diff --git a/xo-reader/include/xo/reader/define_xs.hpp b/xo-reader/include/xo/reader/define_xs.hpp
index 880dd309..4f2eb66a 100644
--- a/xo-reader/include/xo/reader/define_xs.hpp
+++ b/xo-reader/include/xo/reader/define_xs.hpp
@@ -97,6 +97,8 @@ namespace xo {
                                                parserstatemachine * p_psm) override;
             virtual void on_rightparen_token(const token_type & tk,
                                              parserstatemachine * p_psm) override;
+            virtual void on_i64_token(const token_type & tk,
+                                      parserstatemachine * p_psm) override;
             virtual void on_f64_token(const token_type & tk,
                                       parserstatemachine * p_psm) override;
 
diff --git a/xo-reader/include/xo/reader/expect_expr_xs.hpp b/xo-reader/include/xo/reader/expect_expr_xs.hpp
index 646a803a..900ee1de 100644
--- a/xo-reader/include/xo/reader/expect_expr_xs.hpp
+++ b/xo-reader/include/xo/reader/expect_expr_xs.hpp
@@ -41,6 +41,9 @@ namespace xo {
             virtual void on_symbol_token(const token_type & tk,
                                          parserstatemachine * p_psm) override;
 
+            virtual void on_i64_token(const token_type & tk,
+                                      parserstatemachine * p_psm) override;
+
             virtual void on_f64_token(const token_type & tk,
                                       parserstatemachine * p_psm) override;
 
diff --git a/xo-reader/include/xo/reader/exprseq_xs.hpp b/xo-reader/include/xo/reader/exprseq_xs.hpp
index 05d0a6e3..52ec315c 100644
--- a/xo-reader/include/xo/reader/exprseq_xs.hpp
+++ b/xo-reader/include/xo/reader/exprseq_xs.hpp
@@ -10,15 +10,37 @@
 
 namespace xo {
     namespace scm {
+        enum class exprseqtype {
+            /** toplevel interactive sequence.  Most permissive **/
+            toplevel_interactive,
+            /** toplevel non-interactive sequence.
+             *  This used for top-level expressions in a translation unit.
+             **/
+            toplevel_batch,
+            /** nested sequence, for example in function body **/
+            nested,
+        };
+
         /** @class exprseq_xs
          *  @brief parsing state-machine for top-level expression sequence
          *
+         *  expression sequences come in several types:
+         *  1. top-level interactive
+         *  2. top-level batch
+         *  3. nested
+         *
+         *           1 2 3
+         *         +--------
+         *  def    | y y y
+         *  symbol | y n n         1: evaluate as variable
+         *  i64    | y n n         1: evaluate as constant
+         *
          **/
         class exprseq_xs : public exprstate {
         public:
-            exprseq_xs();
+            explicit exprseq_xs(exprseqtype seqtype);
 
-            static void start(parserstatemachine * p_psm);
+            static void start(exprseqtype seqtype, parserstatemachine * p_psm);
 
         public:
             // ----- token input methods -----
@@ -27,6 +49,8 @@ namespace xo {
                                       parserstatemachine * p_psm) override;
             virtual void on_symbol_token(const token_type & tk,
                                          parserstatemachine * p_psm) override;
+            virtual void on_i64_token(const token_type & tk,
+                                      parserstatemachine * p_psm) override;
 
             // ----- victory methods -----
 
@@ -34,9 +58,14 @@ namespace xo {
                                       parserstatemachine * p_psm) override;
             virtual void on_expr(ref::brw<Expression> expr,
                                  parserstatemachine * p_psm) override;
+            virtual void on_expr_with_semicolon(ref::brw<Expression> expr,
+                                                parserstatemachine * p_psm) override;
 
         private:
-            static std::unique_ptr<exprseq_xs> make();
+            static std::unique_ptr<exprseq_xs> make(exprseqtype seqtype);
+
+            /** context for this expression sequence **/
+            exprseqtype xseqtype_;
         };
     } /*namespace scm*/
 } /*namespace xo*/
diff --git a/xo-reader/include/xo/reader/exprstate.hpp b/xo-reader/include/xo/reader/exprstate.hpp
index a6acad1f..467348df 100644
--- a/xo-reader/include/xo/reader/exprstate.hpp
+++ b/xo-reader/include/xo/reader/exprstate.hpp
@@ -12,6 +12,8 @@
 
 namespace xo {
     namespace scm {
+        /** Identify current state in an expression state machine
+         **/
         enum class exprstatetype {
             invalid = -1,
 
