a correction below, with that correct my alternative 2 appears to be equivalent to Sam's answers
On Jan 31, 2012, at 9:20 AM, Allen Wirfs-Brock wrote: > > On Jan 31, 2012, at 8:27 AM, Sam Tobin-Hochstadt wrote: > >> On Tue, Jan 31, 2012 at 9:30 AM, Andy Wingo <[email protected]> wrote: >>> Hello ecmascriptians, >>> >>> I hear that TC39 wants to allow let and const into "classic mode". This >>> sounds like a bad idea to me, but, ok. >> >> Instead, I would say that TC39 wants to eliminate the concept of "classic >> mode". >> >>> If so, can someone say what these expressions would evaluate to, or the >>> errors they would raise: >> >> I don't think we've talked about the behavior of direct |eval| on >> statements with |let|, but here's what I would hope we'd do: > > > But it's an issue that I'ver been thinking about since the last meeting. > > The basic difference between non-strict direct eval and strict direct eval is > that strict creates a new nested environment contour that is used as both the > VariableEnvironement and LexicalEnvironment while non-strict uses the > currently active Variable/Lexical environment. > > It there are two possible semantics for non-strict eval with lexical > declaration that I have come up with are: > 1) same as ES5, it uses/extends the current Variable and Lexical Environments > function f() { > // variable environment > function xVarEnv() {return x} > { > //lexical environment > print(xVarEnv()); //should be: undefined > eval("var x=1"); //create in the variable environment > print(xVarEnv()); //should be: 1 > eval("let x=2"); //create in the lexical environment > eval("print(x)") //should be: 2 > print(x); //should be: 2 > print(xVarEnv()); //should be: 1 > } > print(xVarEnv()); //should be: 1 > ) > > (note that the most interesting example have an inner block) > > 2) It creates a new nested Lexical Environment (for let and const > declarations) but uses the currently active Variable environment. The > lexical environment is discarded when the eval is completed.: > > function f() { > // variable environment > function xVarEnv() {return x} > { > //lexical environment > print(xVarEnv()); //should be: undefined > eval("var x=1"); //create in the variable environment > print(xVarEnv()); //should be: 1 > eval("let x=2; print(x)"); //create in a new lexical environment, > should print: 2 > eval("print(x)"); //should be: 1 > print(x); //should be: 1 > print(xVarEnv()); //should be: 1 > } > print(xVarEnv()); //should be: 1 > ) > > Note that the second alternative requires (for ES5 compatibility, hoisting > functions at the top of the eval code to the Variable Environment. This > would be different from the normal ES6 scoping rules for function > declarations that are logically nested within a block. > > The second alternative is basically half way between a strict and non-strict > eval. By inclination is to stick with the simpler alternative 1 > > >> >>> (function (){eval("let x = 10"); return x;})() >> >> ReferenceError > > alternative 1 would return 10, note that (function (){eval("var x = 10"); > return x;})() must return 10 for compatability\ > alternative 2: return binding of x outside of function. It may be undefined. > It will be a ReferenceError if no out binding exists. >> >>> (function (){var x = 20; eval("let x = 10"); return x;})() >>> (function (){let x = 20; eval("let x = 10"); return x;})() > alternative 1: SyntaxError: duplicate declaration of x > alternative 2: 20 >>> (function (){let x = 20; { eval("let x = 10"); return x;}})() > >>> (function (){ { let x = 20; { eval("let x = 10"); return x;}}})() > alternative 1: 10 > alternative 2: 20 >> >> 20 >> >>> (function (){let x = 20; eval("var x = 10"); return x;})() > alternative 1: SyntaxError: duplicate declaration of x > alternative 2: SyntaxError: duplicate declaration of x >> 10 >> -- > > Allen > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss
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