raise NotImplementedError in case of sprintf is missing

mrblib/print.rb

@@ -48,12 +48,14 @@ module Kernel
     args[0]
   end
 
-  if Kernel.respond_to?(:sprintf)
-    ##
-    # Invoke method +sprintf+ and pass +*args+ to it.
-    # Pass return value to *print* of STDOUT.
-    def printf(*args)
+  ##
+  # Invoke method +sprintf+ and pass +*args+ to it.
+  # Pass return value to *print* of STDOUT.
+  def printf(*args)
+    if Kernel.respond_to?(:sprintf)
       __printstr__(sprintf(*args))
+    else
+      raise NotImplementedError.new('sprintf not available')
     end
   end
 end