from __future__ import annotations
+
+import pytest
+
+from shared.sx.ref.sx_ref import (
+    aser,
+    sx_parse as parse_all,
+    make_env,
+    eval_expr,
+    trampoline,
+    serialize as sx_serialize,
+)
+from shared.sx.types import NIL, EvalError
+
+
+def _render_sx(source: str, env=None) -> str:
+    """Parse SX source and serialize via aser (sync)."""
+    if env is None:
+        env = make_env()
+    exprs = parse_all(source)
+    result = ""
+    for expr in exprs:
+        val = aser(expr, env)
+        if isinstance(val, str):
+            result += val
+        elif val is None or val is NIL:
+            pass
+        else:
+            result += sx_serialize(val)
+    return result
+
+
+# ---------------------------------------------------------------------------
+# Case — matched branch errors must throw, not fall through
+# ---------------------------------------------------------------------------
+
+class TestCaseErrorPropagation:
+    def test_matched_branch_undefined_symbol_throws(self):
+        """If the matched case body references an undefined symbol, the aser
+        must throw — NOT silently skip to :else."""
+        with pytest.raises(Exception, match="Undefined symbol"):
+            _render_sx('(case "x" "x" undefined_sym :else "fallback")')
+
+    def test_else_branch_error_throws(self):
+        with pytest.raises(Exception, match="Undefined symbol"):
+            _render_sx('(case "miss" "x" "ok" :else undefined_sym)')
+
+    def test_matched_branch_nested_error_throws(self):
+        """Error inside a tag within the matched body must propagate."""
+        with pytest.raises(Exception, match="Undefined symbol"):
+            _render_sx('(case "a" "a" (div (p undefined_nested)) :else (p "index"))')
+
+    def test_unmatched_correctly_falls_through(self):
+        """Verify :else works when no clause matches (happy path)."""
+        result = _render_sx('(case "miss" "x" "found" :else "fallback")')
+        assert "fallback" in result
+
+    def test_matched_branch_succeeds(self):
+        """Verify the happy path: matched branch evaluates normally."""
+        result = _render_sx('(case "ok" "ok" (p "matched") :else "fallback")')
+        assert "matched" in result
+
+
+# ---------------------------------------------------------------------------
+# Cond — matched branch errors must throw
+# ---------------------------------------------------------------------------
+
+class TestCondErrorPropagation:
+    def test_matched_branch_error_throws(self):
+        with pytest.raises(Exception, match="Undefined symbol"):
+            _render_sx('(cond true undefined_cond_sym :else "fallback")')
+
+    def test_else_branch_error_throws(self):
+        with pytest.raises(Exception, match="Undefined symbol"):
+            _render_sx('(cond false "skip" :else undefined_cond_sym)')
+
+
+# ---------------------------------------------------------------------------
+# If / When — body errors must throw
+# ---------------------------------------------------------------------------
+
+class TestIfWhenErrorPropagation:
+    def test_if_true_branch_error_throws(self):
+        with pytest.raises(Exception, match="Undefined symbol"):
+            _render_sx('(if true undefined_if_sym "fallback")')
+
+    def test_when_body_error_throws(self):
+        with pytest.raises(Exception, match="Undefined symbol"):
+            _render_sx('(when true undefined_when_sym)')
+
+
+# ---------------------------------------------------------------------------
+# Let — binding or body errors must throw
+# ---------------------------------------------------------------------------
+
+class TestLetErrorPropagation:
+    def test_binding_error_throws(self):
+        with pytest.raises(Exception, match="Undefined symbol"):
+            _render_sx('(let ((x undefined_let_sym)) (p x))')
+
+    def test_body_error_throws(self):
+        with pytest.raises(Exception, match="Undefined symbol"):
+            _render_sx('(let ((x 1)) (p undefined_let_body_sym))')
+
+
+# ---------------------------------------------------------------------------
+# Begin/Do — body errors must throw
+# ---------------------------------------------------------------------------
+
+class TestBeginErrorPropagation:
+    def test_do_body_error_throws(self):
+        with pytest.raises(Exception, match="Undefined symbol"):
+            _render_sx('(do "ok" undefined_do_sym)')
+
+
+# ---------------------------------------------------------------------------
+# Sync aser: components serialize WITHOUT expansion (by design)
+# ---------------------------------------------------------------------------
+
+class TestSyncAserComponentSerialization:
+    """The sync aser serializes component calls as SX wire format without
+    expanding the body. This is correct — expansion only happens in the
+    async path with expand_components=True."""
