Re: ECMAScript: Automatic Semicolon Insertion

[Simon Richter]

Hi,

On Tue, Dec 06, 2016 at 10:52:06PM -0500, Ricky wrote:

> Your syntax implies that [\n] should be treated as [;]. So why not use [\n] 
> as alternative?

Unfortunately, it's not that easy.

    var foo = 4
        + 5

is also allowed. This can lead to interesting hidden bugs[1], but is valid.
Automatic Semicolon Insertion[2] is defined as a last-resort mechanism during
parsing.

I'm currently trying to inline rules, e.g. creating rules like

    variable_statement:
        "var" variable_declaration_list ";" |
        "var" variable_declaration_list after_variable_statement;

    after_variable_statement:
        variable_statement |
        throw_statement |
        ...

listing the alternatives that imply a semicolon there, but this doesn't
account for closing a block with a right brace.

The irony is that the parser does almost the right thing -- it is stuck in
the state

    member_expression:
        primary_expression . |
        member_expression . "." identifier |
        member_expression . "[" assignment_expression_in "]" | ...

where it needs a lookahead token to decide between reducing to
primary_expression and shifting -- and the table lookup here gives the
error. I've also tried

    member_expression:
        primary_expression |
        primary_expression "var" { YYBACKUP(KW_VAR, YYSTYPE()); } |
        ...

which doesn't work, because I cannot back up there, and

    member_expression:
        primary_expression |
        primary_expression error |
        ...

which discards the "var" token.

   Simon

[1] 
https://github.com/GyrosGeier/test262/commit/e9a33d61ac725b4b353a7a20857f04c7d34fed3d
[2] https://es5.github.io/#x7.9