@@ -172,6 +174,10 @@ namespace xo {
             virtual void on_operator_token(const token_type & tk,
                                            parserstatemachine * p_psm);
 
+            /** handle incoming integer-literal token **/
+            virtual void on_i64_token(const token_type & tk,
+                                      parserstatemachine * p_psm);
+
             /** handle incoming floating-point-literal token **/
             virtual void on_f64_token(const token_type & tk,
                                       parserstatemachine * p_psm);
diff --git a/xo-reader/include/xo/reader/paren_xs.hpp b/xo-reader/include/xo/reader/paren_xs.hpp
index 514074d1..0d251a76 100644
--- a/xo-reader/include/xo/reader/paren_xs.hpp
+++ b/xo-reader/include/xo/reader/paren_xs.hpp
@@ -64,6 +64,8 @@ namespace xo {
                                             parserstatemachine * p_psm) override;
             virtual void on_rightparen_token(const token_type & tk,
                                              parserstatemachine * p_psm) override;
+            virtual void on_i64_token(const token_type & tk,
+                                      parserstatemachine * p_psm) override;
             virtual void on_f64_token(const token_type & tk,
                                       parserstatemachine * p_psm) override;
 
diff --git a/xo-reader/include/xo/reader/parser.hpp b/xo-reader/include/xo/reader/parser.hpp
index 30dd94ea..01a3b9b9 100644
--- a/xo-reader/include/xo/reader/parser.hpp
+++ b/xo-reader/include/xo/reader/parser.hpp
@@ -59,10 +59,16 @@ namespace xo {
          *    }
          *
          * Grammar:
-         *   toplevel-program = $expression(1); ..; $expression(n)
+         *   toplevel-program = $toplevel-expression(1); ..; $toplevel-expression(n)
+         *
+         * if interactive:
+         *   toplevel-expression = expression
+         * else
+         *   toplevel-expression = type-decl | define-expr
          *
          *   type-decl        = decltype $typename [<$tp1 .. $tpn>]
-         *   expression       = define-expr
+         *   expression       = type-decl
+         *                       | define-expr
          *                       | literal-expr
          *                       | variable-expr
          *                       | apply-expr
@@ -185,6 +191,10 @@ namespace xo {
              **/
             bool has_incomplete_expr() const;
 
+            /** put parser into state for beginning an interactive session.
+             **/
+            void begin_interactive_session();
+
             /** put parser into state for beginning of a translation unit
              *  (i.e. input stream)
              **/
diff --git a/xo-reader/include/xo/reader/progress_xs.hpp b/xo-reader/include/xo/reader/progress_xs.hpp
index 6e9db243..03bd933e 100644
--- a/xo-reader/include/xo/reader/progress_xs.hpp
+++ b/xo-reader/include/xo/reader/progress_xs.hpp
@@ -89,6 +89,9 @@ namespace xo {
             virtual void on_operator_token(const token_type & tk,
                                            parserstatemachine * p_psm) override;
 
+            virtual void on_i64_token(const token_type & tk,
+                                      parserstatemachine * p_psm) override;
+
             virtual void on_f64_token(const token_type & tk,
                                       parserstatemachine * p_psm) override;
 
diff --git a/xo-reader/include/xo/reader/reader.hpp b/xo-reader/include/xo/reader/reader.hpp
index 28e0b3a6..59e600c1 100644
--- a/xo-reader/include/xo/reader/reader.hpp
+++ b/xo-reader/include/xo/reader/reader.hpp
@@ -67,6 +67,15 @@ namespace xo {
         public:
             reader() = default;
 
+            /** call once before calling .read_expr()
+             *  for a new interactive session
+             **/
+            void begin_interactive_session();
+
+            /** counterpart to .begin_interactive_session()
+             **/
+            reader_result end_interactive_session();
+
             /** call once before calling .read_expr():
              *  1. with new reader
              *  2. if last read_expr() call had eof=true
diff --git a/xo-reader/src/reader/define_xs.cpp b/xo-reader/src/reader/define_xs.cpp
index 19f6d83d..fdd3c410 100644
--- a/xo-reader/src/reader/define_xs.cpp
+++ b/xo-reader/src/reader/define_xs.cpp
@@ -245,6 +245,18 @@ namespace xo {
             this->illegal_input_error(self_name, tk);
         }
 
