Array comprehension syntax
jason.orendorff at gmail.com
Fri Sep 21 16:32:40 PDT 2012
I have a few proposals and questions about ES6 array comprehensions:
1. It seems simpler and more general to accept arbitrary sequences
of 'for' and 'if' clauses:
[x if x !== undefined]
[g for u of users if u.isAdmin() for g of u.groups()]
In the current draft only [Expr ForClause+ IfClause?] is allowed.
2. These comprehensions are permitted:
[EXPR for x of obj if a, b, c]
[EXPR for x of obj if x = 3]
The first is surprising to me for two reasons: first, because
commas in an array literal usually separate array elements,
whereas these are sequencing commas; second, it's odd that
commas are not permitted in the EXPR part of the comprehension
but are permitted in the if-condition.
The second is not so surprising, but it occurs to me that
unparenthesized assignment in this context will usually be a
mistake, so it might be nice to make that a syntax error.
So I propose changing "if Expression" to
"if ConditionalExpression" in the ArrayComprehension
3. SpiderMonkey already supports this nonstandard syntax:
[x for each (x in obj)]
A paren-free ES6 array comprehension could begin with a function
call, like this:
[x for each(x in obj).y of z]
Currently SpiderMonkey treats 'each' as a keyword when it
appears after the 'for' keyword.
The two syntaxes are distinguishable in all cases, right?
It's a little painful to get NotIn right in this case, but we can
hack it. As long as the syntax is unambiguous.
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