Throwing the wrong exception

This line tries to throw the exception of fa. But what if fb was faulted and fa was not?

[louthy]

On a related note, what is the meaning / intention between the classes MTask<> and MTaskFirst<>?

https://github.com/louthy/language-ext/issues/339

[TysonMN]

Actually, most of this is dead code. I used to know this and was just recently reminded of how this works.

The documentation for Task.WhenAll says

If any of the supplied tasks completes in a faulted state, the returned task will also complete in a TaskStatus.Faulted state, where its exceptions will contain the aggregation of the set of unwrapped exceptions from each of the supplied tasks.

Furthermore, when awaiting a failed Task, an exception is thrown. Therefore, most of the method is just dead code. It can be simplified to just

public async Task<A> Apply(Func<A, A, A> f, Task<A> fa, Task<A> fb) 
{
    await Task.WhenAll(fa, fb);
    return f(fa.Result, fb.Result);
}