Error changing Dataset using Part

I'm the developer of Dataset.

Yes, this is a gross documentation oversight. We planned this functionality but had to push it back to a point release. Somehow no-one caught this piece of legacy documentation.

I've filed a bug on the documentation problem right now, it's easy to fix.

As for when L-value assignment will be available, I'm hoping 10.0.1 or 10.0.2, which are in the next month or two. It gets complicated, because you might well want to write things like:

dataset[ Select[#age > 30&] , "salary"] *= 2

That's certainly a powerful kind of operation, but also hard to implement. Even part-like assignments can get complicated when you are assigning multidimensional datasets to each other.

Thanks for trying the functionality, though!

In lieu of Set, the Query syntax offers various ways to update selective elements of a dataset. For example, we can change the value of the field a in the first row like this:

ds[{1 -> (<| #, "a" -> 999|> &)}]

or like this:

ds[{1 -> Query[{"a" -> (999 &)}]}]

Multiple fields can be updated simultaneously:

ds[{1 -> (<| #, "a" -> 999, "b" -> "ZZZ" |> &)}]

We can update selective rows, in this case field "b" in rows with even a:

ds[All, If[EvenQ[#a], <| #, "b" -> "!!!!"|>, #] &]

The accumulation use case can be accomplished like this:

With[{a = ds[Accumulate, "a"]}
, ds @ MapIndexed[<| #, "a" -> a[[First@#2]] |> &]
]

or like this:

Module[{acc = 0}, ds[All, {"a" -> (acc += # &)}]]

Note that none of these operations involve destructively altering the dataset, so they should all read ds = ds[...] if desired. Presumably Set will eventually perform destructive updates in those restricted circumstances that Mathematica tolerates mutation.

Though I don't know what is the efficiency impact of it, a workaround could be converting the Dataset to Association by Normal, making the update on the Association, then converting it back to Dataset.

ds = Dataset[{<|"a" -> 1, "b" -> "x"|>, <|"a" -> 2, "b" -> "y"|>, <|"a" -> 6, "b" -> "z"|>}]

ds = Module[{temp = Normal[ds]},
            temp[[All, "a"]] = Accumulate[temp[[All, "a"]]];
            temp // Dataset]