On Nov 30, 2011, at 7:14 AM, Anton V. Yacenko wrote:
> It's interesting to me why () syntax returns object, and why not {}.
I assume you're talking about generator expressions, which have the syntax of
array comprehensions but using parentheses as their outer delimiter. The idea
originally came from Python, which uses parentheses for generator expressions.
Using {} is an interesting idea, but it could be tricky to fit that into the
grammar, because of the overlap with object literals. Brendan has experimented
with the idea of allowing overlap between different {} forms in other contexts:
http://wiki.ecmascript.org/doku.php?id=strawman:block_vs_object_literal
In another discussion about do-expressions:
http://wiki.ecmascript.org/doku.php?id=strawman:do_expressions
we talked about the possibility of allowing block statements as expressions:
var g = { let x = f(); x * x };
and Allen Wirfs-Brock raised objections that allowing overlap between object
literals and objects whose contents can be expressions will be a problem for
human readers, even if formally unambiguous:
https://mail.mozilla.org/pipermail/es-discuss/2011-November/018257.html
This issue will get worse as we work on extending the grammar of object
literals, for example to allow computed property names in square brackets. In
the context of generator expressions, this would lead to subtle differences
like:
var o = { [f(x)]: 12 }; // object literal with property name
computed from f(x)
var g = { [f(x)] for x of a }; // generator producing a sequence of
singleton arrays containing f(x)
Put differently: we already overload the { } syntax for block statements and
object literals, and we use the context to disambiguate. Overloading them
further, especially within the same context, would lead to visual confusion.
Dave
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