Brendan Eich wrote: > On Oct 15, 2008, at 2:36 PM, Waldemar Horwat wrote: > >> There is no such thing as a "let expression". > > Let expressions > <http://developer.mozilla.org/en/New_in_JavaScript_1.7#let_expressions> in > JS1.7 (Firefox 2+), based on the ES4 proposal > <http://wiki.ecmascript.org/doku.php?id=proposals:block_expressions>. > ES3-ish grammar: > > /LetExpression /*:* > * let*/ /*( VariableDeclarationList/ /*) [lookahead ∉ > {*{*}] AssignmentExpression** > > produced from PrimaryExpression.
That's not a valid grammar. You can't have an AssignmentExpression terminating a PrimaryExpression. It leads to trouble such as: let a = b + c being interpreted as both: (let a = b) + c and: let a = (b + c) >> It's mandatory because the grammar and semicolon insertion rules say >> so. Btw, to properly terminate a lambda expression you'd need *two* >> semicolons. Here's why one would be insufficient: >> >> f = lambda(x) x; >> (a + b) && c; >> >> would parse the body of the lambda as the expression "x;", the "(a + >> b)" as an argument to the lambda, and the rest as applying to the >> result of calling the lambda. What you'd want to write instead would be: >> >> f = lambda(x) x;; > > Expression closures > <http://developer.mozilla.org/en/New_in_JavaScript_1.8#Expression_closures> > in > JS1.8 (Firefox 3+) do not have this problem. Using ES3's notation: > > /FunctionDeclaration /*:* > * function */Identifier /*( */FormalParameterList//opt /*) > */FunctionBody/ > /FunctionExpression /*:* > * function */Identifier//opt /*( */FormalParameterList//opt /*) > */FunctionBody/ > /FormalParameterList /*:* > / Identifier/ > / FormalParameterList /*, */Identifier/ > /FunctionBody /*:* > / { *SourceElements */*}* > * */[lookahead ∉ {*{*}] AssignmentExpression/ That's not a valid grammar. For example, a = function(x) b+c can parse as, among other things: a = (function(x) b) + c which is probably not what you want. Waldemar _______________________________________________ Es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