+
+    def test_component_in_case_serializes_without_expanding(self):
+        """Sync aser should serialize the component call, not expand it."""
+        result = _render_sx(
+            '(do (defcomp ~broken (&key title) (div (p title) (p no_such_helper)))'
+            '    (case "slug" "slug" (~broken :title "test") '
+            '                :else "index"))'
+        )
+        # Component call is serialized as SX, not expanded — no error
+        assert "~broken" in result
+
+    def test_working_component_in_case_serializes(self):
+        result = _render_sx(
+            '(do (defcomp ~working (&key title) (div (p title)))'
+            '    (case "ok" "ok" (~working :title "hello") '
+            '               :else "index"))'
+        )
+        assert "~working" in result
+
+    def test_unmatched_case_falls_through_correctly(self):
+        result = _render_sx(
+            '(do (defcomp ~page (&key x) (div x))'
+            '    (case "miss" "hit" (~page :x "found") '
+            '                :else "index"))'
+        )
+        assert "index" in result
+
+
+# ---------------------------------------------------------------------------
+# Async aser with expand_components=True — the production path
+# ---------------------------------------------------------------------------
+
+class TestAsyncAserComponentExpansion:
+    """Tests the production code path: async aser with component expansion
+    enabled. Errors in expanded component bodies must propagate, not be
+    silently swallowed."""
+
+    def _async_render(self, source: str) -> str:
+        """Render via the async aser with component expansion enabled."""
+        import asyncio
+        from shared.sx.ref.sx_ref import async_aser, _expand_components_cv
+        exprs = parse_all(source)
+        env = make_env()
+
+        async def run():
+            token = _expand_components_cv.set(True)
+            try:
+                result = ""
+                for expr in exprs:
+                    val = await async_aser(expr, env, None)
+                    if isinstance(val, str):
+                        result += val
+                    elif val is None or val is NIL:
+                        pass
+                    else:
+                        result += sx_serialize(val)
+                return result
+            finally:
+                _expand_components_cv.reset(token)
+
+        return asyncio.run(run())
+
+    def test_expanded_component_with_undefined_symbol_throws(self):
+        """When expand_components is True and the component body references
+        an undefined symbol, the error must propagate — not be swallowed."""
+        with pytest.raises(Exception, match="Undefined symbol"):
+            self._async_render(
+                '(do (defcomp ~broken (&key title) '
+                '      (div (p title) (p no_such_helper)))'
+                '    (case "slug" "slug" (~broken :title "test") '
+                '                :else "index"))'
+            )
+
+    def test_expanded_working_component_succeeds(self):
+        result = self._async_render(
+            '(do (defcomp ~working (&key title) (div (p title)))'
+            '    (case "ok" "ok" (~working :title "hello") '
+            '               :else "index"))'
+        )
+        assert "hello" in result
+
+    def test_expanded_unmatched_falls_through(self):
+        result = self._async_render(
+            '(do (defcomp ~page (&key x) (div x))'
+            '    (case "miss" "hit" (~page :x "found") '
+            '                :else "index"))'
+        )
+        assert "index" in result
+
+    def test_hand_written_aser_also_propagates(self):
+        """Test the hand-written _aser in async_eval.py (the production
+        path used by page rendering)."""
+        import asyncio
+        from shared.sx.async_eval import (
+            async_eval_slot_to_sx, RequestContext,
+        )
+        from shared.sx.ref.sx_ref import aser
+
+        env = make_env()
+        # Define the component via sync aser
+        for expr in parse_all(
+            '(defcomp ~broken (&key title) (div (p title) (p no_such_helper)))'
+        ):
+            aser(expr, env)
+
+        case_expr = parse_all(
+            '(case "slug" "slug" (~broken :title "test") :else "index")'
+        )[0]
+        ctx = RequestContext()
+
+        with pytest.raises(Exception, match="Undefined symbol"):
+            asyncio.run(async_eval_slot_to_sx(case_expr, dict(env), ctx))