added (display-if-not)

tk/listlogic.scm

@@ -1,5 +1,6 @@
 (define-module (tk listlogic)
-  #:export (flatten
+  #:export (display-if-not
+            flatten
             lp&
             l&
             if-some
@@ -57,3 +58,14 @@
   (syntax-rules ()
     ((if-some-else $cond $else)
      (if $cond $cond $else))))
+
+;; Prints the message if condition is #f, otherwise prints nothing
+(define-syntax display-if-not
+  (syntax-rules ()
+    ((display-if-not $cond)
+     (syntax-error "Must provide a string to (display-if-not)"))
+    ((display-if-not $cond $message)
+     (begin (if (not $cond) (display $message))
+            (newline)))
+    ((display-if-not $cond . $messages)
+     (display-if-not $cond (string-concatenate/shared '$messages)))))