diff --git a/src/parse/parser-driver.cc b/src/parse/parser-driver.cc
index a603b9c..35ab474 100644
--- a/src/parse/parser-driver.cc
+++ b/src/parse/parser-driver.cc
@@ -2,7 +2,9 @@
 
 namespace abacus::parse {
 
-ParserDriver::ParserDriver() = default;
+ParserDriver::ParserDriver()
+    : parse_trace_p_(std::getenv("PARSE")), scan_trace_p_(std::getenv("SCAN")) {
+}
 
 int ParserDriver::parse(std::string filename) {
     filename_ = std::move(filename);
diff --git a/src/parse/parser-driver.hh b/src/parse/parser-driver.hh
index f909674..d5ba7ea 100644
--- a/src/parse/parser-driver.hh
+++ b/src/parse/parser-driver.hh
@@ -29,8 +29,8 @@ private:
     numeric_type result_{0};
     std::string filename_{};
     yy::location current_location_{};
-    bool parse_trace_p_ = false;
-    bool scan_trace_p_ = false;
+    bool parse_trace_p_;
+    bool scan_trace_p_;
 };
 
 } // namespace abacus::parse