Oh, so subtle. You're right, the missing semi makes it a syntax error. Thanks for clearing that up :)
- peter On Wed, May 24, 2017 at 11:10 AM, Andreas Rossberg <[email protected]> wrote: > Note that the arrow function can only form an ExpressionStatement, which > requires a terminating semicolon. There is none in this example, nor a line > break that would allow one to be inserted implicitly. So when reaching the > first `/` a syntax error is apparent, since there is no way the input can > still form a well-formed statement at that point. > > On 24 May 2017 at 10:32, Peter van der Zee <[email protected]> wrote: >> >> > Unlike an ordinary function expression, which is a PrimaryExpression, an >> > arrow function is merely an AssigmentExpression, so has much lower >> > precedence than any arithmetic operator. >> >> I'm curious how this should be parsed so let's break this down. >> >> Given the following "Script" (I don't think the actual goal matters much >> here): >> ``` >> x=x=>{}/alert(1)/+alert(2)// >> ``` >> >> Script : >> ScriptBody opt >> >> ScriptBody : >> StatementList >> >> StatementList [Yield, Return] : >> StatementListItem [?Yield, ?Return] >> StatementList [?Yield, ?Return] StatementListItem [?Yield, ?Return] >> >> StatementListItem [Yield, Return] : >> Statement [?Yield, ?Return] >> Declaration [?Yield] >> >> Statement [Yield, Return] : >> BlockStatement [?Yield, ?Return] >> VariableStatement [?Yield] >> EmptyStatement >> ExpressionStatement [?Yield] >> ... (trunced) >> >> ExpressionStatement [Yield] : >> [lookahead ∉ { { , function , class , let [ }] Expression [In, ?Yield] ; >> >> Expression [In, Yield] : >> AssignmentExpression [?In, ?Yield] >> Expression [?In, ?Yield] , AssignmentExpression [?In, ?Yield] >> >> AssignmentExpression [In, Yield] : >> ConditionalExpression [?In, ?Yield] >> [+Yield] >> YieldExpression [?In] >> ArrowFunction [?In, ?Yield]LeftHandSideExpression [?Yield] = >> AssignmentExpression [?In, ?Yield] >> LeftHandSideExpression [?Yield] AssignmentOperator >> AssignmentExpression [?In, ?Yield] >> >> I hope we can agree that the leading `x=` is consumed by >> "LeftHandSideExpression [?Yield] AssignmentOperator" in the very last >> rule above. Proceeding with "AssignmentExpression" from the arrow arg. >> >> Note that there is no other rule that applies up to this point. >> >> ArrowFunction [In, Yield] : >> ArrowParameters [?Yield] [no LineTerminator here] => ConciseBody [?In] >> >> ArrowParameters [Yield] : >> BindingIdentifier [?Yield] >> CoverParenthesizedExpressionAndArrowParameterList [?Yield] >> >> Here "CoverParenthesizedExpressionAndArrowParameterList" will consume >> the second `x` and then the only rule in "ArrowFunction" will consume >> the arrow (`=>`). Continueing to parse the remainder >> `{}/alert(1)/+alert(2)//` starting at "ConciseBody". >> >> ConciseBody [In] : >> [lookahead ≠ {] AssignmentExpression [?In] >> { FunctionBody } >> >> Obviously only the second rule applies so we parse the function body >> and the curlies. We parse greedy but the function body is empty so >> only the next two chars are consumed (`{}`). Parser has >> `/alert(1)/+alert(2)//` left to parse and the "Statement" rule has >> depleted it's options. So we go back to "StatementList" and parse >> another statement. This should result in a regular expression, a plus >> operator, a call expression, and a single line comment. >> >> I don't think there's a rule here that allows parsing operators after >> an explicit arrow function body as being part of the arrow function. >> In fact, I remember that this was explicitly designed this way to >> prevent this ambiguity. Beyond that I agree that it parses similar to >> function expressions. >> >> If this was wrong I'd love to know the right way to parse this. >> >> - peter >> >> >> >> On Wed, May 24, 2017 at 9:18 AM, Andreas Rossberg <[email protected]> >> wrote: >> > On 24 May 2017 at 08:57, Gareth Heyes <[email protected]> >> > wrote: >> >> >> >> >> >>> >> >>> you'll get a SyntaxError in all browsers but Edge, which interprets it >> >>> as >> >>> `(x => {}) * alert(1)`. >> >>> >> >>> Given how confusing that expression is, I think that the SyntaxError >> >>> is >> >>> the right choice. >> >> >> >> >> >> Well it is a function expression. So IMO Edge is right. It's >> >> equivalent >> >> to: >> >> x=function(){} * alert(1) >> > >> > >> > Edge is wrong. Unlike an ordinary function expression, which is a >> > PrimaryExpression, an arrow function is merely an AssigmentExpression, >> > so >> > has much lower precedence than any arithmetic operator. The rationale is >> > that its body doesn't necessarily have braces, so `x => x * 1` would be >> > ambiguous. >> > >> > _______________________________________________ >> > es-discuss mailing list >> > [email protected] >> > https://mail.mozilla.org/listinfo/es-discuss >> > > > _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
