Trace by default in REPL-driven development

A repl or nREPL session now turns tail-frame tracing on, so an uncaught error in
evaluated/reloaded code shows a tail-frame backtrace with no JOLT_TRACE set. The
REPL and nREPL catch errors themselves rather than going through the uncaught
reporter, so they now print the history backtrace via a new jolt.host/backtrace-
string (history-only — the live continuation in a REPL is just REPL machinery).

Because the recording is baked in at compile time, only code compiled while a
session is live is traced; reload a namespace to trace already-loaded code.
JOLT_TRACE=1 still forces it on for a whole run (a plain -M:run traces its own
load); JOLT_TRACE=0 forces it off even in a session.

No seed change — jolt.main/jolt.nrepl are runtime-loaded and compile-eval.ss /
source-registry.ss are host files.

docs/tools-deps.md

@@ -190,11 +190,21 @@ binary the frames map to `ns/name (file:line)`; on the runtime eval path they ar
 the surviving fn names. Tail-call optimization erases tail-called frames, so the
 default trace shows only the non-tail spine.
 
-Set `JOLT_TRACE=1` to opt into a fuller **tail-frame history**. Each compiled fn
-then records itself on entry into a bounded ring-of-rings buffer, so the trace
-recovers TCO-elided frames (including the immediate error site) while a tight tail
-loop stays bounded and its non-tail caller context is preserved. It costs a small
-per-call overhead, so it is off by default and never emitted into a built binary.
+A fuller **tail-frame history** recovers the frames TCO erases: each compiled fn
+records itself on entry into a bounded ring-of-rings buffer, so the trace shows
+TCO-elided frames (including the immediate error site) while a tight tail loop
+stays bounded and its non-tail caller context is preserved.
+
+It is **on by default in REPL-driven development** — a `repl` or nREPL session
+turns it on, so an error in code you evaluate or reload shows a tail-frame trace
+with no setup. Because the recording is baked in at compile time, only code
+compiled while a session is live is traced; reload a namespace to trace code that
+was already loaded (e.g. an app's initial `-M:run` load before its nREPL started).
+
+Elsewhere it is off (a small per-call cost, and never emitted into a `jolt build`
+binary). Override with the environment: `JOLT_TRACE=1` forces it on for a whole
+run — including a plain `-M:run`, so the app's own load is traced — and
+`JOLT_TRACE=0` forces it off, even in a REPL/nREPL session.
 
 ## Conformance
 

host/chez/compile-eval.ss

@@ -110,13 +110,24 @@
 ;; older seed during the first re-mint pass.
 (let ((scv (var-deref "jolt.backend-scheme" "set-var-cache!")))
   (when (procedure? scv) (scv #t)))
-;; JOLT_TRACE opts into tail-frame history: the emitter prepends a frame-recording
-;; push to every runtime-compiled fn, and the ring buffer is allocated for this
-;; thread. Off by default, so a normal run emits and pays exactly as before.
-(when (getenv "JOLT_TRACE")
-  (let ((stf (var-deref "jolt.backend-scheme" "set-trace-frames!")))
-    (when (procedure? stf) (stf #t)))
-  (jolt-trace-enable!))
+;; Tail-frame history. Turning it on makes the emitter add a per-fn history push to
+;; every fn compiled AFTERWARD, and allocates this thread's ring. Suppressed when
+;; JOLT_TRACE is explicitly falsey, so JOLT_TRACE=0 disables it even in dev mode.
+(define (jolt-enable-trace!)
+  (let ((e (getenv "JOLT_TRACE")))
+    (unless (and e (member e '("0" "false" "no")))
+      (let ((stf (var-deref "jolt.backend-scheme" "set-trace-frames!")))
+        (when (procedure? stf) (stf #t)))
+      (jolt-trace-enable!))))
+;; Exposed so the REPL / nREPL entrypoints (jolt.main, jolt.nrepl) can turn tracing
+;; on for REPL-driven development without the user setting JOLT_TRACE. Because the
+;; push is baked in at compile time, only code compiled after this call is traced —
+;; which is exactly the code you eval / reload in a live session.
+(def-var! "jolt.host" "enable-trace!" jolt-enable-trace!)
+;; Explicit opt-in for a whole run (JOLT_TRACE=1): enable at load, BEFORE any app
+;; namespace is compiled, so a plain `-M:run` traces the app's own code too.
+(let ((e (getenv "JOLT_TRACE")))
+  (when (and e (not (member e '("0" "false" "no" "")))) (jolt-enable-trace!)))
 
 ;; (with-meta sym m) -> sym, else x — an (ns ^:no-doc name …) yields the name with
 ;; reader metadata as a with-meta form; strip it to read the bare ns symbol.

