Add operands to syntax help message

parse.lisp

@@ -197,19 +197,23 @@ are useful for use in things like syntax explanation messages.")
   "Return whether OPER is a unary operator or not."
   (eq oper 'not))
 
+(defparameter *operand-symbol-table*
+  '((true "t" "true" "⊤" "1")
+    (false "f" "false" "⊥" "0"))
+  "Alist mapping operand symbols (true and false) to their textual
+representations.")
+
 (defun interpret-operand (oper-str)
   "Return a symbol representing OPER-STR, or the string itself if it represents
 a variable."
-  (cond
-    ((member oper-str '("t" "true" "⊤" "1") :test 'equalp)
-     'true)
-    ((member oper-str '("f" "false" "⊥" "0") :test 'equalp)
-     'false)
-    (t
-     (loop for char across oper-str
-           unless (symbol-char-p char)
-             do (return nil)
-           finally (return oper-str)))))
+  (dolist (entry *operand-symbol-table*)
+    (when (member oper-str (cdr entry) :test 'equalp)
+      (return-from interpret-operand (car entry))))
+  ;; check if OPER-STR is a valid variable name
+  (if (or (zerop (length oper-str))
+          (find-if-not 'symbol-char-p oper-str))
+      nil
+      oper-str))
 
 (defun string-first-char-safe (str)
   "Return the first character of STR, or nil if it is empty."