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> OCaml classes allow row polymorphism, but disallow immutable updates (immutable updates meaning create a new object with everything the same, but one element different.)

There's a functional update construct for objects:

http://caml.inria.fr/pub/docs/manual-ocaml/objectexamples.ht...

It works like { r with field = value }.

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That syntax works for records not objects. The {< _ = _ >} syntax is sort-of equivalent except it can only be used within method definitions (!) and is therefore too restricted.

Instead, try taking a look at the store-comonad mechanism I've got below: https://news.ycombinator.com/item?id=9213482

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