Re: Preceding action

[Hans Åberg]

> On 10 Nov 2017, at 19:17, Péter <address@hidden> wrote:
> 
> Can one assume anything about the order of grammar actions?
> At a given action, can one be sure which action was the previous (just 
> finished) action?
> 
> For example, in case of a list:
> 
> list: element {/*action xx*/} | list ',' element {/*action yy*/};
> element: A | B | C | D ;
> 
> At action yy, is it true (always) that the (immediately) preceding action 
> must be an action xx or action yy?
> 
> In other words: can one action start something and subsequent actions 
> continue and finish it? For example, taking the list elements one by one, 
> left to right, and appending them to a list (or concatenating to a buffer).

The $k values of the rule items that come before the action are on the stack 
when the action executes. (One can also have mid-rule actions-see the manual.)

So if you want to build a list, including the empty one, pseudocode for that 
might be, showing off the Bison feature of using action variable names $x, $y, 
etc., which help against numbering errors; and alos defining strings for token 
values, which is more flexible than just using single character tokens:

%token LEFT_BRACKET "["
%token RIGHT_BRACKET "]"
%token COMMA ","

%%
  list:
    "[" sequence "]"

  sequence:
      %empty { $$ = new list; }
    | sequence[x] "," element[y] { $$ = $x.append($y); }
  ;