Stream/Iterable.firstWhere inconsistency (orElse vs. defaultValue)

[DartBot]

This issue was originally filed by @seaneagan

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I wanted to call firstWhere on a variable which could either be an Iterable or a Stream , but since I needed to have a fallback, I needed orElse / defaultValue, but since they are named differently I would have to test if the variable is an Iterable or or Stream first.

[lrhn]

That's an annoying difference. Thanks for noticing!
Not something we can change right now without making a breaking change, so it'll have to be a 2.0 change if we do the rename.

Same problem for lastWhere, obviously.
I think the async version should be changed to orElse. It's shorter.

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Added this to the 2.0 milestone.
Added Library-Async, Area-Library, Accepted labels.

[DartBot]

This comment was originally written by @seaneagan

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A related annoyance: Not being able to call map on a value which may either be a Stream or Iterable when the mapping function may return a Future, due to the split between Stream.map and Stream.mapAsync. And same issue for expand/expandAsync and Stream methods which don't allow their callbacks to return Futures e.g. firstWhere, see issue #8247 and issue #12295.

[DartBot]

This comment was originally written by @seaneagan

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For example, here's what I would want for Stream.firstWhere:

firstWhere(Stream stream, bool test(item), { orElse() }) async {
  await for (var i in this) {
    if (await test(i) return i;
  }
  return await orElse();
}

[DartBot]

This comment was originally written by @seaneagan

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Correction:

Future<T> firstWhere(test(item), { orElse() }) async {
  await for (var i in this) {
    if (await test(i) return i;
  }
  return await orElse();
}

[dgrove]

Are any further changes planned here for 2.0 @lrhn?

[lrhn]

This has almost been fixed, but we still need to remove the defaultValue parameter from Stream.

[dgrove]

Any updates here? We are 2 days out from the Dart2 Beta 3 milestone.

[lrhn]

The defaultValue parameter has been removed, so this has been fixed.