I'm very excited about the brand new [`Dataset`](http://reference.wolfram.com/language/ref/Dataset.html) function. I have played with it in [Wolfram Cloud][1], and haven't figured out how can I add a new column into an existing `Dataset`. Here is an example:

    data={<|"col1"->1,"col2"->2|>,<|"col1"->3,"col2"->4|>,<|"col1"->5,"col2"->6|>};
    ds=Dataset[data]
Now I can play with `ds` columns. For example, I can easily make calculations between columns using they names like: 

    ds[All, (#col1+#col2&)]

> {3, 7, 11}

Another way is:

    ds[All, <|"col3"-> (#col1+#col2&)|>]

> <|col3->3,col3->7,col3->11|>

Now, how can I update `ds`, to append the brand new calculated column as "col3"? I tried:

    Join[ds,ds[All, <|"col3"-> (#col1+#col2&)|>],2]

without success. It would be magic if I could just do something like:

    ds[All, "col3"]=ds[All, (#col1+#col2&)]

But do not work either.




  [1]: http://www.wolframcloud.com