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Did you have a previous experience of something easier? May you share about it?

by antpls 8y ago

magicalhippo 8y ago
I'm curious about this as well. When writing my own interpreter it didn't take me long before I opted to use the visitor pattern, and once I did it felt like a natural fit.
chubot 8y ago
Not the OP, but I took a survey of the /r/programminglanguages reddit, where basically everybody has to deal with the "visitor issue". https://www.reddit.com/r/ProgrammingLanguages/comments/7w3oeh/whats_your_experience_with_visitors/ https://www.reddit.com/r/ProgrammingLanguages/comments/7w3oe... The top comment supported the use of the visitor pattern, but I would say that MOST people in the thread did NOT use visitors. You only really need a visitor if you have more than one traversal. And even then you don't necessarily need it.
criddell 8y ago
What is the actual problem? Is it multiple dispatch? I think some languages (Lisp may be one) handle it natively. Visitor is the only way I know of dealing with it in C++. Once you do it enough, Visitor starts to feel like a pretty natural way to extend a class without modifying the class.
- jcelerier 8y ago
  > Visitor is the only way I know of dealing with it in C++. Once you do it enough, Visitor starts to feel like a pretty natural way to extend a class without modifying the class. Are you talking about traditonal OOP visitor pattern ? It's pretty terrible, when compared to std::variant ; imagine for instance that you want to have a way to draw the rect that contains a given shape without changing your shape class: https://godbolt.org/g/34KyMw https://godbolt.org/g/34KyMw
  - criddell 8y ago
    If your interface is that consistent, why not just make the bounding box drawing function a template? Visitor is nice when some kind of transformation needs to be applied to the visited objects.
isaachier 8y ago
If using C++ I'd usually just opt for a type enum and switch on it. Visitor is a better way of using type to dispatch, but harder to see control flow.

Did you have a previous experience of something easier? May you share about it?

by antpls 8y ago

Did you have a previous experience of something easier? May you share about it?

I'm curious about this as well. When writing my own interpreter it didn't take me long before I opted to use the visitor pattern, and once I did it felt like a natural fit.

chubot 8y ago

Not the OP, but I took a survey of the /r/programminglanguages reddit, where basically everybody has to deal with the "visitor issue". https://www.reddit.com/r/ProgrammingLanguages/comments/7w3oeh/whats_your_experience_with_visitors/ https://www.reddit.com/r/ProgrammingLanguages/comments/7w3oe... The top comment supported the use of the visitor pattern, but I would say that MOST people in the thread did NOT use visitors. You only really need a visitor if you have more than one traversal. And even then you don't necessarily need it.

criddell 8y ago

What is the actual problem? Is it multiple dispatch? I think some languages (Lisp may be one) handle it natively. Visitor is the only way I know of dealing with it in C++. Once you do it enough, Visitor starts to feel like a pretty natural way to extend a class without modifying the class.

jcelerier 8y ago
> Visitor is the only way I know of dealing with it in C++. Once you do it enough, Visitor starts to feel like a pretty natural way to extend a class without modifying the class. Are you talking about traditonal OOP visitor pattern ? It's pretty terrible, when compared to std::variant ; imagine for instance that you want to have a way to draw the rect that contains a given shape without changing your shape class: https://godbolt.org/g/34KyMw https://godbolt.org/g/34KyMw
- criddell 8y ago
  If your interface is that consistent, why not just make the bounding box drawing function a template? Visitor is nice when some kind of transformation needs to be applied to the visited objects.

isaachier 8y ago

If using C++ I'd usually just opt for a type enum and switch on it. Visitor is a better way of using type to dispatch, but harder to see control flow.