[namelosw]

> I believe that "single source of truth" is a principle that should always be followed

Theoretically and conceptually I agree. But in practice there are a lot of programming languages aren’t as expressive. People prefer codebases with duplications rather than visitor patterns everywhere. In essence, visitor pattern is a tool to solve multi-dimensional abstraction problems, just like type classes in Haskell or CLOS in Common Lisp. But it’s so verbose and non-straightforward so more often than not it’s not worth it even conceptually it’s a legit case for “single source of truth”.

[gf000]

Visitor pattern is there due to a very simply reason. You have n datatypes with m functions. FP languages makes adding a new row to this nm table easy, OOP languages makes adding a new column easy (that is, without changing every* use site as well).

Visitor pattern makes the row addition case possible for OOP languages, that's it.