Associativity of fold-expressions

Question

N4191 proposed fold-expressions to C++. The definition there was that

(args + ...)

is a left-fold (i.e. (((a0 + a1) + a2) + ...),

Answer 1

I can't speak for the proposal, but the new, swapped definitions seem natural to me. My rationale for that is that (... + args) is a sub-expression of a left fold, and (args + ...) is a sub expression of a right fold. In fact, the former is the final segment, and the latter is the initial segment of the expression (I may not be using the correct terminology).

Here is how I would illustrate the expansion of the fold from the syntax:

Left fold

                     (... + args)
             (... + args) + a999)
     (... + args) + a998) + a999)

((...((a0 + a1) + a2)...) + a999)

Right fold

(args + ...)
(a0 + (args + ...)
(a0 + (a1 + (args + ...)

(a0 + (...(a997 + (a998 + a999))...))