Implement unwinding mechanism, repl example

examples/echo.lysp

@@ -0,0 +1,4 @@
+;; vi:ft=lisp:sw=2:ts=2
+
+(print (length *args*))
+(print *args*)

examples/repl.lysp

@@ -0,0 +1,36 @@
+;; vi:ft=lisp:sw=2:ts=2
+
+(defun cadr (x) (car (cdr x)))
+
+(defun map-ok-err (f-ok f-err result)
+  (if (= (car result) 'ok)
+    `(ok ,(f-ok (cadr result)))
+    `(err ,(f-err (cadr result)))
+    )
+  )
+(defun map-ok (f-ok result) (map-ok-err f-ok identity result))
+(defun map-err (f-err result) (map-ok-err identity f-err result))
+
+(defun repl-print (value)
+  (print "==>" value)
+  )
+(defun repl-eval-print (expression)
+  (map-ok-err repl-print repl-eval-error (unquote (eval expression)))
+  )
+
+(defun repl-eval-error (error)
+  (print "Evaluation error:")
+  (print error)
+  )
+(defun repl-read-error (error)
+  (print "Parse error:")
+  (print error)
+  )
+
+(loop
+  (let (expression (read))
+    (if expression NIL (return))
+    (setq expression (unquote expression))
+    (map-ok-err repl-eval-print repl-read-error expression)
+    )
+  )