Task ate my data! Unexpected results at runtime. No errors in core stack.

Description of the issue

Converting an existing async function to the new F# 6 task CE throws up some unexpected and undesirable behaviour:

• Runtime failure, not compiler failure
• No errors thrown in core stack at all, just lost data

Steps required to reproduce the problem:

The attached ZIP file contains two .NET Interactive Notebooks, test_async.dib, which is the original function using async, and test_task.dib, which uses the new task CE. Running them yields the results described. The data array gets swallowed up.

test.dib.zip

Other notes:

• Weird error if you remove unused fromAccount variable declaration: System.InvalidOperationException: An attempt was made to transition a task to a final state when it had already completed

Expected result

True, [ ( 2022-06-28 18:06:32Z, 3, 4, 5, { Some(6): Value: 6 }, { Some(1): Value: 1 }, True, ( { Some(7): Value: 7 }, True, 2022-06-28 18:06:32Z ) ), ( 2022-06-28 18:06:32Z, 3, 4, 5, { Some(6): Value: 6 }, { Some(1): Value: 1 }, True, ( { Some(7): Value: 7 }, True, 2022-06-28 18:06:32Z ) ), ( 2022-06-28 18:06:32Z, 3, 4, 5, { Some(6): Value: 6 }, { Some(1): Value: 1 }, True, ( { Some(7): Value: 7 }, True, 2022-06-28 18:06:32Z ) ) ], 3

Actual result

True, null, 3

Known workarounds

1. Change task to async -> Works, but reverts to slower workflow. The code has no real async operations.
2. Move mapPrioritiesTransfers tasks inside match conditions -> Works, but high overhead to convert code
3. remove minOrder and set isPrio to true -> Works, but altered behaviour

There is no clear understanding of when we need a workaround for what, leading to a lack of ability to trust the new task-construct, so it seems rather fragile.

Related information

• Win 11
• .NET SDK 6
• F# 6.0
• VS2022 and VSCode

[dsyme]

OK I have this fixed - at least the data corruption part (caused by incorrect codegen).

The bug will occur when a nested task fails state machine compilation. A realiable workaround is if a nested task gives FS3511 you should move it out to a separate function.

The failure of state machine compilation is separate issue.

[dsyme]

@simontreanor

1. What exact version of .NET notebooks do you have please?
2. This bug looks highly relevant, as it is related to uses of Array.map in tasks https://github.com/dotnet/fsharp/issues/12359, it has been fixed but it’s possible the fix is not yet in .NET Notebooks.
3. Could you try replacing Array.map and perhaps any other inlined Array functions with your own non-inlined versions

@Thorium What’s your exact VS or VSCode or .NET SDK version please

BTW I’d imagine a fix is to move the chunks of synchronous code out of task { ... }