Record invariants vs constructor/destructor identity

[Victor Nazarov]

Recent discussion on amber experts list considers particularities of
constructor overriding for records.
There seems to be a tension between two requirements:

1. Records should be it's state only state and nothing but state
2. Records should enforce some requirements considering state

(There was a third requirement that records should allow derived state that
facilitate more optimizations, but I won't touch this point in this
message).

What I'd like to point out is that ML-family languages already implement
first maxima that "Records should be it's state only state and nothing but
state" and simultaneously deals with enforcement of invariants.
And I think the was it is implemented in languages like Haskell,
StandardML, etc is quite successful.

In ML-laguages constructor and destructor ALWAYS form an identity and users
are quite accustomed to and rely on this. But types like `Rational` needs
to preprocess it's data to be useful and you can do it in ML-languages. To
do it, you just need to (TM) use a mechanism that you already have: private
constructor. So `Rational` becomes:

    private-constructor record Rational(int num, int denom) {
          public static Rational makeRational(int num, int denom) {
              int gcd = gcd(num, denim);
              num /= gcd;
              denom /= gcd;
              return new Rational(num, denom);
          }
    }

User knows that constructor/destructor identity always holds in any order,
user gets access to all internal representation, but user can't break class
invariant because they don't have access to direct constructor call.
User model remains clear and simple.

The following properties always hold

      record Foo(int x, int y)
      assert new Foo(1, 2).x() == 1
      assert new Foo(1, 2).y() == 2

but sometimes user can't create every representable record instance.
Factory-method is here to moderate instance creation. And factory methods
are already known mechanism for Java-programmers.

--
Victor Nazarov