Do you define "predicate function" here as "method that returns a boolean value"? If so, then it reveals something about the object state- information that should be encapsulated.
in reply to Re: Re: Re: Short Refactoring Tip: let the object do it for you
in thread Short Refactoring Tip: let the object do it for you
A predicate method (or a query method, for that matter) defends encapsulation. The existence of the method says nothing about how the target object derives an answer. It could have the answer in a hash, it could calculate it lazily, it could delegate to another object. Whatever. The point is that the client doesn't know. Coupling is kept down. And, assuming that the question lies within the province of the target object to answer, cohesion is kept up.
I see that what you're really arguing for is reducing the need for "can you do this? yes? then do it" types of protocols. Fine; I agree. But in arguing that point, you're either misunderstanding or misrepresenting encapsulation.