[issue43892] Make match patterns explicit in the AST

[Nick Coghlan]

Nick Coghlan <ncoghlan at gmail.com> added the comment:

The draft PR moved because I messed up the previous branch name: https://github.com/ncoghlan/cpython/pull/9/files

It's to the point where everything except symtable.c compiles.

I put an initial skeleton of a validator in, but only enough to check that the parallel sequence lengths in the class and mapping pattern nodes are consistent - there's still plenty to be done in Batuhan's PR (e.g. because the AST constant folding now just delegates to the expression folding functions for MatchValue values and MatchMapping keys, this iteration only enforces the restrictions on permitted subexpressions in the surface syntax).


In addition to Brandt's MatchStar suggestion, I've also tweaked the MatchClassparameter names to better match PEP 634 (the old names were based on PEP 642's attribute pattern concept, so "extra_*" made sense, but "kwd_*" makes more sense for PEP 634):

    pattern = MatchAlways
         | MatchValue(expr value)
         | MatchConstant(constant value)
         | MatchSequence(pattern* patterns)
         | MatchMapping(expr* keys, pattern* patterns)
         | MatchClass(expr cls, pattern* patterns, identifier* kwd_attrs, pattern* kwd_patterns)

         | MatchStar(identifier? target)
         -- A NULL entry in the MatchMapping key list handles capturing extra mapping keys

         | MatchAs(pattern? pattern, identifier target)
         | MatchOr(pattern* patterns)
          attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)

I think the idea of making the MatchMapping node signature "MatchMapping(expr* keys, pattern* patterns, identifier? rest)" has merit, but I'd like to get this initial iteration reviewed first to minimise the diff in the compiler_pattern_mapping implementation (right now it is fairly clear that the code generation for mapping patterns hasn't changed, but that will become significantly less obvious if/when the "**rest" handling changes away from working the same way _PyAST_Dict works)

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