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
State 18 conflicts: 1 reduce/reduce
Grammar
0 $accept: Program $end
1 Program: Statement
2 | Program Statement
3 Statement: FOR '(' Expression ';' Expression ';' Expression ')' Statement
4 | FOR '(' Expression IN Expression ')' Statement
5 | Expression ';'
6 Expression: Expression IN Primary
7 | Primary
8 Primary: NUMBER
9 | IDENT
Terminals, with rules where they appear
$end (0) 0
'(' (40) 3 4
')' (41) 3 4
';' (59) 3 5
error (256)
FOR (258) 3 4
IDENT (259) 9
IN (260) 4 6
NUMBER (261) 8
Nonterminals, with rules where they appear
$accept (10)
on left: 0
Program (11)
on left: 1 2, on right: 0 2
Statement (12)
on left: 3 4 5, on right: 1 2 3 4
Expression (13)
on left: 6 7, on right: 3 4 5 6
Primary (14)
on left: 8 9, on right: 6 7
state 0
0 $accept: . Program $end
FOR shift, and go to state 1
IDENT shift, and go to state 2
NUMBER shift, and go to state 3
Program go to state 4
Statement go to state 5
Expression go to state 6
Primary go to state 7
state 1
3 Statement: FOR . '(' Expression ';' Expression ';' Expression ')'
Statement
4 | FOR . '(' Expression IN Expression ')' Statement
'(' shift, and go to state 8
state 2
9 Primary: IDENT .
$default reduce using rule 9 (Primary)
state 3
8 Primary: NUMBER .
$default reduce using rule 8 (Primary)
state 4
0 $accept: Program . $end
2 Program: Program . Statement
$end shift, and go to state 9
FOR shift, and go to state 1
IDENT shift, and go to state 2
NUMBER shift, and go to state 3
Statement go to state 10
Expression go to state 6
Primary go to state 7
state 5
1 Program: Statement .
$default reduce using rule 1 (Program)
state 6
5 Statement: Expression . ';'
6 Expression: Expression . IN Primary
IN shift, and go to state 11
';' shift, and go to state 12
state 7
7 Expression: Primary .
$default reduce using rule 7 (Expression)
state 8
3 Statement: FOR '(' . Expression ';' Expression ';' Expression ')'
Statement
4 | FOR '(' . Expression IN Expression ')' Statement
IDENT shift, and go to state 2
NUMBER shift, and go to state 3
Expression go to state 13
Primary go to state 7
state 9
0 $accept: Program $end .
$default accept
state 10
2 Program: Program Statement .
$default reduce using rule 2 (Program)
state 11
6 Expression: Expression IN . Primary
IDENT shift, and go to state 2
NUMBER shift, and go to state 3
Primary go to state 14
state 12
5 Statement: Expression ';' .
$default reduce using rule 5 (Statement)
state 13
3 Statement: FOR '(' Expression . ';' Expression ';' Expression ')'
Statement
4 | FOR '(' Expression . IN Expression ')' Statement
6 Expression: Expression . IN Primary
IN shift, and go to state 15
';' shift, and go to state 16
state 14
6 Expression: Expression IN Primary .
$default reduce using rule 6 (Expression)
state 15
4 Statement: FOR '(' Expression IN . Expression ')' Statement
6 Expression: Expression IN . Primary
IDENT shift, and go to state 2
NUMBER shift, and go to state 3
Expression go to state 17
Primary go to state 18
state 16
3 Statement: FOR '(' Expression ';' . Expression ';' Expression ')'
Statement
IDENT shift, and go to state 2
NUMBER shift, and go to state 3
Expression go to state 19
Primary go to state 7
state 17
4 Statement: FOR '(' Expression IN Expression . ')' Statement
6 Expression: Expression . IN Primary
IN shift, and go to state 11
')' shift, and go to state 20
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)
state 19
3 Statement: FOR '(' Expression ';' Expression . ';' Expression ')'
Statement
6 Expression: Expression . IN Primary
IN shift, and go to state 11
';' shift, and go to state 21
state 20
4 Statement: FOR '(' Expression IN Expression ')' . Statement
FOR shift, and go to state 1
IDENT shift, and go to state 2
NUMBER shift, and go to state 3
Statement go to state 22
Expression go to state 6
Primary go to state 7
state 21
3 Statement: FOR '(' Expression ';' Expression ';' . Expression ')'
Statement
IDENT shift, and go to state 2
NUMBER shift, and go to state 3
Expression go to state 23
Primary go to state 7
state 22
4 Statement: FOR '(' Expression IN Expression ')' Statement .
$default reduce using rule 4 (Statement)
state 23
3 Statement: FOR '(' Expression ';' Expression ';' Expression . ')'
Statement
6 Expression: Expression . IN Primary
IN shift, and go to state 11
')' shift, and go to state 24
state 24
3 Statement: FOR '(' Expression ';' Expression ';' Expression ')' .
Statement
FOR shift, and go to state 1
IDENT shift, and go to state 2
NUMBER shift, and go to state 3
Statement go to state 25
Expression go to state 6
Primary go to state 7
state 25
3 Statement: FOR '(' Expression ';' Expression ';' Expression ')' Statement
.
$default reduce using rule 3 (Statement)
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