Articulating the ineffable. Programming language theory 
cognitive psychology. PhD @Stanford
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Maybe there’s a better term than “extensible record” for this sort of language feature. Now I’m curious if there are some interesting power using ECS for this sort of thing similar to that which you shared with visitor pattern 
I think so. Ex: common ECS pattern is tags, or data-less components (newtypes). You can dynamically add/remove a tag, and query for elements with a tag: Query<Entity, With<Tag>>. A static extensible system would need Option<Tag> on every entity, so more boilerplate.
Also, a key part of ECS is struct of arrays data layout. Extensible records with subtyping don’t get you this for free. ECS has a central World that stores multiple entities permitting SoA optimization.
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