diff --git a/include/xo/reader/exprstate.hpp b/include/xo/reader/exprstate.hpp
index fb541cd9..e36972fa 100644
--- a/include/xo/reader/exprstate.hpp
+++ b/include/xo/reader/exprstate.hpp
@@ -63,7 +63,7 @@ namespace xo {
             return os;
         }
 
-        class parserstatemachine;
+        class parserstatemachine; /* see parserstatemachine.hpp */
         class exprstatestack;
 
         class formal_arg;
@@ -91,8 +91,7 @@ namespace xo {
              *  forward instructions to parent parser
              **/
             void on_input(const token_type & tk,
-                          exprstatestack * p_stack,
-                          rp<Expression> * p_emit_expr);
+                          parserstatemachine * p_psm);
 
             /** update exprstate in response to a successfully-parsed subexpression **/
             virtual void on_expr(ref::brw<Expression> expr,
diff --git a/src/reader/exprstate.cpp b/src/reader/exprstate.cpp
index 71f0eb50..d5f72881 100644
--- a/src/reader/exprstate.cpp
+++ b/src/reader/exprstate.cpp
@@ -247,20 +247,20 @@ namespace xo {
 
         void
         exprstate::on_input(const token_type & tk,
-                            exprstatestack * p_stack,
-                            rp<Expression> * p_emit_expr)
+                            parserstatemachine * p_psm)
         {
             constexpr bool c_debug_flag = true;
             scope log(XO_DEBUG(c_debug_flag));
             log && log(xtag("tk", tk));
             log && log(xtag("state", *this));
 
-            parserstatemachine psm(p_stack, p_emit_expr);
+            auto p_stack = p_psm->p_stack_;
+            auto p_emit_expr = p_psm->p_emit_expr_;
 
             switch (tk.tk_type()) {
 
             case tokentype::tk_def:
-                this->on_def_token(tk, &psm);
+                this->on_def_token(tk, p_psm);
                 return;
 
             case tokentype::tk_lambda: