quit repl from step mode

src/libexpr/eval.cc

@@ -438,6 +438,7 @@ EvalState::EvalState(
     , store(store)
     , buildStore(buildStore ? buildStore : store)
     , debugStop(false)
+    , debugQuit(false)
     , regexCache(makeRegexCache())
 #if HAVE_BOEHMGC
     , valueAllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))

src/libexpr/eval.hh

@@ -116,6 +116,7 @@ public:
     RootValue vImportedDrvToDerivation = nullptr;
 
     bool debugStop;
+    bool debugQuit;
     std::list<DebugTrace> debugTraces;
 
     void debug_throw(Error e);

src/libexpr/primops.cc

@@ -718,6 +718,15 @@ static RegisterPrimOp primop_break({
           auto &dt = state.debugTraces.front();
           debuggerHook(&error, dt.env, dt.expr);
 
+          if (state.debugQuit) {
+              // if the user elects to quit the repl, throw an exception.
+              throw Error(ErrorInfo{
+                  .level = lvlInfo,
+                  .msg = hintfmt("quit from debugger"),
+                  .errPos = pos,
+              });
+          }
+
           // returning the value we were passed.
           v = *args[0];
         }