host/chez/smoke.sh

@@ -224,5 +224,25 @@ else
   fails=$((fails + 1))
 fi
 
+# REPL-driven development traces by default: an error in an evaluated form shows a
+# tail-frame backtrace with no JOLT_TRACE set. rb tail-calls ra tail-calls +, all
+# TCO-elided from the continuation — only the history recovers them.
+repl_err="$(printf '(defn ra [x] (+ x 1))\n(defn rb [x] (ra x))\n(rb :nan)\n:exit\n' | bin/joltc repl 2>&1)"
+if printf '%s' "$repl_err" | grep -q '  trace:' && printf '%s' "$repl_err" | grep -q 'rb'; then
+  pass=$((pass + 1))
+else
+  echo "  FAIL: a REPL error should show a tail-frame trace by default"
+  printf '%s\n' "$repl_err" | sed 's/^/    | /'
+  fails=$((fails + 1))
+fi
+# JOLT_TRACE=0 opts out — no trace in the REPL.
+repl_off="$(printf '(defn ra [x] (+ x 1))\n(defn rb [x] (ra x))\n(rb :nan)\n:exit\n' | JOLT_TRACE=0 bin/joltc repl 2>&1)"
+if printf '%s' "$repl_off" | grep -q '  trace:'; then
+  echo "  FAIL: JOLT_TRACE=0 should suppress the REPL trace"
+  fails=$((fails + 1))
+else
+  pass=$((pass + 1))
+fi
+
 echo "cli smoke: $pass passed, $fails failed"
 [ "$fails" -eq 0 ]

host/chez/source-registry.ss

@@ -145,10 +145,11 @@
 ;;      TCO-truncated non-tail spine.
 ;; Each frame maps to "ns/name (file:line)" when registered, else its bare name.
 ;; #f when neither source yields a frame (the caller then prints just the location).
-(define (jolt-backtrace-string v)
+;; The tail-frame history ring rendered as a backtrace, or #f when tracing is off /
+;; empty. A mapped frame is kept; else drop plumbing (same rule as the continuation
+;; path) so the two sources read consistently.
+(define (jolt-history-backtrace)
   (let* ((hist (jolt-trace-snapshot))
-         ;; a mapped frame is always kept; else drop plumbing (same rule as the
-         ;; continuation path) so the two backtrace sources read consistently
          (recs (let loop ((ns hist) (acc '()))
                  (if (null? ns)
                      (reverse acc)
@@ -156,12 +157,23 @@
                        (loop (cdr ns)
                              (if (or src (not (srcreg-plumbing-name? nm)))
                                  (cons (cons nm src) acc) acc)))))))
-    (if (pair? recs)
-        (jolt-render-recs recs)
-        (let ((k (jolt-error-continuation v)))
-          (and k
-               (let ((recs (jolt-frame-records k)))
-                 (and (pair? recs) (jolt-render-recs recs))))))))
+    (and (pair? recs) (jolt-render-recs recs))))
+
+(define (jolt-backtrace-string v)
+  (or (jolt-history-backtrace)
+      (let ((k (jolt-error-continuation v)))
+        (and k
+             (let ((recs (jolt-frame-records k)))
+               (and (pair? recs) (jolt-render-recs recs)))))))
+
+;; Exposed for the REPL / nREPL error paths, which catch errors themselves instead
+;; of going through the uncaught reporter. Returns the "  trace:\n<frames>" block
+;; from the tail-frame HISTORY only — the live continuation in a REPL is just the
+;; REPL's own machinery — or nil when tracing is off (so a caller can when-let).
+(def-var! "jolt.host" "backtrace-string"
+  (lambda ()
+    (let ((bt (jolt-history-backtrace)))
+      (if bt (string-append "  trace:\n" bt) jolt-nil))))
 
 ;; Render an uncaught jolt throw (any value, not just a Chez condition) to a port:
 ;; an ex-info shows its message + ex-data (+ a host cause); anything else is

jolt-core/jolt/main.clj

@@ -147,6 +147,9 @@
   ;; loaded — same context a run gets, so (require '[some.lib]) works in the REPL.
   (try (apply-project! (deps/resolve-project (project-dir)))
        (catch :default _ nil))
+  ;; REPL-driven development: trace by default so an uncaught error in evaluated
+  ;; code shows a tail-frame backtrace, no JOLT_TRACE needed (JOLT_TRACE=0 opts out).
+  (jolt.host/enable-trace!)
   (println (str ";; jolt " (version) " repl — :repl/quit or ^D to exit"))
   (loop []
     (let [form (repl-read-form)]
@@ -160,7 +163,9 @@
                  (catch :default e
                    (println "error:" (or (ex-message e)
                                          (try ((resolve 'jolt.host/condition-message) e) (catch :default _ nil))
-                                         (pr-str e)))))
+                                         (pr-str e)))
+                   (when-let [bt (jolt.host/backtrace-string)]
+                     (print bt))))
             (recur)))))))
 
 ;; A deps.edn :tasks entry: a string is a shell command; a map is {:main-opts …}.

jolt-core/jolt/nrepl.clj

@@ -188,7 +188,10 @@
               (try (when (and ns-str (not (str/blank? ns-str)) (find-ns (symbol ns-str)))
                      (in-ns (symbol ns-str)))
                    (reset! result (load-string code))
-                   (catch :default e (reset! err (err-msg e)))))]
+                   (catch :default e
+                     (reset! err (str (err-msg e)
+                                      (when-let [bt (jolt.host/backtrace-string)]
+                                        (str "\n" bt)))))))]
     {:value (when (nil? @err) (pr-str @result))
      :out out
      :ns (str (ns-name *ns*))
@@ -277,6 +280,11 @@
   no-op."
   ([port] (start port nil))
   ([port middleware]
+   ;; An nREPL session is REPL-driven development: trace by default so an uncaught
+   ;; error in code evaluated over the connection shows a tail-frame backtrace, with
+   ;; no JOLT_TRACE needed. Covers both `--nrepl-server` and an app that starts its
+   ;; own server under `-M:run` (reload a namespace to trace already-loaded code).
+   (jolt.host/enable-trace!)
    (let [handler (build-handler (resolve-middleware (or middleware [])))
          fd (listen-socket port)                  ; throws on bind/listen failure
          stopped (atom false)]