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Comment by TeMPOraL | Hacker News Reader

In such case I think I'd go for an internal-DRYing + copy-on-write approach. That is, two identical classes or entry points, one for each format; internally, they'd share all the common code. Over time, if something changes in one format but not the other, that piece of code gets duplicated and then changed, so the other format retains the original code, which it now owns.

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I believe this very method is very common in games - you have similar logic for entities, but some have divergences that could occur in unknown ways after playtesting or future development.

Tho if done haphazardly by someone inexperienced, you might end up with subtle divergences that might look like they're meant to be copies, and debugging them in the future by another developer (without the history or knowledge) can get hard.

Then someone would wonder why there are these two very similar pieces of code, and mistakenly try to DRY it in the hopes of improving it, causing subtle mistakes to get introduced...

I prefer the FP approach of separating data and logic. you could end up with a box of functions (logic) that can be reused by the different "entities".

Last time i checked the FP world is slowly producing ECS frameworks that are needed to make the game performant. They used to be nearly C++ (or OO) exclusive.

Is an entity component system really functional programming? I had the sense that functional programming was more about writing functions that are pure and referentially transparent, and making data immutable normally, which can make code simpler and more modular. It tends to use higher-order functions (recursive operators) more than direct recursion, because it's easier to verify correctness then. Rather than imperative loops and mutation, the meat of the program consists of the composition of many different functions, both small and large.

Entity component systems are pretty cool, as is functional programming, but I don't see the relation.

In addition, object-oriented languages seem well-suited to making entity component systems. There are some tutorials on them in different object-oriented programming languages:

C++: https://austinmorlan.com/posts/entity_component_system/

C#: https://matthall.codes/blog/ecs/

Common Lisp: https://edoput.it/2023/11/19/data-oriented-clos.html

> In such case I think I'd go for an internal-DRYing + copy-on-write approach.

I agree. The primary risk of presented by DRY is tight coupling code which only bears similarities at a surface level. Starting off by explicitly keeping the externa bits separate sounds like a good way to avoid the worst tradeoff.

Nevertheless I still prefer the Write Everything Twice (WET) principle, which means mostly the same thing, but following a clear guideline: postpone all de-duplication efforts until it's either obvious there's shared code (semantics and implementation) in >2 occurrences, and always start by treating separate cases as independent cases.

This is really good advice and a great way to think about it.

I like that approach.

I've had the mantra "inheritance is only for code reuse" and it's never steered me wrong.

Inheritance is only good for code reuse, and it’s a trick you only get to use once for each piece of code, so if you use it you need to be absolutely certain that the taxonomy you’re using it to leverage code across is the right one.

All ‘is-a so it gets this code’ models can be trivially modeled as ‘has-a so it gets this code’ patterns, which don’t have that single-use constraint… so the corollary to this rule tends towards ‘never use inheritance’.

Single use? No way that's what multiple inheritance and mixins are for. Inheritance being only for code reuse is explicitly about not creating a taxonomy. No more is-a just, "I need this code here." Hey this thing behaves like a mapping inherit from the MutipleMapping and get all the usual mapping methods for free. Hey this model needs created/updated_at, inherit from ChangeTracking and get those fields and helper methods for free.

Has-a doesn't make sense for code like the literal text reuse. It makes sense for composition and encapsulation.

Edit: I'm now realizing that Python has one of the only sane multiple inheritance implementations. It's no wonder the rest of y'all hate it.

It seems like OP is describing a shared interface, not necessarily inheritance.

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