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Date: Sun, 13 Nov 2022 14:08:01 -0600
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Subject: [RFC] Asymmetric Visibility, with readonly
From: larry@garfieldtech.com ("Larry Garfield")

Hi folks.  Ilija is nearly done with the implementation for asymmetric visibility and flushing out edge cases, but we've run into one design question we'd like feedback on.

There's two design decisions we've made at this point, both of which we think are logical and reasonable:

1. If specified, the set visibility must be tighter than the get visibility.  So `protected protected(set)` and `protected public(set)` are not permitted, for instance.

2. `readonly` is a "write once" flag that may be combined with asymmetric visibility.  If no set visibility is specified, `readoly` implies `private(set)`, but a different set visibility may also be provided.

These are both reasonable rules.  However, it creates a conflict.  Specifically, in the following cases:

public public(set) readonly string $foo

protected protected(set) readonly string $foo

These would be the only way to have a non-private-set readonly property.  While the first is in practice quite unlikely, the second has valid use cases.  (In particular, a base class that provides properties expected to be set by a child constructor, and then used by a method in the parent class.)  However, it would not be allowed under the rules above.  Working around it would require specifying `public protected(set) readonly...`, which means exposing a property that likely should not be exposed.

That creates an odd situation where readonly and asymmetric visibility may only be combined "sometimes."  That is not deesireable.  The only way to combine them in their current form is to allow `protected protected(set)` only if readonly is in use, which is excessively complicated both to implement and to explain/document/use.

We see two possible ways to resolve this conflict:

1. Relax the set-is-tighter restriction.  That would allow `protected protected(set)` etc. on any property.  It wouldn't be particularly useful unless readonly is being used, but it would be syntactically legal and behave as you'd expect.  We could still disallow "set is more permissive" combinations (eg, `private public(set)`), as those have no apparent use case.

2. Disallow readonly and asymmetric visibility being combined, because readonly already has a hard-coded implied asymmetric visibility.  This option removes some potential use cases (they would most likely drop the readonly), but has the upside that it's easier to re-allow at some point in the future.

3. Some other brilliant idea we've not thought of.


Both are viable approaches with pros and cons.  We're split on which way to go with this, so we throw it out to the group for feedback.  Which approach would you favor, or do you have some other brilliant idea to square this circle?


-- 
  Larry Garfield
  larry@garfieldtech.com