API for internally calculated fields

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
```julia
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. 