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Comment by ceronman | Hacker News Reader

The visitor pattern is very common in programming language implementations. I've seen it in the Rust compiler, in the Java Compiler, in the Go compiler and in the Roslyn C# compiler. Also used extensively in JetBrains' IDEs.

What do you have against this pattern? Or what is a better alternative?

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Visitor is heavy of code pattern that can be replaced by elegant, readable switch with exhaustive check, so all operations available by "Kind" enum are covered.

This wasn't available in Javs at the time. You're free to rewrite it with pattern matching (like the book, quite literally, leaves as an exercise for the reader).

Exhaustive switch with tail-calling makes for a very fast and readable interpreter.

Roslyn has visitor pattern combined with the 'Kind' enumeration you mentioned. You can either choose to visiti a SyntaxNode of a certain type, or override the generic version and decide what you want to do based on that enumeration.

C# doesnt have exhaustive switch over enums.

It needs to get "closed enum" lang. feature.

Exhaustive enums (or type switches) are not a requirement, and are infact harmful - imagine if they add a new kind of syntax node to the language, now your analyzer no longer compiles unless you add a default case - which is very easy to add in C# as well.

Unless you add default... or handle such case, as expected.

Ofc you can use this feature wrong and abuse default case, but in general this is very good since it prevents you about missing places to add handling and screams at you at comp time instead of runtimr

A switch or pattern matching approach is useful, but not practical for some cases. For example, there are cases where you are interested in only a single kind of node in the three, for those cases the Visitor pattern is very helpful, while doing pattern matching is cumbersome because you have to match and check almost every node kind. That's why, for example, the Rust compiler still uses the visitor pattern for certain things, and pattern matching for others.

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