API for internally calculated fields

[tpapp]

I read the docs but it is still not clear how one should define the relevant interface for types that have internally calculated fields. Eg

struct Foo
    a
    b
    c
    Foo(a, b) = new(a, b, hypot(a, b))
end

I want the user to be able to modify (= create a new immutable with) a and b, and pretend that c does not even exist.

[jw3126]

I agree this is a pain point. What I generally recommend is using setproperties/getproperties for public access and constructorof for private access. See also #54 (comment)

[tpapp]

Sorry if I am missing the obvious, but could you please provide a suggestion to deal with the above struct so that eg
Accessors.@optic _.a etc works to modify a?

[jw3126]

Did you try to overload ConstructionBase.setproperties?