用于JavaScript / ECMAScript数组文字生产的LOOKAHEADs [英] LOOKAHEADs for the JavaScript/ECMAScript array literal production

问题描述

我目前正在实现JavaScript / ECMAScript 5.1 使用JavaCC解析器，并且 ArrayLiteral 生产。

I currently implementing a JavaScript/ECMAScript 5.1 parser with JavaCC and have problems with the ArrayLiteral production.

ArrayLiteral :
    [ Elision_opt ]
    [ ElementList ]
    [ ElementList , Elision_opt ]

ElementList :
    Elision_opt AssignmentExpression
    ElementList , Elision_opt AssignmentExpression

Elision :
    ,
    Elision ,

我有三个问题，我将一个接一个地问他们。

I have three questions, I'll ask them one by one.

这是第二个。

我已将此生产简化为以下形式：

I have simplified this production to the following form:

ArrayLiteral:
    "[" ("," | AssignmentExpression ",") * AssignmentExpression ? "]"

请查看第一个问题是否正确：

Please see the first question on whether it is correct or not:

如何简化JavaScript / ECMAScript数组文字的生成？

现在我已经尝试在JavaCC中实现它，如下所示：

Now I have tried to implement it in JavaCC as follows:

void ArrayLiteral() :
{
}
{
    "["
    (
        ","
    |   AssignmentExpression()
        ","
    ) *
    (
        AssignmentExpression()
    ) ?
    "]"
}

JavaCC抱怨，或 AssignmentExpression （其内容）。显然，需要 LOOKAHEAD 规范。我花了很多时间试图弄清楚 LOOKAHEAD 的问题，尝试了类似的事情

JavaCC complains about ambiguous , or AssignmentExpression (its contents). Obviously, a LOOKAHEAD specification is required. I have spent a lot of time trying to figure the LOOKAHEADs out, tried different things like

• LOOKAHEAD（AssignmentExpression（），）在（...）*
中

• LOOKAHEAD（AssignmentExpression（）]） in （...）？

• LOOKAHEAD (AssignmentExpression() ",") in (...)*
• LOOKAHEAD (AssignmentExpression() "]") in (...)?

和其他一些变体，但我无法摆脱JavaCC警告。

and a few other variations, but I could not get rid of the JavaCC warning.

我不明白为什么它不起作用：

I fail to understand why this does not work:

void ArrayLiteral() :
{
}
{
    "["
    (
        LOOKAHEAD ("," | AssignmentExpression() ",")
        ","
    |   AssignmentExpression()
        ","
    ) *
    (
        LOOKAHEAD (AssignmentExpression() "]")
        AssignmentExpression()
    ) ?
    "]"
}

好， AssignmentExpression （）本身是模棱两可的，但结尾的， 或] 在 LOOKAHEAD 中应该清楚应该选择哪个选项-还是我在这里弄错了？

Ok, AssignmentExpression() per se is ambiguous, but the trailing "," or "]" in LOOKAHEADs should make it clear which of the choices should be taken - or am I mistaken here?

对于此产品，正确的 LOOKAHEAD 规范是什么样的？

What would a correct LOOKAHEAD specification for this production look like?

更新

不幸的是，这不起作用：

This did not work, unfortunately:

void ArrayLiteral() :
{
}
{
    "["
    (
        ","
    |
        LOOKAHEAD (AssignmentExpression() ",")
        AssignmentExpression()
        ","
    ) *
    (
        AssignmentExpression()
    ) ?
    "]"
}

警告：

Warning: Choice conflict in (...)* construct at line 6, column 5.
         Expansion nested within construct and expansion following construct
         have common prefixes, one of which is: "function"
         Consider using a lookahead of 2 or more for nested expansion.

第6行是（ code> LOOKAHEAD 。常见的前缀函数 只是 AssignmentExpression 的可能起始位置。

Line 6 is ( before the first LOOKAHEAD. The common prefix "function" is simply one of the possible starts of AssignmentExpression.

推荐答案

这是另一种方法，它的优点是可以识别哪些逗号表示未定义的元素而没有

Here is yet another approach. It has the advantage of identifying which commas indicate an undefined elements without using any semantic actions.

void ArrayLiteral() : {} { "[" MoreArrayLiteral() }

void MoreArrayLiteral() : {} {
    "]"
|    "," /* undefined item */ MoreArrayLiteral()
|    AssignmentExpression() ( "]" |  "," MoreArrayLiteral() )
}

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