CSHARP-4883: Support SkipWhile and TakeWhile methods in LINQ. #1613
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| var method = expression.Method; | ||
| var arguments = expression.Arguments; | ||
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| if (method.IsOneOf(__skipWhileOrTakeWhileMethods)) |
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I know this is the pattern used in other expression translators but I wonder if we should switch to a negative if and throw the ExpressionNotSupportedException here to avoid the indenting.
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Sometimes there's more than one if statement. I'm fine with the current pattern.
| var valuePredicateField = AstExpression.GetField(valueVar, "predicate"); | ||
| var valueCountField = AstExpression.GetField(valueVar, "count"); | ||
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| var reduceAst = AstExpression.Reduce( |
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What's the min version/wire version required for this reduce/while/predicate MQL construct? Do we want to throw if it's not supported?
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The server documentation is very unhelpful for determining when a feature was first introduced. Sometimes we just have to let the patch build run and see if something fails on older servers.
Based on this patch build looks like all the MQL operators used in this PR are supported on all the servers we still support.
Also, we never throw when something is not supported. We let the server do that.
| var whileBinding = AstExpression.VarBinding(whileVar, reduceAst); | ||
| var whileCountField = AstExpression.GetField(whileVar, "count"); | ||
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| var sliceAst = method switch |
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I understand the slicing here and the idea of a reduce above but when the predicate stops matching how does it terminate/stop counting ones that match after?
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The predicate field of the accumulator value starts out true and stays true as long as the predicate keeps returning true. The count field indicates how many times the predicate has returned true so far.
Once the predicate returns false the accumulator predicate field is set to false, and once that happens, we never execute the predicate or increment the count again.
The first case of the $switch is where this happens. When the predicate field is false we just keep returning { predicate : false, count : n } over and over until the array is exhausted. There is no way in MQL to short circuit the reduction to ignore the rest of the array.
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