Grammar question about ArrowFormalParameters
Allen Wirfs-Brock
allen at wirfs-brock.com
Fri Nov 14 09:20:56 PST 2014
On Nov 13, 2014, at 7:53 PM, Cyrus Najmabadi wrote:
> Specifically, while the spec says that :
>
> SomeProd[Yield,GeneratorParameter]
>
> Is equivalent to:
>
> 1) SomeProd
> 2) SomeProd_Yield
> 3) SomeProd_GeneratorParameter
> 4) SomeProd_Yield_GeneratorParameter
>
> It’s doesn’t seem like it is actually possible for any path of productions from the root to ever end with “3) SomeProd_GeneratorParameter”. Whereas we can see paths to every other production.
>
> Does that match your expectation? If not, what’s a path that could lead to such a state.
Yes, that's right. As a grammar production, "Yield" means that `yield` is to be interpreted as the yield keyword and not as an identifier. "GeneratorParameter" means that the context of the parameterized prodution is the formal parameter list of a generator function. The [Yield,GeneratorParameter]] production is used for situations where `yield` should not be treated as an identifier but where use of the yield operator is also illegal. The only such situation is the formal parameter list of generator functions.
It is never the case that the context is the formal parameter list of a generator and `yield` is treated as an identifier.
Allen
>
> Thanks!
>
> -- Cyrus
>
> From: Jason Freeman
> Sent: Thursday, November 13, 2014 7:10 PM
> To: Cyrus Najmabadi; Allen Wirfs-Brock
> Cc: es-discuss list
> Subject: RE: Grammar question about ArrowFormalParameters
>
> Thanks Allen for answering all of our questions. I have a theory that I wanted to run by you. It seems that the Yield and GeneratorParameter flags as a pair have only 3 possible states:
> · Yield and GeneratorParameter are both on
> · Yield and GeneratorParameter are both off
> · Yield is on, but GeneratorParameter is off
>
> Would you say that is correct?
>
> Thanks,
> Jason
>
> From: Cyrus Najmabadi
> Sent: Thursday, November 13, 2014 4:45 PM
> To: Allen Wirfs-Brock
> Cc: Jason Freeman; es-discuss list
> Subject: RE: Grammar question about ArrowFormalParameters
>
> That seems reasonable* Allen.
>
> And I appreciate confirmation that our reading of the spec is in line with yours.
>
> -- Cyrus
>
> * I personally prefer the approach that moving to new language constructs may come with additional restrictions that are more onerous. But that’s just me. For example, I don’t mind that Classes force ‘strict mode’ within them, and that you would not be able to copy “function (yield) { }” from outside the class to inside a class method. But that’s just me. It wouldn’t be javascript if there wasn’t lots of fun complexity for us compiler writers J
>
> From: Allen Wirfs-Brock [mailto:allen at wirfs-brock.com]
> Sent: Thursday, November 13, 2014 3:57 PM
> To: Cyrus Najmabadi
> Cc: Jason Freeman; es-discuss list
> Subject: Re: Grammar question about ArrowFormalParameters
>
>
> On Nov 13, 2014, at 3:21 PM, Cyrus Najmabadi wrote:
>
>
> 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 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
>
> This would permit:
>
> function *g() {
> yield (yield) => 42;
> }
>
> Because 'yield' is an identifier if ArrowFormalParameters isn't parameterized with [Yield]. Because arrow functions are syntactically so tightly integrated into expressions I didn't think it would be a good idea to allow that usage.
>
>
>
>
>
> 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();
> }
>
>
> yes, this is legal. It is JS legacy that 'yield' is a regular identifier (not a reserved word) in non strict mode code. We've tried to minimize the number of "micro-modes" so we didn't add additional special cases to make things like the above illegal. The function keyword strongly delimits the start of a new function definition. Identical rules are applied to all non-strict FunctionDeclaration and FunctionExpressions . We special case arrow functions because they lack that strong syntactic delimiter and seem more like part of the enclosing expression. Basically, definitions starting with 'function' can be though of as self-contained entities.
>
>
>
>
>
> 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?
>
> see above
>
> Allen
>
>
>
> 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 inhttp://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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