Within a generator body Arrow function formals treat yield as keyword but function declarations and expressions do not
Is this intentional?
14.1 Function Definitions
---
FunctionDeclaration[Yield, Default] :
function BindingIdentifier[?Yield] ( FormalParameters ) { FunctionBody }
[+Default] function ( FormalParameters ) { FunctionBody }
FunctionExpression :
function BindingIdentifieropt ( FormalParameters ) { FunctionBody }
---
14.2 Arrow Function Definitions
---
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] )
---
Following the production rules for StrictFormalParameters and FormalParameters
you can end up at SingleNameBinding
SingleNameBinding[Yield, GeneratorParameter] :
[+GeneratorParameter] BindingIdentifier[Yield] Initializer[In]opt
[~GeneratorParameter] BindingIdentifier[?Yield] Initializer[In, ?Yield]opt
This looks like it suggests following:
var yield;
function* gf() {
function f(yield = yield) { } // valid parse, both yields are identifiers,
second binds to the global var
var f = function (yield = yield) { }; // same deal
var a = (yield = yield) = { }; // first yield is treated as keyword,
doesn't parse, second is treated as identifier and again binds to the global var
}
Binding to the global var in the default argument expressions is weird and
perhaps confusing but acceptable I suppose.
The inconsistency of the formal name treatment between functions and arrow
functions feels wrong to me given that the formal names are created in
non-generator scopes.
Ian
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RE: Specification of use before declaration errors
Wait, so is there no variable shadowing allowed then?
13.1.1http://people.mozilla.org/~jorendorff/es6-draft.html#sec-block-static-semantics-early-errors
Static Semantics: Early Errors
Block : { StatementList }
* It is a Syntax Error if the LexicallyDeclaredNames of StatementList
contains any duplicate entries.
* It is a Syntax Error if any element of the LexicallyDeclaredNames of
StatementList also occurs in the VarDeclaredNames of StatementList.
StatementList can contain Blocks whose LexicallyDeclaredNames and
VarDeclaredNames algorithms return the values for their StatementLists, so it
recursively collects the names from all nested lexical and var declarations.
It looks like VarDeclaredNames is missing a definition for VariableStatement
because I can't see any way for the bound names of a VariableStatement to get
added to VarDeclaredNames lists.
But barring for the moment that I cannot find an algorithm definition that adds
BoundNames of a VariableStatement to VarDeclaredNames, this second early error
bullet implies that shadowing of bound names is not allowed at all.
Is this correct?
Ian
From: Allen Wirfs-Brock [mailto:al...@wirfs-brock.com]
Sent: Friday, November 8, 2013 4:16 PM
To: Ian Halliday
Cc: es-discuss@mozilla.org
Subject: Re: Specification of use before declaration errors
On Nov 8, 2013, at 3:35 PM, Ian Halliday wrote:
Hello es-discuss,
I'm having difficulty figuring out where the ES6 draft spec specifies that a
use before declaration error should be thrown. My last understanding of the
temporal dead zone was that ECMAScript would always issue a use before
declaration error at runtime, regardless whether it can be statically
determined or not. However it seems like the evaluation semantics of var
declarations, for example, do not lead to any line that throws a ReferenceError.
That is, consider this code:
function f() {
{
var x = 5;
let x;
declaring the same name using a let and a var is an early error:
http://people.mozilla.org/~jorendorff/es6-draft.html#sec-block-static-semantics-early-errors
or for the function level
http://people.mozilla.org/~jorendorff/es6-draft.html#sec-function-definitions-static-semantics-early-errors
}
}
f();
I think the var declaration creates a binding for x in the function's lexical
environment, but then binds to the x in the block's environment for the
initialization. As such, the initialization should throw a use before
declaration error. But this is what I cannot find in the spec. Maybe I am
wrong about the semantics here?
because an early error exists, the surround script or module is never evaluated.
Allen
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RE: Specification of use before declaration errors
Oh, is shadowing a let declaration with a var declaration a syntax error?
E.g.
{
let x;
{
var x;
}
}
From: es-discuss [mailto:es-discuss-boun...@mozilla.org] On Behalf Of Ian
Halliday
Sent: Wednesday, November 13, 2013 4:14 PM
To: Allen Wirfs-Brock
Cc: es-discuss@mozilla.org
Subject: RE: Specification of use before declaration errors
Then for 13.1.8 shouldn't there be something like this
StatementListItem : Statement
1. If Statement is a Block then return a new empty List.
2. Else return VarDeclaredNames of Statement
defined in order to prevent the var names from spreading into enclosing blocks?
I might be misunderstanding VarDeclaredNames. I am guessing that it should be
a collection of all the names declared via var declaration statements, i.e.
VariableStatement, but there isn't a definition of VarDeclaredNames for
VariableStatement.
