Grammar question about ArrowFormalParameters

[Cyrus Najmabadi]

Hey Allen,

We have a followup question.  This time it's more about understanding the rationale behind things, rather than asking what the spec is explicitly stating.

Specifically, I'm curious why the spec says this in "14.2.1<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-arrow-function-definitions-static-semantics-early-errors> Static Semantics: Early Errors":

If the [Yield] grammar parameter is present for CoverParenthesizedExpressionAndArrowParameterList[Yield] return the result of parsing the lexical token stream matched by CoverParenthesizedExpressionAndArrowParameterList[Yield] using ArrowFormalParameters[Yield, GeneratorParameter] as the goal symbol

Instead of:

If the [Yield] grammar parameter is present for CoverParenthesizedExpressionAndArrowParameterList[Yield] return the result of parsing the lexical token stream matched by CoverParenthesizedExpressionAndArrowParameterList[Yield] using ArrowFormalParameters as the goal symbol


Your example seems to exemplify the issue, but we're not sure we understand why arrow function are treated this way.  For example, you say (and we agree) that this should be illegal as per the spec:

var yield = 42;
function *g() {
   var f = (arg=yield) => arg;  //it  is a syntax error to use 'yield' within an arraw parameter list inside a generator function
   yield f();
}

However, it appears as if the following would be legal:

var yield = 42;
function *g() {
   var f = function (arg=yield) { return arg;}
   yield f();
}

This is because of the following rule:

FunctionExpression : See 14.1
function BindingIdentifieropt ( FormalParameters ) { FunctionBody }

Which explicitly does not use [Yield] or [Generatorparameter] on the FormalParameters.


1)      do you think that code should be legal or not.  If not, what part of the spec does make it illegal?

2)      If it is legal, do you think that Arrow Functions and normal function expressions should behave differently here?  If so, why?

Thanks!

            -- Cyrus




From: es-discuss [mailto:es-discuss-bounces at mozilla.org] On Behalf Of Cyrus Najmabadi
Sent: Thursday, November 13, 2014 2:45 PM
To: Allen Wirfs-Brock
Cc: Jason Freeman; es-discuss list
Subject: RE: Grammar question about ArrowFormalParameters

Hi Allen,

I see.  This was the part that was missed:

If the [Yield] grammar parameter is present for CoverParenthesizedExpressionAndArrowParameterList[Yield] return the result of parsing the lexical token stream matched by CoverParenthesizedExpressionAndArrowParameterList[Yield] using ArrowFormalParameters[Yield, GeneratorParameter] as the goal symbol

That does clear up the grammar question.

Thanks much!

             -- Cyrus



From: Allen Wirfs-Brock [mailto:allen at wirfs-brock.com]
Sent: Thursday, November 13, 2014 2:25 PM
To: Cyrus Najmabadi
Cc: es-discuss list; Jason Freeman
Subject: Re: Grammar question about ArrowFormalParameters


On Nov 12, 2014, at 4:46 PM, Cyrus Najmabadi wrote:

Hey ES6ers,

I'm currently implementing some of the ES6 support for the next version of TypeScript.  The part I'm looking at right now is generators and yield expressions.  So far we feel fairly comfortable with the grammar and understand the implications of the [Yield] and [GeneratorParameter].  One spec issue that is getting us to scratch our heads though is this section:

When the production
ArrowParameters[Yield] : CoverParenthesizedExpressionAndArrowParameterList[?Yield]

is recognized the following grammar is used to refine the interpretation of
CoverParenthesizedExpressionAndArrowParameterList:

ArrowFormalParameters[Yield, GeneratorParameter] :
    ( StrictFormalParameters[?Yield, ?GeneratorParameter] )

The issue relates to the [GeneratorParamater] parameter on ArrowFormalParameters.  We can't see any path through the grammar that could ever end up enabling this parameter.  While CoverParenthesizedExpressionAndArrowParameterList picks up the 'yield' parameter from ArrowParameters, there seems to be nothing related to 'GeneratorParameter'.

See static semantic rules of http://people.mozilla.org/~jorendorff/es6-draft.html#sec-arrow-function-definitions-static-semantics-early-errors and the second algorithm in http://people.mozilla.org/~jorendorff/es6-draft.html#sec-static-semantics-coveredformalslist


We also find the presence of this grammar parameter here to be somewhat odd as arrow function can't be generators.

It is dealing with code such as this:

var yield = 42;
function *g() {
   var f = (arg=yield) => arg;  //it  is a syntax error to use 'yield' within an arraw parameter list inside a generator function
   yield f();
}


Is this an issue with the spec?  Or is there some subtlety here that we've missed that enables this parameter?

It's subtle.

Allen

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