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10  Expressions

A nest or a single ellipsis is allowed in some expression contexts, and causes ambiguity in others. For example, in a sequence expr, the nonterminal expr must be instantiated as an explicit C-language expression, while in an array reference, expr1 [ expr2 ], the nonterminal expr2, because it is delimited by brackets, can be also instantiated as , representing an arbitrary expression. To distinguish between the various possibilities, we define three nonterminals for expressions: expr does not allow either top-level nests or ellipses, nest_expr allows a nest but not an ellipsis, and dot_expr allows both. The EXPR macro is used to express these variants in a concise way.

expr   ::=  EXPR(expr)
nest_expr   ::=  EXPR(nest_expr)
|NEST(nest_expr, exp_whencode)
dot_expr   ::=  EXPR(dot_expr)
|NEST(dot_expr, exp_whencode)
|... [exp_whencode]
EXPR(exp)   ::=  exp assign_op exp
|exp metaidAssignOp exp
|exp++
|exp
|unary_op exp
|exp bin_op exp
|exp metaidBinOp exp
|expdot_exprexp
|(type) exp
|exp [dot_expr]
|expid
|exp -> id
|exp([PARAMSEQ(arg, exp_whencode)])
|id
|(type) { COMMA_LIST(init_list_elem) }
 |metaidExp
 |metaidConst
|const
|(dot_expr)
|OR(exp)
arg   ::=  nest_expr
|metaidExpList
exp_whencode   ::=  when != expr
assign_op   ::=  = | -= | += | *= | /= | %=
|&= | |= | ^= | <<= | >>=
bin_op   ::=  * | / | % | + | -
|<<| >>| ^ | & | |
|< | > | <= | >= | == | != | && | ||
unary_op   ::=  ++ || & | * | + | - | !

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