On Nov 25, 2013, at 4:21 PM, Kevin Smith wrote: > > This makes for wtfjs additions, but they all seem non-wtf on reflection (or > did to us when Waldemar threw them up on a whiteboard last week). By non-wtf, > I mean anyone who groks that yield is reserved only in function* can work > them out. > > The star after function really helps. ES5's "use strict" directive prologue > in the body applying to its left (even in ES5 -- duplicate formals are a > strict error) is goofy. > > Agree on all counts, but not quite understanding yet. > > Say I'm parsing this, and the token stream is paused at the "#": > > function(a = # yield > > I assume that we're not un-reserving yield in strict mode. That means that I > don't know whether to treat `yield` as an identifier or reserved word until I > get to that goofy prologue. > > Previously, I've just naively parsed parameter initializers using the same > strictness as the code which contains the function. >
Here is a simplified explanation of how it actually works in the specification: 1) 'yield' is a reserved keyword at the level of the lexical grammar [1]. This applies to both strict and non-strict mode. 2) The GeneratorDeclaration and GeneratorExpression productions includes the [Yield] grammar parameter on their RHS FormalParameter and FunctionBody nonterminals. [2] This parameterization makes 'yield' legal as an AssignmentExpression operator [3]. That use of 'yield' is only enabled within generator definitions and is independent of the strictness. 3) When the [Yield] parameter is not present, the 'yield' keyword is one of the expansions of the IdentifierReference (via UnresolvedIdentifier) [4] and BindingIdentifier[5] productions. This permits 'yield' to be appear outside of Generator definitions in any syntactically allowed identifier position. Again, this is independent of strictness. 4) There are static semantic early error rules [6], [7] that require an error if 'yield' actually shows up in an IdentiferReference or IdentiferBinding expansion within strict code. Allen [1]: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-keywords [2]: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-generator-function-definitions [3]: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-assignment-operators [4]: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-identifier-reference [5]: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-let-and-const-declarations [6]:http://people.mozilla.org/~jorendorff/es6-draft.html#sec-identifier-reference-static-semantics-early-errors [7]: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-let-and-const-declarations-static-semantics-early-errors > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss
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