Will Crichton

The benefit of traits vs. classes is that I can add *functions* to existing objects without needing to modify a monolithic definition. The benefit of ECS vs. structs is that I can add *data* to existing objects without needing to modify a monolithic definition.

I feel like ECS is a hacky workaround for a language’s lack of extensible records in the same way the “visitor pattern” is a hacky workaround for a language’s lack of algebraic data types and pattern matching. Or is there something else there?

I’ve never seen a language with extensible records, so can’t say 100%. I searched and found Elm, but it just lets you define type aliases. You would need structural subtyping to make that work as traits do. Are you thinking of a specific lang?

The most experience I have is with OCaml: OCaml’s object system can be used as extensible records but they’re a bit limited because you aren’t allowed to refer to the row variable. I’ve heard Purescript does a really good job with their extensible records as well.