On Oct 13, 2010, at 11:56 PM, Erik Corry wrote:
The semantics for const in Harmony are likely to be silently different
in Harmony from the semantics it has in non-strict current
implementations. (In particular the current const is hoisted to the
surrounding function, whereas the one in Harmony won't, so the
shadowing will be different.)
Agreed so far. IIRC const in Opera is just a synonym for var.
Given this silent behavioural change it would be advantageous to flush
out existing uses of const before Harmony arrives and gives it a new
meaning.
Flush out means make developers find all such uses, and do what?
Forbidding const in strict mode would seem to be a way to do that.
If only I knew what flush out means. Users may or may not rewrite complex
const-using code just to enable strict mode.
The problem is even more acute for let-based code -- Mozilla JS (Firefox FE and
add-on code) uses let heavily. It's a sunk cost, arguably a good sunk cost
(mostly future-proof, even with let becoming like let* not hoisted to block
top). I doubt people will rewrite to var (which requires careful analysis to
avoid collisions where there used to be shadowing). They simply won't use
strict mode.
But const or let, the idea that use strict is so attractive it will cause
re-sinking sunk costs seems farfetched to me. We'll find out, though, because I
do agree we should make Firefox's strict mode ban let, const, and function in
block for now. I hope JSC and V8 do likewise with their IE-workalike
function-in-block extension.
/be
14. okt. 2010 03.01 skrev Brendan Eich bren...@mozilla.com:
On Oct 13, 2010, at 5:35 PM, Brendan Eich wrote:
On Oct 13, 2010, at 4:31 PM, Mark S. Miller wrote:
* ES5/strict implementations as deployed in non-beta releases should obey
http://wiki.ecmascript.org/doku.php?id=conventions:recommendations_for_implementors.
If they don't, then either a separate opt-in will be necsary or we will
have to retreat from some of the accepted harmony proposals. For example, if
browsers deploy a harmony-incompatible semantics for let, const, and
nested named functions, and if ES5/strict code comes to depend on those
non-standard extensions, then without an additional opt-in it becomes
impossible to deploy harmony without breaking that code.
The point about let, const, and function in block is good but I'll take it
up in a separate message.
Here are my thoughts. Reactions, advice, alternatives I've missed all
welcome:
1. No one can claim to have future-proof (Harmony-compatible) let until we
really draft it better. Same for const. At the last TC39 meeting I proposed
we drop hoisting to block-top and go with Scheme let* or C++
declaration-scope semantics, and everyone agreed (with great relief). But we
haven't draft-spec'ed or prototyped this at all. It needs to be more solid.
2. Everyone wants to promote use strict but with Firefox 4 betas
supporting most but not all of it, and the (still being debugged) complete
patch just landed on WebKit nightlies, it is premature to try to get
developers slapping the directive on new code. This already bit back due to
concatenation with old code, and with lack of validation. We do not want to
risk creating pressure for quirks mode strict mode!
3. Firefox's let and const support could create quirks-mode pressure on
other browsers. This happens, and not just on non-IE, lesser-market-share
browsers (Mozilla gets pressure from WebKit all the time, mostly for the
good!). So there' s a risk here. But:
4. Forbidding our let, const, and function-substatement extensions under
use strict -- especially forbidding let and const -- will harm uptake of
validated strict mode usage in Mozilla-specific JS (add-ons, our front end
JS, middle-end JS components, etc.). Maybe that's ok, we can try to use our
long-standing strict option (which gives warnings by default, and overlaps
ES5 strict mode but is not the same as it). This harm has to count for
something, even if hard to quantify -- the risk in 3 is hard to measure too
(probably small).
5. You didn't mention 'yield', which ES5 strict makes a future reserved
word, but which we implement in a possibly Harmonious way. We could play it
safe in terms of future proofing, again at the risk of dampening use
strict uptake in Mozilla-specific JS. Or we could gamble that we are
already future-proof.
My tentative conclusion is that we should make ES5 strict forbid let, const,
and function in block (or any substatement) in Firefox 4. I would hope
(haven't tested it) that JSC will do likewise with function in block.
Mozilla JS hackers can lean on the strict option and its warnings, which
would deviate further from ES5 strict by not banning let, const, and
function in block.
On yield, I'd like to let it ride. It's a good bet IMHO. But who knows?
http://wiki.ecmascript.org/doku.php?id=proposals:versioning
(Collin Jackson added to this; I wrote the original version.)
