Bottom Exception when first Either in LINQ from expression is in Left state

First up, this is an awesome Library and it is making me love C# again.

I have noticed an issue when using nested Monads in LINQ from select statements.

In my case I’m nesting an Either inside a Task, however if the unwrapping is done in a single LINQ step a Bottom Exception occurs.

So given:

var failableTask = fun((Either<string, int> value) =>
    Task.Run(() =>
    {
        Thread.Sleep(3000);
        return value;
    }));

The following will work as expected:

var result = await from a in failableTask(34)
                   from b in failableTask("This will work as expected")
                   select a + b;

However, if the first Either is in it’s Left state a Bottom Exception occurs:

var result = await from a in failableTask("This will cause a Bottom Exception")
                   from b in failableTask(3)
                   select a + b;

If I unwrap the nested Monads separately it works as expected:

var result = await from a in failableTask("This will work as expected")
                   from b in failableTask(3)
                   select from a1 in a
                          from b1 in b
                          select a1 + b1;

I haven’t yet explored what functions are called under the hood to debug this further just yet.

[louthy]

This issue is fixed in v2.2.17-beta

[louthy]

Ok, leave it with me I’ll put in some more tests.