diff --git a/include/xo/parser/reader.hpp b/include/xo/parser/reader.hpp
index 39dcd6b8..b36489d6 100644
--- a/include/xo/parser/reader.hpp
+++ b/include/xo/parser/reader.hpp
@@ -6,10 +6,27 @@
 #pragma once
 
 #include "parser.hpp"
+#include "xo/expression/Expression.hpp"
 #include "xo/tokenizer/tokenizer.hpp"
 
 namespace xo {
     namespace scm {
+        /** @class parse_result
+         *  @brief Result object returned from reader::read_expr
+         **/
+        struct reader_result {
+            using Expression = xo::ast::Expression;
+            using span_type = span<const char>;
+
+            /** parsed schematica expression **/
+            rp<Expression> expr_;
+            /** span giving text input consumed to construct expr,
+             *  including any leading whitespace.
+             *  This is the span returned in result of tokenizer<char>::scan()
+             **/
+            span_type rem_;
+        };
+
         /**
          *  Use:
          *  @code
@@ -21,7 +38,7 @@ namespace xo {
          *        // eof: true if no more input will be forthcoming from this stream
          *        eof = ins.eof();
          *
-         *        for (auto rem = input; ; !rem.empty()) {
+         *        for (auto rem = input; !rem.empty();) {
          *            // res: (parsed-expr, used)
          *            auto res = rdr.read_expr(rem, eof);
          *
@@ -39,18 +56,32 @@ namespace xo {
          *  @endcode
          **/
         class reader {
+        public:
+            using tokenizer_type = tokenizer<char>;
+            using span_type = tokenizer_type::span_type;
+
         public:
             reader() = default;
 
+            /** Try to read one expression from @p input.
+             *  Return struct containing parsed expression
+             *  and span of characters comprising that expression
+             *
+             *  @param input  Supply this input span of chars
+             *  @param eof.  True if input stream supplying @p input
+             *         reports end-of-file immediately after the last char
+             *         in @p input.
+             **/
+            reader_result read_expr(const span_type & input, bool eof);
+
         private:
             /** tokenizer: text -> tokens **/
-            tokenizer tokenizer_;
+            tokenizer_type tokenizer_;
 
-            /** parser: tokens -> expressions (TODO: reanme ->reader) **/
+            /** parser: tokens -> expressions **/
             parser parser_;
         };
     } /*namespace scm*/
 } /*namespace xo*/
 
-
-/* end Repl.hpp */
+/* end reader.hpp */
diff --git a/src/parser/CMakeLists.txt b/src/parser/CMakeLists.txt
index 2af43636..a97b50fa 100644
--- a/src/parser/CMakeLists.txt
+++ b/src/parser/CMakeLists.txt
@@ -2,7 +2,8 @@
 
 set(SELF_LIB xo_parser)
 set(SELF_SRCS
-    parser.cpp)
+    parser.cpp
+    reader.cpp)
 
 xo_add_shared_library4(${SELF_LIB} ${PROJECT_NAME}Targets ${PROJECT_VERSION} 1 ${SELF_SRCS})
 xo_dependency(${SELF_LIB} xo_expression)
diff --git a/src/parser/reader.cpp b/src/parser/reader.cpp
new file mode 100644
index 00000000..84d00b30
--- /dev/null
+++ b/src/parser/reader.cpp
@@ -0,0 +1,75 @@
+/* @file reader.cpp */
+
+#include "reader.hpp"
+
+namespace xo {
+    namespace scm {
+        reader_result
+        reader::read_expr(const span_type & input_arg, bool eof)
+        {
+            span_type input = input_arg;
+
+            /* input text-span consumed by this call.
+             * Always comprises some number (possibly 0)
+             * of complete tokens,  along with any leading
+             * whitespace
+             */
+            span_type expr_span = input.prefix(0ul);
+
+            while (!input.empty()) {
+                /* read one token from input */
+                auto sr = this->tokenizer_.scan2(input, eof);
+                const auto & tk = sr.first;
+                const span_type & used_span = sr.second;
+
+                input = input.after_prefix(used_span);
+                expr_span += used_span;
+
+                if (tk.is_valid()) {
+                    /* forward just-read token to parser */
+                    auto expr = this->parser_.include_token(tk);
+
+                    if (expr) {
+                        /* token completes an expression -> victory */
+                        return reader_result(expr, expr_span);
+                    } else {
+                        /* token did not complete an expression
+                         * (e.g. token for '[')
+                         *
+                         * input span may contain more tokens -> iterate
+                         */
+                        input = input.after_prefix(used_span);
+                    }
+                } else {
+                    assert(input.empty());
+
+                    /* no more tokens in input */
+                    break;
+                }
+            }
+
+            /* control here: either
+             * 1. input.empty (perhaps ate some whitespace,  ok)
+             * 2. missing or incomplete token (ok unless eof)
+             */
+            if (eof) {
+                if (parser_.has_incomplete_expr()) {
+                    throw std::runtime_error
+                        ("reader::read_expr"
+                         ": eof reached with incomplete expression");
+                }
+
+                if (tokenizer_.has_prefix()) {
+                    throw std::runtime_error
+                        ("reader::read_expr"
+                         ": unintelligible input recognized at eof");
+                }
+            }
+
+            return reader_result(nullptr, expr_span);
+        }
+
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/* end reader.cpp */