The whole point is to separate behavior from the datastructure. Yes, this doesn't fit nicely into OO principles, but strictly that's true any time you externally query an object for state and make a conditional decision based on the result.
Inevitably, any method (visitor, pattern matching, if elsif) of customizable walking of a tree datastructure separates behavior from the tree object. That's the point, though, again.
Now, you can say wanting to do that (customizable tree walking) in itself is an antipattern and shouldn't be done. That just doesn't match well with reality.
I've seen many cases where the visitor pattern is about the best one can do. Yes, it becomes ugly but that's usually due to the complexity of the attempted tree walking, e.g. for transformation.
A lot of the alternative approaches in the functional world become ugly too once there's a need to offer maximum customizability, e.g. in terms of order of evaluation.
If you are able to sufficiently constrain the domain you can get away with some simplied templates, but where I see visitors most often is in API's wanting to offer any kind of tree walking.
Inevitably, any method (visitor, pattern matching, if elsif) of customizable walking of a tree datastructure separates behavior from the tree object. That's the point, though, again.
Now, you can say wanting to do that (customizable tree walking) in itself is an antipattern and shouldn't be done. That just doesn't match well with reality.
I've seen many cases where the visitor pattern is about the best one can do. Yes, it becomes ugly but that's usually due to the complexity of the attempted tree walking, e.g. for transformation.
A lot of the alternative approaches in the functional world become ugly too once there's a need to offer maximum customizability, e.g. in terms of order of evaluation.
If you are able to sufficiently constrain the domain you can get away with some simplied templates, but where I see visitors most often is in API's wanting to offer any kind of tree walking.