[1] https://ericlippert.com/2015/04/27/wizards-and-warriors-part...
[2] https://ericlippert.com/2015/05/11/wizards-and-warriors-part...
The abstraction he offered, of having a tree and then visiting every node in that tree, made the most sense.
> One of the most common usages of this pattern is for the scenario where you’ve got a complex tree where tree nodes can be of many types, and you wish to traverse the tree — that is, visit every node in the tree — taking some actions or performing some computations as you go. The “double dispatch” part of it is that the actions you take as you visit nodes in the tree can depend on the runtime types of both the nodes in the tree, and the code performing the action.
That sounds like a parser. It accepts an AST and evaluates productions on its way through. Saying "actions you take ... depend on the runtime types of ... nodes ... and the code performing the action" sounds similar to sorting out how to mash tree-traversal and productions together.
In my compilers class we first wrote a code linter for our "Wombo" language (a simplified Java). Then we wrote something that built an AST. The core tree traversal code was the same, but the "productions" paired with nodes were different.