[[!comment format=mdwn username="joey" subject="""comment 1""" date="2015-09-30T14:53:02Z" content=""" My suggestion in the specific case of the sbuild property would be to generate the file whenever the property that generates the chroot has to make a change. The `onChange` combinator accomplishes that. But if you really want to only run the property if the file doesn't exist, you can do that by using the `check` combinator. For example: check (not <$> doesFileExist f) (createtarball f) """]]