permit with a class which is not a subtype is allowed

[John Rose]

On Sep 3, 2019, at 8:25 AM, forax at univ-mlv.fr wrote:
> 
> Conceptually, i've a hard time to think that i want simultaneously a closed hierarchy and let the subtype open by specifying only it's name that may exist or not or that may not visible.

This is a very narrow corner case.  There are known-useful wider
corner cases involving an open subtype, as follows:

public sealed interface Container permits TrustedC, GeneralC { }

private interface TrustedC { }
//or non-exported public interface TrustedC { }
//or public final class TrustedC { }

public interface GeneralC { }
//or public abstract class GeneralC { }

Here, TrustedC is a constrained implementation of Container, while
GeneralC is unconstrained.  (In the case of an abstract class, it is
differently constrained).

An API which accepts a Container operand can test whether it is
of the trusted subtype and perform optimized access that assumes
that the operand behaves properly.  Meanwhile, users of the API
can create less-constrained implementations of the same interface.
Operands of those less-constrained implementations can be treated
with more care, perhaps by making temporary copies if race conditions
are a concern.

The above design pattern would allow us to define things like
immutable lists with controlled access for untrusted implementors.

The design pattern is not exactly what you requested, Remi, but it
is similar enough to suggest that there might be times when the
“more open” arms of the pattern (GeneralC) could be in different
packages or even modules, which implies a sort of loose coupling.

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