These are some impressions looking at what I expect from the language, and how some things in the specification can cause confusion.
I would have contributed here during the discussions, but I discovered the mailing lists just a couple of days ago. I expect the compiler's interpretation of program-code text to be close to my intuitive understanding of what the text says. It's very unfortunate if keywords have unexpected meanings that cause mysterious side effects. If I learn that ECMAScript will let me change my var(iables) into const(ants) I expect this to turn them into constants, in the sense that trying to change their value will be considered an error. It's very disappointing that by default they are instead defined to have the baffling and mysterious behavior of silently ignoring an attempt to change them, acting as if no error had occurred. You'll have to keep this oddity in mind at all times, and even then errors related to this will sometimes cause symptoms to appear far from where the error is, costing quite some time to explore. Why doesn't my program change its behavior even though I'm provoking changes? Where in this big program's complicated sequence of events is the change silently, secretly lost? If instead you use var, at least the problems that can come from this will tend to give symptoms closely connected to the incorrect change in the value. So this is a disappointing red flag: Don't use const, it is likely to cause baffling problems and unlikely to help. Unfortunately there's another problem with const that is much more important. I often use constants for conditional settings: if (Debugging) { var DatabaseName = "TestDatabase"; var DisplayCount = 5; } else { var DatabaseName = "RealDatabase"; var DisplayCount = 15; } The redundant "var"s are a defensive habit, omitting them would be a warning about accesses outside the current scope. If I haven't been warned, and hear that ECMAScript understands "const", I expect that replacing "var" with "const" will change the above from variables into constants. The keyword in no way suggests that it will hide them from view. If they disappear I'll inevitably consider such a completely unrelated side effect a compiler bug. Because of this I'm unhappy about the conclusions of ES3.1 that the visibility scope of "const" should be the enclosing brace- delimited block. Such intricate semantics hidden in words that express something completely unrelated will make the language seem difficult and fraught with hidden surprises. I much prefer what ES4 says in various places on the website: that you express this localness with "let const" and "let function". One block-scope keyword for all block-scope visibility. Consistency and clarity. However this brings me to the unfortunate word "let". Although this word has a precise and clear technical meaning for the initiate, for us in the unwashed masses I can't see that the English word "let" gives even the remotest suggestion of local containment. In fact it suggests very clearly that it's related to the "=" that so often follows: if (x == 5) { let y = 3; } "If x is 5, then let y equal 5." There's an almost inescapably strong suggestion that "let" is a phrasing of the assignment expression, and therefore can't have anything to do with the braces. I think ECMAScript should be easily accessible to us in the unwashed masses. It becomes much more intuitively accessible if it uses a word that strongly implies localness: { if (x == 5) { local y = 3; } local const Debugging = false; for (local Key in List) ++List [Key]; } You get plain English sentences that express quite accurately what they're supposed to mean. The programmer won't be the least surprised if a value gets hidden by "local". When people want to write let expressions, if they have to write "local" instead of "let" I don't think this will cause problems. I'm sure the initiate are sophisticated enough that they can adapt to this. Apart from this, I think the scoping arrangements would become significantly simpler and clearer if the language made a very clear, really visible, intuitively accessible distinction between two different types of block, and allowed you to choose either type of block wherever this made sense. My suggestion is to introduce a clearly distinct new and better block. This block should be delimited by {{ and }} if it's at all possible, and I think it is. No keyword, just {{ and }}. This better block would bind vars, consts and functions, just like function scopes do. In fact function scopes and {{ }} would be the same thing, as seen by the programmer. An important advantage with {{ }} is that you can keep everything contained without tedious and error-prone repetition of local (or let) everywhere. And the scoping is prominently visible and clearly structured. It may seem odd that I say that adding yet another scoping construct would make it simpler and easier to learn, but if it's built this way it becomes conceptually clear, free of hidden intricacies, easy to explain. The delimiters {{ and }} suggest that you are walling things in with thicker walls, so that hoisting can't get past. Nicely intuitive. Especially for programmers who put braces at left it becomes very clear indeed. And I'm sure syntax-coloring editors will help making the scoping clear at a glance. The terminology might distinguish between the two types of block by talking about strong and weak blocks, where strong means thick walls that you can't hoist out of, and weak means that the block can only capture things that are marked local (or let), and everything else gets hoisted out. In fact I think a terminology with strong versus weak blocks is clearer than the current terminology, where one type of block is called block and the other is called variable object. I'm sorry if this comes across as a series of complaints. All in all I'm delighted with the many enticing improvements! But listing all the nice things here wouldn't make for interesting reading. And so it may sound much more negative than my overall delighted and enthusiastic feelings. -- Ingvar von Schoultz ------- (My quirky use of capitals in code comes from my opinion that reserved and predefined words should all start with lowercase, and user-defined should all start with uppercase, because this will easily and elegantly prevent a host of name-collision problems when things like programming languages are upgraded with new labels.) _______________________________________________ Es4-discuss mailing list Es4-discuss@mozilla.org https://mail.mozilla.org/listinfo/es4-discuss