break() primop; step and go debug commands

src/libcmd/repl.cc

@@ -439,7 +439,11 @@ bool NixRepl::processLine(string line)
              << "  :d <cmd>      Debug mode commands\n"
              << "  :d stack      Show call stack\n"
              << "  :d env        Show env stack\n"
-             << "  :d error      Show current error\n";
+             << "  :d error      Show current error\n"
+             << "  :d go         Go until end of program, exception, or builtins.break().\n"
+             << "  :d step       Go one step\n"
+             ;
+
     }
 
     else if (command == ":d" || command == ":debug") {
@@ -476,6 +480,16 @@ bool NixRepl::processLine(string line)
                 notice("error information not available");
             }
         }
+        else if (arg == "step") {
+            // set flag and exit repl.
+            state->debugStop = true;
+            return false;
+        }
+        else if (arg == "go") {
+            // set flag and exit repl.
+            state->debugStop = false;
+            return false;
+        }
     }
 
     else if (command == ":a" || command == ":add") {

src/libexpr/eval.cc

@@ -417,6 +417,7 @@ EvalState::EvalState(
     , repair(NoRepair)
     , store(store)
     , buildStore(buildStore ? buildStore : store)
+    , debugStop(true)
     , regexCache(makeRegexCache())
     , baseEnv(allocEnv(128))
     , staticBaseEnv(new StaticEnv(false, 0))
@@ -930,7 +931,7 @@ DebugTraceStacker::DebugTraceStacker(EvalState &evalState, DebugTrace t)
 :evalState(evalState), trace(t)
 {
     evalState.debugTraces.push_front(t);
-    if (debuggerHook)
+    if (evalState.debugStop && debuggerHook)
         debuggerHook(0, t.env, t.expr);
 }
 

src/libexpr/eval.hh

@@ -115,6 +115,7 @@ public:
     RootValue vCallFlake = nullptr;
     RootValue vImportedDrvToDerivation = nullptr;
 
+    bool debugStop;
     std::list<DebugTrace> debugTraces;
 
 private:

src/libexpr/primops.cc

@@ -710,6 +710,28 @@ static RegisterPrimOp primop_genericClosure(RegisterPrimOp::Info {
     .fun = prim_genericClosure,
 });
 
+static RegisterPrimOp primop_break({
+    .name = "break",
+    .args = {},
+    .doc = R"(
+      In debug mode, pause Nix expression evaluation and enter the repl.
+    )",
+    .fun = [](EvalState & state, const Pos & pos, Value * * args, Value & v)
+    {
+        // PathSet context;
+        // string s = state.coerceToString(pos, *args[0], context);
+        if (debuggerHook && !state.debugTraces.empty())
+        {
+          auto &dt = state.debugTraces.front();
+          // std::optional<ErrPos> pos;
+          // const Expr &expr;
+          // const Env &env;
+          // hintformat hint;
+          debuggerHook(nullptr, dt.env, dt.expr);
+        }
+    }
+});
+
 static RegisterPrimOp primop_abort({
     .name = "abort",
     .args = {"s"},