alt Hacker NewsIf you can use functions in the same way objects are used, there’s no need for visitor objects.
There’s a reason why everything is a Lisp. All of the patterns are obvious with its primitives, while higher level primitives like classes, interfaces hide that there’s data and there’s behavior/effects.
Visitor objects are needed when you want, at runtime, to decide what code to execute based on the types of two parameters of a function (regular OOP virtual dispatch can only do this based on the type of one argument, the one before the dot). While you can model this in different ways, there is nothing in "plain" Lisp (say, R7RS Scheme) that makes this particularly simple.
Common Lisp does have a nicer solution to this, in the form of CLOS generic functions. In CLOS, methods are defined based on the classes of all arguments, not just the first one like in traditional object systems. Combined with inheritance, you can implement the whole thing with the minimal amount of code. But it's still an OOP system designed specifically for this.