+        void
+        define_xs::on_i64_token(const token_type & tk,
+                                parserstatemachine * /*p_psm*/)
+        {
+            constexpr bool c_debug_flag = true;
+            scope log(XO_DEBUG(c_debug_flag));
+
+            constexpr const char * self_name = "define_xs::on_i64";
+
+            this->illegal_input_error(self_name, tk);
+        }
+
         void
         define_xs::on_f64_token(const token_type & tk,
                                 parserstatemachine * /*p_psm*/)
diff --git a/xo-reader/src/reader/expect_expr_xs.cpp b/xo-reader/src/reader/expect_expr_xs.cpp
index 1f07d131..c4a56f30 100644
--- a/xo-reader/src/reader/expect_expr_xs.cpp
+++ b/xo-reader/src/reader/expect_expr_xs.cpp
@@ -172,6 +172,17 @@ namespace xo {
             return;
         }
 
+        void
+        expect_expr_xs::on_i64_token(const token_type & tk,
+                                     parserstatemachine * p_psm)
+        {
+            constexpr bool c_debug_flag = true;
+            scope log(XO_DEBUG(c_debug_flag));
+
+            progress_xs::start
+                (Constant<int64_t>::make(tk.i64_value()),
+                 p_psm);
+        }
 
         void
         expect_expr_xs::on_f64_token(const token_type & tk,
diff --git a/xo-reader/src/reader/exprseq_xs.cpp b/xo-reader/src/reader/exprseq_xs.cpp
index 421b037c..e650e238 100644
--- a/xo-reader/src/reader/exprseq_xs.cpp
+++ b/xo-reader/src/reader/exprseq_xs.cpp
@@ -3,25 +3,28 @@
 #include "exprseq_xs.hpp"
 #include "parserstatemachine.hpp"
 #include "exprstatestack.hpp"
+#include "exprseq_xs.hpp"
+#include "expect_expr_xs.hpp"
 #include "define_xs.hpp"
 #include "expect_symbol_xs.hpp"
 
 namespace xo {
     namespace scm {
         std::unique_ptr<exprseq_xs>
-        exprseq_xs::make()
+        exprseq_xs::make(exprseqtype seqtype)
         {
-            return std::make_unique<exprseq_xs>(exprseq_xs());
+            return std::make_unique<exprseq_xs>(exprseq_xs(seqtype));
         }
 
         void
-        exprseq_xs::start(parserstatemachine * p_psm)
+        exprseq_xs::start(exprseqtype seqtype, parserstatemachine * p_psm)
         {
-            p_psm->push_exprstate(exprseq_xs::make());
+            p_psm->push_exprstate(exprseq_xs::make(seqtype));
         }
 
-        exprseq_xs::exprseq_xs()
-            : exprstate(exprstatetype::expect_toplevel_expression_sequence)
+        exprseq_xs::exprseq_xs(exprseqtype x)
+            : exprstate(exprstatetype::expect_toplevel_expression_sequence),
+              xseqtype_{x}
         {
         }
 
@@ -51,6 +54,24 @@ namespace xo {
             this->illegal_input_error(c_self_name, tk);
         }
 
+        void
+        exprseq_xs::on_i64_token(const token_type & tk,
+                                 parserstatemachine * p_psm)
+        {
+            constexpr bool c_debug_flag = true;
+            scope log(XO_DEBUG(c_debug_flag));
+
+            constexpr const char * c_self_name = "exprseq_xs::on_i64_token";
+
+            if (xseqtype_ == exprseqtype::toplevel_interactive)
+            {
+                expect_expr_xs::start(p_psm);
+                p_psm->top_exprstate().on_i64_token(tk, p_psm);
+            } else {
+                this->illegal_input_error(c_self_name, tk);
+            }
+        }
+
         void
         exprseq_xs::on_typedescr(TypeDescr /*td*/,
                                  parserstatemachine * /*p_psm*/)
@@ -73,6 +94,21 @@ namespace xo {
             *p_emit_expr = expr.promote();
         } /*on_expr*/
 
+        void
+        exprseq_xs::on_expr_with_semicolon(ref::brw<Expression> expr,
+                                           parserstatemachine * p_psm)
+        {
+            /* toplevel expression sequence accepts an
+             * arbitrary number of expressions.
+             *
+             * semicolons are sometimes mandatory to avoid ambiguity.
+             */
+
+            auto p_emit_expr = p_psm->p_emit_expr_;
+
+            *p_emit_expr = expr.promote();
+        }
+
     } /*namespace scm*/
 } /*namespace xo*/
 
diff --git a/xo-reader/src/reader/exprstate.cpp b/xo-reader/src/reader/exprstate.cpp
index 77480dcb..8145c550 100644
--- a/xo-reader/src/reader/exprstate.cpp
+++ b/xo-reader/src/reader/exprstate.cpp
@@ -251,6 +251,18 @@ namespace xo {
             this->illegal_input_error(self_name, tk);
         }
 
