Visitor pattern is extremely useful in some areas, such as compiler development.

That’s only true in languages that do not have Algebraic Data Types and pattern matching, which nowadays is a minority of languages (even Java has it).

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cyberax 9 days ago | parent [-]

Visitors additionally allow you to decouple graph traversal from the processing. It is still needed even in the languages with pattern matching.

There's also the question of exhaustiveness checking. With visitors, you can typically opt-in to either checking that you handle everything. Or use the default no-ops for anything that you're not interested in.

So if you look at compilers for languages with pattern matching (e.g. Rust), you still see... visitors! E.g.: https://github.com/rust-lang/rust/blob/64a99db105f45ea330473...

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brabel 9 days ago | parent [-]

The example you posted is very interesting as it used both a visitor and ADTs. It seems the need for the Visitor comes from the generics in this case? Probably a Rust specific limitation. I don’t understand why you mention exhaustiveness though, it’s obviously easy have comprehensive or partial matching with ADT.

	▲	cyberax 9 days ago \| parent [-]
		No. The code can be rewritten without visitors using iterators for traversal, for example). But it'll look badly. Visitors in the linked example are real classic visitors. The code _within_ the visitor methods, of course, uses pattern matching, but the pattern itself is not materially different from C++. Exhaustiveness checking for pattern matching is also "best effort" for complex matching.