Array comprehension syntax
allen at wirfs-brock.com
Sat Sep 22 09:11:03 PDT 2012
On Sep 21, 2012, at 4:32 PM, Jason Orendorff wrote:
> 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.
Allowing an arbitrary sequence of |for| and |if| clauses is arguably more complex and harder to understand than the current proposal for a sequence of |for| clauses followed by a single optional |if| clause.
My personal bias, is that comprehensions are just sugar that are best used to express relatively simple and common construction use cases. Complicated sequences of |for| and |if| clauses will be rarely seen and hence less understandable than the equivalent explicit looping expansions.
> 2. These comprehensions are permitted:
> [EXPR for x of obj if a, b, c]
> [EXPR for x of obj if x = 3]
see separate response
> 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.
For-of was introduced to be a more generalizable equivalent to for-each-in. I don't think we want to go back on that design decision and it isn't clear in the above suggestion what the alternative would be to
[x for x of keys(obj)] // or [x for x of obj.keys()]
[x for keys( x of obj)]
There seems to be all sort of complications for the form you suggesting. For example, must the function that is called be identified by a PrimaryExpressioin? Is only one argument allowed? Is the binding declaration of "x" always the first argument position. Is something the the |of| keyword in a parameter posEtc.
I find it bad enough that the Expression that references the for clause bound identifiers is to the left of the bindings. I think burying bindings in augment positions would make things even worse.
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