diff --git a/repl/repl.scm b/repl/repl.scm
new file mode 100644
index 0000000..ce79a52
--- /dev/null
+++ b/repl/repl.scm
@@ -0,0 +1,214 @@
+;; -*- Mode: Irken -*-
+
+(include "self/lisp_reader.scm")
+
+;; --- s-expression input ---
+
+(define (find-and-read-file path)
+  (raise (:NoIncludeFiles))
+  )
+
+(define (file/read-line file)
+  (let loop ((ch (file/read-char file))
+         (r '()))
+    (if (eq? ch #\newline)
+    (list->string (reverse r))
+    (loop (file/read-char file) (list:cons ch r)))))
+
+(define (ask prompt file)
+  (printf prompt) (flush)
+  (file/read-line file))
+
+;; --- universal datatype ---
+;;
+;; this datatype covers all the types known by the interpreter.
+;; 
+
+(datatype univ
+  (:int int)
+  (:char char)
+  (:string string)
+  (:bool bool)
+  (:symbol symbol)
+  (:undef)
+  (:list (list univ))
+  (:function (list symbol) sexp)
+  )
+
+;; how to print out a universal value
+
+(define univ-repr
+  (univ:int n)    -> (format (int n))
+  (univ:char n)   -> (format (char n))
+  (univ:string s) -> (format (string s))
+  (univ:bool b)   -> (format (bool b))
+  (univ:symbol s) -> (format (sym s))
+  (univ:undef)    -> "#u"
+  (univ:list subs) 
+  -> (format "(" (join univ-repr " " subs) ")")
+  (univ:function rands body)
+  -> (format "<function (" (join symbol->string " " rands) ") " (repr body) ">")
+  )
+
+;; lexical environment.
+;;
+;; a 'rib' consists of a set of variable bindings.
+;; when a function is called, the arguments are evaluated
+;;  and bound to their formal variable names in a new rib,
+;;  which is pushed onto the environment for that call.
+
+;; the keys are symbols, the values are a record type containing a single
+;;  field named 'val', of type 'univ'.
+;;  the values must be placed in some kind of mutable storage, for set! to work.
+
+(datatype env
+  (:empty)
+  (:rib (alist symbol {val=univ}) env)
+  )
+
+(define (repl-error what)
+  (printf "error: " what "\n")
+  (univ:undef)
+  )
+
+(define namespace (env:empty))
+
+(define (get-cell name env)
+  (match env with
+    (env:empty) 
+    -> (maybe:no)
+    (env:rib rib next)
+    -> (match (alist/lookup rib name) with
+         (maybe:no) -> (get-cell name next)
+         (maybe:yes cell) -> (maybe:yes cell)
+         )
+    ))
+
+(define (varref name env)
+  (match (get-cell name env) with
+    (maybe:no) -> (repl-error (format "undefined variable: " (sym name)))
+    (maybe:yes {val=val}) -> val
+    ))
+
+(define (varset name val env)
+  (match (get-cell name env) with
+    (maybe:no) ;; not defined, create a top-level entry
+    -> (let ((cell {val=val}))
+         (match namespace with
+           (env:empty) 
+           -> (set! namespace (env:rib (alist:entry name cell (alist:nil)) (env:empty)))
+           (env:rib rib next)
+           -> (set! namespace (env:rib (alist:entry name cell rib) next))
+           ))
+    (maybe:yes cell)
+    -> (set! cell.val val)
+    )
+  (univ:undef)
+  )
+
+;; evaluate a primitive operator (one starting with '%')
+
+(define eval-prim
+  '%+ (arg0 arg1) env
+  -> (let ((a (eval arg0 env))
+           (b (eval arg1 env)))
+       (match a b with
+         (univ:int a) (univ:int b) -> (univ:int (+ a b))
+         _ _ -> (repl-error (format "bad args: " (univ-repr a) " " (univ-repr b)))
+         ))
+  '%- (arg0 arg1) env
+  -> (let ((a (eval arg0 env))
+           (b (eval arg1 env)))
+       (match a b with
+         (univ:int a) (univ:int b) -> (univ:int (- a b))
+         _ _ -> (repl-error (format "bad args: " (univ-repr a) " " (univ-repr b)))
+         ))
+  prim _ _
+  -> (repl-error (format "unknown prim: " (sym prim)))
+  )
+
+(define (make-function rands body)
+  (let ((formals '()))
+    (for-list rand rands
+      (match rand with
+        (sexp:symbol name)
+        -> (begin (PUSH formals name) (univ:undef))
+        _ -> (repl-error (format "bad formals: " (repr (sexp:list rands))))
+        ))
+    (univ:function (reverse formals) body)
+    ))
+
+;; build an environment rib for these formals
+;;  and operands, ready to be pushed onto the lexical
+;;  environment.
+(define (eval-args fun formals rands env)
+  (let loop ((formals formals)
+             (rands rands)
+             (rib (alist:nil)))
+    (match formals rands with
+      (name . formals) (arg . rands) 
+      -> (loop formals rands (alist:entry name {val=(eval arg env)} rib))
+      () ()
+      -> (maybe:yes rib)
+      _ _
+      -> (maybe:no)
+      )))
+
+(define (eval-apply rator rands env)
+  (match (eval rator env) with
+    (univ:function formals body)
+    -> (match (eval-args rator formals rands env) with
+         (maybe:no) -> (repl-error (format "wrong number of arguments to function " (repr rator)))
+         (maybe:yes new-rib)
+         -> (eval body (env:rib new-rib env))
+         )
+    op -> (repl-error (format "operator is not a function: " (univ-repr op)))
+    ))
+
+;; top-level eval function
+
+(define (eval exp env)
+  (match exp with
+    ;; self-evaluating expressions
+    (sexp:int n)    -> (univ:int n)
+    (sexp:char ch)  -> (univ:char ch)
+    (sexp:string s) -> (univ:string s)
+    (sexp:bool b)   -> (univ:bool b)
+    (sexp:undef)    -> (univ:undef)
+    ;; variable lookup
+    (sexp:symbol s) -> (varref s env)
+    ;; variable assignment
+    (sexp:list ((sexp:symbol 'set!) (sexp:symbol name) val))
+    -> (varset name (eval val env) env)
+    ;; lambda
+    (sexp:list ((sexp:symbol 'lambda) (sexp:list formals) body))
+    -> (make-function formals body)
+    ;; application
+    (sexp:list (rator . rands))
+    -> (match rator with
+         (sexp:symbol name)
+         -> (if (starts-with (symbol->string name) "%")
+                (eval-prim name rands env)
+                (eval-apply rator rands env))
+         _ -> (eval-apply rator rands env))
+    ;; anything else...  
+    exp -> (repl-error (format "bad/unknown expression: " (repr exp)))
+    ))
+
+(define (setup-initial-environment)
+  (varset 'x (univ:int 34) namespace)
+  )
+
+(define (read-eval-print-loop stdin)
+  (setup-initial-environment)
+  (let loop ((line (ask "> " stdin)))
+    (match (string-length line) with
+      0 -> #u
+      _ -> (begin
+         ;;(printf "line = '" line "'\n")
+         (for-list exp (read-string line)
+           (printf (univ-repr (eval exp namespace)) "\n"))
+         (loop (ask "> " stdin)))
+      )))
+
+(read-eval-print-loop (file/open-stdin))