+        void
+        exprstate::on_i64_token(const token_type & tk,
+                                parserstatemachine * /*p_psm*/)
+        {
+            constexpr bool c_debug_flag = true;
+            scope log(XO_DEBUG(c_debug_flag));
+
+            constexpr const char * self_name = "exprstate::on_i64";
+
+            this->illegal_input_error(self_name, tk);
+        }
+
         void
         exprstate::on_f64_token(const token_type & tk,
                                 parserstatemachine * /*p_psm*/)
@@ -284,7 +296,7 @@ namespace xo {
                 return;
 
             case tokentype::tk_i64:
-                assert(false);
+                this->on_i64_token(tk, p_psm);
                 return;
 
             case tokentype::tk_f64:
diff --git a/xo-reader/src/reader/paren_xs.cpp b/xo-reader/src/reader/paren_xs.cpp
index 2c4ca484..357d377a 100644
--- a/xo-reader/src/reader/paren_xs.cpp
+++ b/xo-reader/src/reader/paren_xs.cpp
@@ -170,6 +170,18 @@ namespace xo {
             }
         }
 
+        void
+        paren_xs::on_i64_token(const token_type & tk,
+                               parserstatemachine * /*p_psm*/)
+        {
+            constexpr bool c_debug_flag = true;
+            scope log(XO_DEBUG(c_debug_flag));
+
+            constexpr const char * c_self_name = "paren_xs::on_i64";
+
+            this->illegal_input_error(c_self_name, tk);
+        }
+
         void
         paren_xs::on_f64_token(const token_type & tk,
                                parserstatemachine * /*p_psm*/)
diff --git a/xo-reader/src/reader/parser.cpp b/xo-reader/src/reader/parser.cpp
index e71deb40..4250e028 100644
--- a/xo-reader/src/reader/parser.cpp
+++ b/xo-reader/src/reader/parser.cpp
@@ -29,6 +29,16 @@ namespace xo {
             return !xs_stack_.empty();
         }
 
+        void
+        parser::begin_interactive_session() {
+            /* note: not using emit expr here */
+            parserstatemachine psm(&xs_stack_,
+                                   &env_stack_,
+                                   nullptr /*p_emit_expr*/);
+
+            exprseq_xs::start(exprseqtype::toplevel_interactive, &psm);
+        }
+
         void
         parser::begin_translation_unit() {
             /* note: not using emit expr here */
@@ -36,7 +46,7 @@ namespace xo {
                                    &env_stack_,
                                    nullptr /*p_emit_expr*/);
 
-            exprseq_xs::start(&psm);
+            exprseq_xs::start(exprseqtype::toplevel_batch, &psm);
         }
 
         rp<Expression>
diff --git a/xo-reader/src/reader/progress_xs.cpp b/xo-reader/src/reader/progress_xs.cpp
index 40d857b6..44938fca 100644
--- a/xo-reader/src/reader/progress_xs.cpp
+++ b/xo-reader/src/reader/progress_xs.cpp
@@ -401,6 +401,22 @@ namespace xo {
             }
         }
 
+        void
+        progress_xs::on_i64_token(const token_type & tk,
+                                  parserstatemachine * p_psm)
+        {
+            constexpr bool c_debug_flag = true;
+            scope log(XO_DEBUG(c_debug_flag));
+
+            constexpr const char * self_name = "progress_xs::on_i64";
+
+            if (this->op_type_ == optype::invalid) {
+                this->illegal_input_error(self_name, tk);
+            } else {
+                exprstate::on_i64_token(tk, p_psm);
+            }
+        }
+
         void
         progress_xs::on_f64_token(const token_type & tk,
                                   parserstatemachine * p_psm)
diff --git a/xo-reader/src/reader/reader.cpp b/xo-reader/src/reader/reader.cpp
index eb012541..bc9de1ce 100644
--- a/xo-reader/src/reader/reader.cpp
+++ b/xo-reader/src/reader/reader.cpp
@@ -4,6 +4,16 @@
 
 namespace xo {
     namespace scm {
+        void
+        reader::begin_interactive_session() {
+            parser_.begin_interactive_session();
+        }
+
+        reader_result
+        reader::end_interactive_session() {
+            return this->read_expr(span_type(nullptr, nullptr), true /*eof*/);
+        }
+
         void
         reader::begin_translation_unit() {
             parser_.begin_translation_unit();