Few more updates to PEP 544: Protocols (#246)

* Drop Set's from protocols; add adapters to rejected ideas

* Add link to runtime implementation

pep-0544.txt

@@ -527,7 +527,7 @@ inferred from a protocol definition. Examples::
   new_sender = sender  # OK, type checker finds that 'Sender' is contravariant.
 
   class Proto(Protocol[T]):
-      attr: T
+      attr: T  # this class is invariant, since it has a mutable attribute
 
   var: Proto[float]
   another_var: Proto[int]
@@ -789,20 +789,19 @@ Changes in the typing module
 
 The following classes in ``typing`` module will be protocols:
 
-* ``Hashable``
-* ``SupportsAbs`` (and other ``Supports*`` classes)
+* ``Callable``
+* ``Awaitable``
 * ``Iterable``, ``Iterator``
+* ``AsyncIterable``, ``AsyncIterator``
+* ``Hashable``
 * ``Sized``
 * ``Container``
 * ``Collection``
 * ``Reversible``
 * ``Sequence``, ``MutableSequence``
-* ``AbstractSet``, ``MutableSet``
 * ``Mapping``, ``MutableMapping``
-* ``AsyncIterable``, ``AsyncIterator``
-* ``Awaitable``
-* ``Callable``
 * ``ContextManager``, ``AsyncContextManager``
+* ``SupportsAbs`` (and other ``Supports*`` classes)
 
 Most of these classes are small and conceptually simple. It is easy to see
 what are the methods these protocols implement, and immediately recognize
@@ -1119,6 +1118,39 @@ This was rejected for the following reasons:
   it has an unsafe override.
 
 
+Support adapters and adaptation
+-------------------------------
+
+Adaptation was proposed by PEP 246 (rejected) and is supported by
+``zope.interface``, see https://docs.zope.org/zope.interface/adapter.html.
+Adapters is quite an advanced concept, and PEP 484 supports unions and
+generic aliases that can be used instead of adapters. This can be illustrated
+with an example of ``Iterable`` protocol, there is another way of supporting
+iteration by providing ``__getitem__`` and ``__len__``. If a function
+supports both this way and the now standard ``__iter__`` method, then it could
+be annotated by a union type::
+
+  class OldIterable(Sized, Protocol[T]):
+      def __getitem__(self, item: int) -> T: ...
+
+  CompatIterable = Union[Iterable[T], OldIterable[T]]
+
+  class A:
+      def __iter__(self) -> Iterator[str]: ...
+  class B:
+      def __len__(self) -> int: ...
+      def __getitem__(self, item: int) -> str: ...
+
+  def iterate(it: CompatIterable[str]) -> None:
+      ...
+
+  iterate(A())  # OK
+  iterate(B())  # OK
+
+Since there is a reasonable alternative for such cases with existing tooling,
+it is therefore proposed not to include adaptation in this PEP.
+
+
 Backwards Compatibility
 =======================
 
@@ -1145,6 +1177,9 @@ https://github.com/ilevkivskyi/typeshed/tree/protocols. Installation steps::
   git fetch proto && git checkout proto/protocols
   cd .. && git add typeshed && sudo python3 -m pip install -U .
 
+The runtime implementation of protocols in ``typing`` module is
+found at https://github.com/ilevkivskyi/typehinting/tree/protocols.
+
 
 References
 ==========