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Not quite sure what you're talking about but it sounds like you want lenses.

Mutability is definitely not less hassle or time if you account for the dramatic difference in error rates and maintenance costs.

If you add a new member to a type aren't you usually describing new behavior? Isn't it sensible to have to implement that behavior everywhere?

What language don't you have to do that in where not doing so also won't change behavior and result in an error?