It's not really named based on the types, but more on the expected use. For example, it's not a predicate because it returns a Boolean, it's a predicate because it's a statement about the information passed in. A predicate could take multiple arguments (and depending on the domain could theoretically return something other than a Boolean, though that's more rare and domain specific), and a function that returns a Boolean may not nesessarily be a predicate.
So, to answer your question, how to name your alias depends on it's expected use. As @Landry Norris said, transform[ation] and mutator make sense when you're respectively transforming or mutating the value, but if you have no information about the use, your best bet would probably just be "function" or a synonym (operation, action, etc.).