Merge pull request #20 from jolt-lang/stage2-task3-deliberate-fallback

Stage 2 Task 3: interpreter fallback is deliberate-only

src/jolt/backend.janet

@@ -352,20 +352,36 @@
   # while it runs — so h/current-ns must read this instead of ctx-current-ns.
   (put (ctx :env) :compile-ns (ctx-current-ns ctx))
   (def av (ns-find (ctx-find-ns ctx "jolt.analyzer") "analyze"))
+  # Pre-kernel bootstrap: ensure-analyzer is gated until the kernel tier loads
+  # (see api/load-core-overlay!), so a compile request from an earlier tier (e.g.
+  # 00-syntax's destructure defn) finds no analyzer. That fallback is DESIGNED —
+  # route it through the sanctioned punt channel rather than crashing on a nil var.
+  (unless av (error "jolt/uncompilable: analyzer not built (pre-kernel bootstrap)"))
   (def r ((var-get av) ctx form))
   (put (ctx :env) :compile-ns nil)
   r)
 
+# The analyzer's deliberate punt signal — (uncompilable why) throws the string
+# "jolt/uncompilable: <why>". Anything else escaping the compile step is an
+# unexpected compiler error, not a punt.
+(defn- uncompilable-error? [err]
+  (and (or (string? err) (buffer? err))
+       (string/has-prefix? "jolt/uncompilable" (string err))))
+
 (defn compile-and-eval
   "Self-hosted compile path: analyze (portable Clojure) -> IR -> Janet -> eval.
-  Hybrid: only the compile step (analyze+emit) is guarded — a form the analyzer
-  can't handle throws and falls back to the interpreter; runtime errors in
-  compiled code propagate (no double-eval, no hidden errors)."
+  The interpreter fallback is DELIBERATE-ONLY (Stage 2): only an analyzer punt
+  (jolt/uncompilable — the curated stateful/letrec set) falls back; any other
+  compile-step error is a compiler bug and propagates rather than being silently
+  hidden by interpretation. Runtime errors in compiled code propagate as before
+  (no double-eval, no hidden errors)."
   [ctx form]
   (def compiled (protect (emit-ir ctx (analyze-form ctx form))))
   (if (compiled 0)
     (eval (compiled 1) (comp/ctx-janet-env ctx))
-    (eval-form ctx @{} form)))
+    (if (uncompilable-error? (compiled 1))
+      (eval-form ctx @{} form)
+      (error (compiled 1)))))
 
 (defn analyzer-built? [ctx]
   (> (length ((ctx-find-ns ctx "jolt.analyzer") :mappings)) 0))

test/unit/compile-fallback-test.janet

@@ -0,0 +1,41 @@
+# Stage 2 Task 3: the compile path's interpreter fallback is DELIBERATE-ONLY.
+#
+# compile-and-eval used to wrap the compile step in a blanket protect — ANY
+# failure (including a genuine compiler bug) silently fell back to the
+# interpreter, hiding the bug behind a correct-looking result. Now only the
+# analyzer's deliberate punt signal ("jolt/uncompilable: …", raised for the
+# curated stateful/letrec set) may fall back; any other compile-step error
+# propagates. Verified here by stubbing jolt.analyzer/analyze.
+
+(use ../../src/jolt/types)
+(use ../../src/jolt/api)
+(use ../../src/jolt/reader)
+(import ../../src/jolt/backend :as backend)
+
+(def ctx (init))
+
+# 1. A deliberate punt (letfn needs letrec IR) falls back and evaluates correctly.
+(assert (= 3 (backend/compile-and-eval ctx (parse-string "(letfn [(f [n] (+ n 1))] (f 2))")))
+        "deliberate uncompilable punt falls back to the interpreter")
+
+(assert (backend/analyzer-built? ctx) "analyzer built")
+(def analyze-var (ns-find (ctx-find-ns ctx "jolt.analyzer") "analyze"))
+(def real-analyze (var-get analyze-var))
+
+# 2. A NON-punt compile error must propagate — even though the interpreter could
+#    evaluate the form fine, it must NOT be silently used (that hides compiler bugs).
+(var-set analyze-var (fn [ctx form] (error "boom: simulated compiler bug")))
+(def r (protect (backend/compile-and-eval ctx (parse-string "(+ 1 2)"))))
+(assert (not (r 0)) "non-uncompilable compile error must propagate, not silently interpret")
+(assert (string/find "boom" (string (r 1))) "the original error is surfaced")
+
+# 3. The punt marker is the one sanctioned fallback channel.
+(var-set analyze-var (fn [ctx form] (error "jolt/uncompilable: stubbed")))
+(assert (= 3 (backend/compile-and-eval ctx (parse-string "(+ 1 2)")))
+        "uncompilable punt falls back to the interpreter")
+
+# Restore the real analyzer and confirm the pipeline still works.
+(var-set analyze-var real-analyze)
+(assert (= 7 (backend/compile-and-eval ctx (parse-string "(+ 3 4)"))) "restored analyzer compiles")
+
+(print "All compile-fallback tests passed!")