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Allow pattern case conditions in while statements. #3927

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lrhn opened this issue Jun 20, 2024 · 3 comments
Closed

Allow pattern case conditions in while statements. #3927

lrhn opened this issue Jun 20, 2024 · 3 comments
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feature Proposed language feature that solves one or more problems patterns Issues related to pattern matching.

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@lrhn
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lrhn commented Jun 20, 2024

Dart allows if (e case p when c) ....
It should also allow while (e case p when c) ....

The true branch is the loop body, which is where the bound values are available.

An example use could be finding the last element of a linked list:

  while (cursor.next case var next?) cursor = next;

Generally if and while are mostly symmetric, and the behavior here can be emulated by:

while (true) {
  if (cursor.next case var next?) cursor = next;
  else break;
}

(Even if Dart doesn't make (e case p when c) a general boolean expression, it can still safely and meaningfully be allowed in a few more cases, mainly [conditional expression][#2664] and while conditions. I doesn't make sense for do/while since the true branch isn't dominated by the condition.)

@lrhn lrhn added feature Proposed language feature that solves one or more problems patterns Issues related to pattern matching. labels Jun 20, 2024
@rrousselGit
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What about for?

for (...; something case ... ;  ...) {

}

If we can do it for while, I think it'd make sense here too.

@lrhn
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lrhn commented Jun 20, 2024

The for should work too. It's a little crowded syntactically, and it's not completely clear whether any bound variables are in scope for the increment-expressions or not. Still, the check being true dominates the body and increments, so it should probably just work for those.

@leafpetersen
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I think this is already covered by #2536 .

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Labels
feature Proposed language feature that solves one or more problems patterns Issues related to pattern matching.
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