Support list comprehensions

[DartBot]

This issue was originally filed by [email protected]

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Support list comprehensions a la Python, or else list somewhere why you won't support them.

Rationale:
 - They are voted the number one favorite language feature of Pythonistas, which will help with adoption and evangelism.
 - They can be implemented as syntactic sugar and require no new keywords.
 - They allow for clearer and shorter looping constructs.
 - They allow for a functional style without explicit use of map() and filter().

You will gain converts who love list comprehensions and are tired of them not existing in JavaScript.

I'm sure you know the format, but for everyone else: the list comprehension

  [ expr1 for a in list1 if expr2 ]

is roughly equivalent to the JavaScript

  (function() {
    var result = [];
    for (var i = 0; i < list1.length; i++) {
      if (expr2)
        result.append(expr1); // can refer to a
    return result;
  )()

and multiple 'for' subclauses are supported.

[DartBot]

This comment was originally written by [email protected]

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To be precise, list1 can be any iterable, so the syntactic un-sugaring would use a for..in loop rather than a for(;;) loop.

[dgrove]

Removed Type-Defect label.
Added Type-Enhancement, Area-Language, Triaged labels.

[gbracha]

Set owner to @gbracha.
Added Accepted label.

[anders-sandholm]

Short function syntax may come in handy.

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Added WontFix label.

[DartBot]

This comment was originally written by [email protected]

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@sandholm, thanks for the update. Could you post a comment with the short function syntax equivalent of

  [ emp.salary / 12 for emp in employees if not emp.retired ]

for comparison?

[whesse]

My guess is that he means to use short functions in the filter and map:

employees.filter((emp) => !emp.retired ).map((emp) => emp.salary / 12 );

This isn't too bad, but I still prefer list comprehensions.

[DartBot]

This comment was originally written by @seaneagan

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If this bug (or a similar one) ever gets revived, ought to add an async version as well:

[await for e in stream if (e.isEven) e * e]

Note that this follows the ES6 format, which works better with "await for" to match the async loop syntax:

await for(var e in stream) ...

I would have preferred "on" which would work better with the python style:

[e * e on e in stream if e.isEven]