libexpr: refactor fallibly doing stuff on debug traces (NFC)

Change-Id: I009ce2ea424938507223fc6b3cf3b1236a6a6964

lix/libexpr/eval.cc

@@ -2924,6 +2924,15 @@ Expr & Evaluator::parseStdin()
     );
 }
 
+std::optional<DebugTrace const *> Evaluator::nextDebugTrace() const
+{
+    if (!debug) {
+        return std::nullopt;
+    }
+
+    return debug->traces().next();
+}
+
 
 kj::Promise<Result<EvalPaths::PathResult<SourcePath, ThrownError>>>
 EvalPaths::findFile(const std::string_view path)

lix/libexpr/eval.hh

@@ -568,6 +568,9 @@ public:
      */
     void evalLazily(Expr & e, Value & v);
 
+    /** If debugging is enabled, returns the next trace. Otherwise, std::nullopt. */
+    std::optional<DebugTrace const *> nextDebugTrace() const;
+
 private:
     Expr * parse(
         char * text,

lix/libexpr/primops.cc

@@ -629,7 +629,7 @@ static void prim_genericClosure(EvalState & state, Value * * args, Value & v)
 
 static void prim_break(EvalState & state, Value * * args, Value & v)
 {
-    if (auto trace = state.ctx.debug ? state.ctx.debug->traces().next() : std::nullopt) {
+    if (auto const trace = state.ctx.nextDebugTrace()) {
         auto error = EvalError(ErrorInfo {
             .level = lvlInfo,
             .msg = HintFmt("breakpoint reached"),
@@ -758,12 +758,13 @@ static void prim_trace(EvalState & state, Value * * args, Value & v)
         printError("trace: %1%", Uncolored(args[0]->str()));
     else
         printError("trace: %1%", Uncolored(ValuePrinter(state, *args[0])));
-    if (auto last = evalSettings.builtinsTraceDebugger && state.ctx.debug
-            ? state.ctx.debug->traces().next()
-            : std::nullopt)
-    {
-        state.ctx.debug->onEvalError(nullptr, (*last)->env, (*last)->expr);
+
+    if (evalSettings.debuggerOnTrace) {
+        if (auto const trace = state.ctx.nextDebugTrace()) {
+            state.ctx.debug->onEvalError(nullptr, (*trace)->env, (*trace)->expr);
+        }
     }
+
     state.forceValue(*args[1], noPos);
     v = *args[1];
 }

lix/libexpr/settings/debugger-on-trace.md

@@ -1,6 +1,6 @@
 ---
 name: debugger-on-trace
-internalName: builtinsTraceDebugger
+internalName: debuggerOnTrace
 type: bool
 default: false
 ---