[v5] One item lag in StreamT when used with Source

[louthy]

The new Source<A> type yields a StreamT when .Await<IO>() is called.

Posting a value to the Source (with Post) doesn't immediately yield a value downstream. It seems to yield when a subsequent item is posted.

static StreamT<IO, Unit> show(Source<string> source) =>
    from v in source.Await<IO>()
    from _ in writeLine(v)
    where false
    select unit;

It seems the item makes it all the way to the M.Pure(MList<A>.Cons(iter.Current, next(iter))); in Lift(IEnumerable) but no value yields.

I suspect that the IO monad doesn't evaluate until subsequent binds/combines, which means we need another item to come thru to get the first one.

Debugging this was an absolute sh!t which is turning my head inside out. So logging this for now so I can come back to it fresh.

The test case is Streams sample "0. Source stream"