Allen Wirfs-Brock <mailto:[email protected]>
September 3, 2013 10:45 AM
Yup, we went off-track on this at the meeting. But Waldemar's point
about arrow functions is still valid. We are going to need to have
ArrowFunctionNoIn to disallow things like:
for (f = x => x in foo;;)
For sure.
/be
allen
Brendan Eich <mailto:[email protected]>
September 1, 2013 7:42 PM
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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Allen Wirfs-Brock <mailto:[email protected]>
September 1, 2013 5:52 PM
Right, this point didn't come up when we talked about eliminating the
NoIn productions at the last TC39 meeting:
https://github.com/rwaldron/tc39-notes/blob/master/es6/2013-07/july-23.md#41-es6-status-report
Presumably the original motivation for the NoIn productions was to
resolve the ambiguity that the introduction of the ES3 'in' operator
created for the ES1 production:
IterationStatement : 'for' '(' 'var' Identifier [Initializer] 'in'
Expression ')' Statement
when parsing something like:
for (var x = a in b in c) ;
it could be either:
for (var x = (a in b) in c) ;
or
for (var x = a in (b in c)) ;
Because we agreed for ES6 to eliminate the optional Initializer from
for-in/for-of statements this ambiguity is no longer a possibility and
the NoIn productions are not needed to resolve them. At the last
meeting the discussion was about the grammar simplification benefits
of completely eliminating the NoIn productions. I did this in the
rev17 ES6 draft.
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 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.
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.
My preference is to simply allow the use of the 'in' operator in the
first expression of a for(;;) statement. 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
Brendan Eich <mailto:[email protected]>
August 31, 2013 9:21 PM
Allen, are you doing this some other way? Static semantics can't do
it, we need parametric productions or else ye olde NoIn splitting.
/be
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André Bargull <mailto:[email protected]>
August 31, 2013 4:21 AM
The NoIn grammar productions have been removed in rev17. Does this
mean that `for (a in b;;);` is now a valid (C-style) for-loop instead
of a SyntaxError?
Thanks,
André
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