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 :)
From: Allen Wirfs-Brock [mailto:[email protected]]
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<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
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:[email protected]] 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:[email protected]]
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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