Why is declaration-seq in the C++ standard written this way?

Question

declaration-seq:
   declaration
   declaration-seq declaration

not this way:

declaration-seq:
   declaration
   declaration declaration-seq

Are the two definitions interchangeable? What is the difference between them?

Answer 1

This is a vestige of C++'s C heritage. The C grammar is (almost) LALR(1), and therefore uses left recursion as much as possible. The C++ grammar is not even vaguely LALR anymore, but many of the rules are still written in the form an LALR parser would prefer, because there's no reason to change them—any parser algorithm powerful enough to handle C++ doesn't care which type of syntactic recursion is used.