Oh, is "shadowing" a let declaration with a var declaration a syntax error?
E.g.
{
let x;
{
var x;
}
}
From: es-discuss [mailto:[email protected]] On Behalf Of Ian
Halliday
Sent: Wednesday, November 13, 2013 4:14 PM
To: Allen Wirfs-Brock
Cc: [email protected]
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.
13.1.8<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-block-static-semantics-vardeclarednames>
Static Semantics: VarDeclaredNames
See also:
13.0.1<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-statement-semantics-static-semantics-vardeclarednames>,
13.5.1<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-if-statement-static-semantics-vardeclarednames>,
13.6.1.1<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-do-while-statement-static-semantics-vardeclarednames>,
13.6.2.1<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-while-statement-static-semantics-vardeclarednames>,
13.6.3.1<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-for-statement-static-semantics-vardeclarednames>,
13.6.4.3<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-for-in-and-for-of-statements-static-semantics-vardeclarednames>,
13.10.2<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-with-statement-static-semantics-vardeclarednames>,
13.11.4<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-switch-statement-static-semantics-vardeclarednames>,
13.12.2<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-labelled-statements-static-semantics-vardeclarednames>,
13.14.2<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-try-statement-static-semantics-vardeclarednames>,
14.1.11<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-function-definitions-static-semantics-vardeclarednames>,
14.4.10<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-generator-function-definitions-static-semantics-vardeclarednames>,
14.5.14<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-class-definitions-static-semantics-vardeclarednames>,
15.1.0.12<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-module-semantics-static-semantics-vardeclarednames>,
15.2.5<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-scripts-static-semantics-vardeclarednames>.
Block : { }
1. Return a new empty
List<http://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
List<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-list-and-record-specification-type>.
From: Allen Wirfs-Brock [mailto:[email protected]]
Sent: Wednesday, November 13, 2013 3:49 PM
To: Ian Halliday
Cc: [email protected]<mailto:[email protected]>
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;
}
13.1.1<http://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:[email protected]]
Sent: Friday, November 8, 2013 4:16 PM
To: Ian Halliday
Cc: [email protected]<mailto:[email protected]>
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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