Removal of NoIn grammar productions and for-loop parsing

Brendan Eich brendan at mozilla.com
Sun Sep 1 19:42:28 PDT 2013


Another item from the day of the July meeting that I happened to miss.

Allen Wirfs-Brock wrote:
> However, at the meeting, we did not discussion the fact that in ES3 
> NoIn was also used in:
>
> IterationStatement : 'for' '(' [ExpressionNoIn]   ';'  [Expression] 
>  ';' [Expression] ')' Statement
>
> This makes statements like this:
>
> for (a in b ;;) ;
>
> illegal in ES3.  This was presumably done for nanny reasons

No, not for nanny reasons.

> as using the 'in' operator in this position isn't ambiguous. The 
> availability of the NoIn productions also made it easy to express such 
> a restriction.

Consider this toy grammar:

%token FOR
%token IDENT
%token IN
%token NUMBER

%%

Program:
         Statement
|       Program Statement
;

Statement:
         FOR '(' Expression ';' Expression ';' Expression ')' Statement
|       FOR '(' Expression IN Expression ')' Statement
|       Expression ';'
;

Expression:
         Expression IN Primary
|       Primary
;

Primary:
         NUMBER
|       IDENT
;

This grammar is ambiguous: as bison(1) says,

state 18

     6 Expression: Expression IN Primary .
     7           | Primary .

     IN        reduce using rule 6 (Expression)
     IN        [reduce using rule 7 (Expression)]
     ')'       reduce using rule 7 (Expression)
     $default  reduce using rule 6 (Expression)

See attached file for full output.


> But if we eliminate the NoIn productions it's no longer so easy to 
> impose that restriction.  I may be able to come up with some other 
> static semantic mechanism to express that restriction but it will have 
> complexity similar to the NoIn productions.

You can'd do this via static semantics, as I said in my last message. We 
need an LR(1) grammar, that was always a consensus requirement.

> My preference is to simply allow the use of the 'in' operator in the 
> first expression of a for(;;) statement.

Ambiguity is not a matter of preference. We need to validate the ES6 
grammar. Until then, please put back the NoIn productions. They were not 
there only because of the silly and unwanted initialiser option for 'for 
(var x = y of z)'. They were there because for-in and for(;;) have 
prefixes in common up to arbitrary lookahead.

/be


>  This is what the rev17 grammer does.  As it is currently illegal in 
> ES<=5.1,  allowing 'in' use in that context is an extension rather 
> than a breaking change. 'a in 'b may not be very useful in that 
> position but neither is 'a + b'.  The simplification of the expression 
> grammar is a pretty big win both now and for future extensions.
>
> Allen
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