PEP 642: Add notes on reference implementation status (#1686)

pep-0642.rst

@@ -11,7 +11,7 @@ Content-Type: text/x-rst
 Requires: 634
 Created: 26-Sep-2020
 Python-Version: 3.10
-Post-History: 25-Oct-2020
+Post-History:
 Resolution:
 
 Abstract
@@ -44,6 +44,10 @@ pattern syntax:
 * The `_` identifier is no longer syntactically special (it is a normal capture
   pattern, just as it is an ordinary assignment target)
 
+Note: the reference implementation for this PEP is being built on the reference
+implementation for PEP 634. Once the implementation reaches a usable state,
+the PEP will be published to python-dev and discuss.python.org.
+
 
 Relationship with other PEPs
 ============================
@@ -628,6 +632,18 @@ In particular, if static analysers can't follow certain kinds of dynamic checks,
 then they can limit the permitted expressions at analysis time, rather than the
 compiler restricting them at compile time.
 
+Reference Implementation
+========================
+
+A reference implementation for this PEP is being derived from Brandt Bucher's
+reference implementation for PEP 634 [3_].
+
+Relative to the text of this PEP, the draft reference implementation will
+retain literal patterns as implemented for PEP 634 (as this PEP only removes
+them as redundant given constraint patterns, it doesn't inherently conflict with
+them). Value patterns, wildcard patterns, and mapping patterns are being updated
+to follow this PEP rather than PEP 634.
+
 
 Acknowledgments
 ===============
@@ -649,6 +665,9 @@ References
 .. [2] Declined pull request proposing to list this as a Rejected Idea in PEP 622
    https://github.com/python/peps/pull/1564
 
+.. [3] PEP 634 reference implementation
+   https://github.com/python/cpython/pull/22917
+
 
 .. _Appendix A:
 
@@ -665,10 +684,6 @@ Notation used beyond standard EBNF is as per PEP 534:
 - ``SEP.RULE+`` is shorthand for ``RULE (SEP RULE)*``
 - ``!RULE`` is a negative lookahead assertion
 
-Note: this still needs a reference implementation to confirm that the proposed
-      grammar is correct (in particular, ensuring the new wildcard and
-      constraint pattern definitions aren't considered ambiguous by the parser)
-
 ::
 
     match_stmt: "match" subject_expr ':' NEWLINE INDENT case_block+ DEDENT