13.1.8http://people.mozilla.org/~jorendorff/es6-draft.html#sec-block-static-semantics-vardeclarednames
Static Semantics: VarDeclaredNames
See also:
13.0.1http://people.mozilla.org/~jorendorff/es6-draft.html#sec-statement-semantics-static-semantics-vardeclarednames,
13.5.1http://people.mozilla.org/~jorendorff/es6-draft.html#sec-if-statement-static-semantics-vardeclarednames,
13.6.1.1http://people.mozilla.org/~jorendorff/es6-draft.html#sec-do-while-statement-static-semantics-vardeclarednames,
13.6.2.1http://people.mozilla.org/~jorendorff/es6-draft.html#sec-while-statement-static-semantics-vardeclarednames,
13.6.3.1http://people.mozilla.org/~jorendorff/es6-draft.html#sec-for-statement-static-semantics-vardeclarednames,
13.6.4.3http://people.mozilla.org/~jorendorff/es6-draft.html#sec-for-in-and-for-of-statements-static-semantics-vardeclarednames,
13.10.2http://people.mozilla.org/~jorendorff/es6-draft.html#sec-with-statement-static-semantics-vardeclarednames,
13.11.4http://people.mozilla.org/~jorendorff/es6-draft.html#sec-switch-statement-static-semantics-vardeclarednames,
13.12.2http://people.mozilla.org/~jorendorff/es6-draft.html#sec-labelled-statements-static-semantics-vardeclarednames,
13.14.2http://people.mozilla.org/~jorendorff/es6-draft.html#sec-try-statement-static-semantics-vardeclarednames,
14.1.11http://people.mozilla.org/~jorendorff/es6-draft.html#sec-function-definitions-static-semantics-vardeclarednames,
14.4.10http://people.mozilla.org/~jorendorff/es6-draft.html#sec-generator-function-definitions-static-semantics-vardeclarednames,
14.5.14http://people.mozilla.org/~jorendorff/es6-draft.html#sec-class-definitions-static-semantics-vardeclarednames,
15.1.0.12http://people.mozilla.org/~jorendorff/es6-draft.html#sec-module-semantics-static-semantics-vardeclarednames,
15.2.5http://people.mozilla.org/~jorendorff/es6-draft.html#sec-scripts-static-semantics-vardeclarednames.
Block : { }
1. Return a new empty
Listhttp://people.mozilla.org/~jorendorff/es6-draft.html#sec-list-and-record-specification-type.
StatementList : StatementList StatementListItem
1. Let names be VarDeclaredNames of StatementList.
2. Append to names the elements of the VarDeclaredNames of
StatementListItem.
3. Return names.
StatementListItem : Declaration
1. Return a new empty
Listhttp://people.mozilla.org/~jorendorff/es6-draft.html#sec-list-and-record-specification-type.
From: Allen Wirfs-Brock [mailto:al...@wirfs-brock.com]
Sent: Wednesday, November 13, 2013 3:49 PM
To: Ian Halliday
Cc: es-discuss@mozilla.orgmailto:es-discuss@mozilla.org
Subject: Re: Specification of use before declaration errors
On Nov 13, 2013, at 3:41 PM, Ian Halliday wrote:
Wait, so is there no variable shadowing allowed then?
this is saying that things like the following are illegal:
{var x;
let x;
}
But shadowing, like the following is fine:
var x;
{let x;
}
13.1.1http://people.mozilla.org/~jorendorff/es6-draft.html#sec-block-static-semantics-early-errors
Static Semantics: Early Errors
Block : { StatementList }
* It is a Syntax Error if the LexicallyDeclaredNames of StatementList
contains any duplicate entries.
* It is a Syntax Error if any element of the LexicallyDeclaredNames of
StatementList also occurs in the VarDeclaredNames ofStatementList.
StatementList can contain Blocks whose LexicallyDeclaredNames and
VarDeclaredNames algorithms return the values for their StatementLists, so it
recursively collects the names from all nested lexical and var declarations.
It looks like VarDeclaredNames is missing a definition for VariableStatement
because I can't see any way for the bound names of a VariableStatement to get
added to VarDeclaredNames lists.
But barring for the moment that I cannot find an algorithm definition that adds
BoundNames of a VariableStatement to VarDeclaredNames, this second early error
bullet implies that shadowing of bound names is not allowed at all.
Is this correct?
Ian
From: Allen Wirfs-Brock [mailto:al...@wirfs-brock.com]
Sent: Friday, November 8, 2013
Should arrow functions have arguments or capture arguments bindings
The draft spec has a note in 9.2.13 Function Declaration Instantiation about
arrow functions and arguments:
Issure: should concise methods also not get an arguments object?
From rev16 it appears.
Section 9.2.13 also has a large banner stating that it is old and will change
re concensus made in the Sept 2013 TC39 meeting.
I can't see any mention of arguments and arrow functions in the Sept 2013
meeting notes so is it safe to assume this behavior hasn't changed? Given the
note is this still an open issue?
If arguments is not added to the arrow function's var bindings, does this mean
an arrow function should capture variables named arguments from the enclosing
scope?
E.g.
function f () {
var x = () = arguments; // captures f()'s implicit arguments
{
let arguments;
var y = () = arguments; // captures lexical local variable
}
}
Ian
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Specification of use before declaration errors
Hello es-discuss,
I'm having difficulty figuring out where the ES6 draft spec specifies that a
use before declaration error should be thrown. My last understanding of the
temporal dead zone was that ECMAScript would always issue a use before
declaration error at runtime, regardless whether it can be statically
determined or not. However it seems like the evaluation semantics of var
declarations, for example, do not lead to any line that throws a ReferenceError.
That is, consider this code:
function f() {
{
var x = 5;
let x;
}
}
f();
I think the var declaration creates a binding for x in the function's lexical
environment, but then binds to the x in the block's environment for the
initialization. As such, the initialization should throw a use before
declaration error. But this is what I cannot find in the spec. Maybe I am
wrong about the semantics here?
If I am not wrong then this raises the question of order of operations. Is the
RHS evaluated first, then the error is thrown? Or is the LHS name evaluated
first, throwing if it is a binding that is currently in its TDZ?
E.g. is g() called here before throwing?
function g() {
console.log('hello');
return 0;
}
function f() {
{
var x = g();
let x;
}
}
f();
How about for-in/of loops? Is g() called here?
function g() {
console.log('hello');
return { a: 1, b: 2, c: 3 };
}
function f() {
{
for (var x in g()) {
console.log('unreachable');
}
let x;
}
}
Ian Halliday
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