what is the purpose of the expression grammar

Expression:
	AssignExpression
	AssignExpression , Expression

?

(emphasis on the comma)

Ellery Newcomer wrote:
> what is the purpose of the expression grammar
> 
> Expression:
> 	AssignExpression
> 	AssignExpression , Expression
> 
> ?
> 
> (emphasis on the comma)

To allow two expressions separated by a comma to be another expression. This is only useful if the expression before the last comma has side-effects of course, since its value isn't used. Such an expression returns the result of its right-hand side.
For example: (foo(), bar()) is an expression that calls foo(), then calls bar() and evaluates to the return value of bar().

This is mostly used in the increment clause of a for loop, like this:
    for (int i = 0; i < max; i++, someOtherCounter++)

Frits van Bommel:
> To allow two expressions separated by a comma to be another expression. This is
> only useful if the expression before the last comma has side-effects of course,
> since its value isn't used. Such an expression returns the result of its
> right-hand side.
> For example: (foo(), bar()) is an expression that calls foo(), then calls bar()
> and evaluates to the return value of bar().
> This is mostly used in the increment clause of a for loop, like this:

Outside for loops is a bug-prone and not much useful feature. (In python the comma is more useful and safer).

Bye,
bearophile