What does bitwise_or mean in PEG grammar? More accurately, there are lots of bitwise_or in contexts where it's not even parsing for | yet there are occurrences. Does bitwise_or serve any other purpose in PEG other than being the | in Python?
Example extracted from Python PEG:-
comparison[expr_ty]:
| a=bitwise_or b=compare_op_bitwise_or_pair+ {
_PyAST_Compare(
a,
CHECK(asdl_int_seq*, _PyPegen_get_cmpops(p, b)),
CHECK(asdl_expr_seq*, _PyPegen_get_exprs(p, b)),
EXTRA) }
| bitwise_or
Note the word bitwise_or here. The question is about that not the vertical bar in PEG.
The "bitwise or operator" aka | has the lowest precedence of regular binary operators. The only binary operators with lower precedence are comparison operators, which are subject to chaining – for example, a < b < c is roughly equivalent to a < b and b < c – and thus behave specially.
For a PEG parser, precedence is usually encoded using precedence climbing. That means a lower precedence clause matches either itself or the next precedence clause. As such, the operator precedence "| < ^ < & < ..." is encoded in a ladder of bitwise or, bitwise xor, bitwise and, and so on:
bitwise_or:
| bitwise_or '|' bitwise_xor
| bitwise_xor
bitwise_xor:
| bitwise_xor '^' bitwise_and
| bitwise_and
bitwise_and:
| bitwise_and '&' shift_expr
| shift_expr
This makes bitwise_or the "entry point" that matches all binary operators: it may defer to bitwise_xor, which may defer to bitwise_and, and so on to the highest-precedence operator. Notably, it means the grammar rule bitwise_or can match input that does not contain the operation "bitwise or" – for example, bitwise_or matches a ^ b.
Thus, bitwise_or is used in any position a binary operator may occur.