Followup to discuss some open issues from the thread, evident in the message cited below:
1. Unifying var scope and let scope at top level of a function could be done, with tolerable restrictions: given let x at top level in a function body, existence of formal parameter x, and redeclaration of let x by top-level var x, are static errors. 2. Unifying var scope and let scope at top level in global code cannot work since var x creates a DontDelete property of the global object if no property x exists, else redeclares an existing property x having arbitrary attributes. We could make let at top level in global code an error. Making such a let bind in an implicit block seems bad because inconsistent with 1, because implicit, and because it's more complicated to implement and specify than making global top-level let an error. Any pragma of the "use lexical scope" kind: http://wiki.ecmascript.org/doku.php?id=strawman:lexical_scope would remove this restriction. Comments? /be On Oct 13, 2008, at 6:41 PM, Brendan Eich wrote: > On Oct 13, 2008, at 6:08 PM, Waldemar Horwat wrote: > >> Brendan Eich wrote: >>>> Because presumably the "let x:t" became "var x:t" and var can't >>>> have >>>> types? >>> >>> Why can't var have a type annotation? >> >> Because a function can have many var declarations for the same >> variable and because you can use the variable before any of the var >> declarations are evaluated. You can work out the implications >> easily enough. This has been brought up at meetings before. > > Of course, ES4 had var x:t all over, but as you noted, different rules > for evaluating t. Still, with new syntax comes the opportunity for > reformed semantics, including restrictions on any "bad" var abusages > we would like to move away from. > > So my question remains (amended to be clear about what is the same): > why does this mean we cannot equate let and var scope at the top > level? I'm not talking about allowing them to mix in bad ways, or mix > at all (see below). I'm talking about not having an implicit block > around top-level function and global code. > > >> You wrote that you'd like let to become var (and unlike let as used >> within an independent block) if used at the top level of a function: > > Ok, sorry for being unclear. I do not mean to translate top-level > 'let' to 'var' and free let bindings from necessary restrictions. I do > mean that let binds in the variable object, and let usage restricts > other usage. > > We can forbid mixed var x and let x at top level. We can require the > same single definition for any let x at any level. We can forbid > arguments usage if arguments[i] could alias a let binding, since let > is new (along with rest and optional parameters to enable arguments > deprecation). > > But must we have an implicit block around programs and function bodies > that contain let declarations, distinct from the variable scope > (object)? > > >> What did you mean by "had better fail to compile"? Other than the >> type annotation, there is nothing about >> >> function f() { >> x = 15; >> ... >> var t = some_runtime_expression; >> ... >> var x:t = ... >> } >> >> that ought to fail to compile. > > The assignment to x in that temporal dead zone before t's initializer > has been evaluated. > > Why is this different if you s/var x/let x/? > > /be > > _______________________________________________ > 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
