Not quite, but what I'm asking for isn't much different than adding a column to a 2-D array which can be a bit tedious with
MapThread[Append,{twoDarray,newColumn}]
I can use MapThread if I convert the datasets to associations, do the MapThread, then turn it back into an association
Dataset@MapThread[Append,
Normal /@ {dataset, dataset[All, <|"d" -> #a + #b|> &]}]
Using Transpose instead
The route that seems a little more intuitive is to use Transpose. So for adding a column to a 2-D array
Transpose[Append[Transpose[twoDarray],newColumn]]
Which is analogous to the approach I'm using with "joinDataset". I think Datasets are a great way to keep track of a lot of data in a human-usable form.
Extracting column/row with key
One other thing I wonder about is, how do I retain the key when extracting a single value from the Dataset? So for instance, say I want to take dataset[All,"b"] and add it to a different Dataset dataset2? I can't do this:
dataset2 = dataset[All, {"a", "c"}]
joinDataset[{dataset2, dataset[All, "b"]}]
I have to map an association to each element first
joinDataset[{dataset2, <|"b" -> #|> & /@ dataset[All, "b"]}]
