refactor: asListDS

[timcadman]

asListDS is currently an aggregate function rather than an assign function. Whilst refactoring I unwittingly returned the whole value to the user as an invisible behaviour. It would be safer if we rewrote as an assign function.

[StuartWheater]

In which version did this change occur?

[timcadman]

Only on v7.0-dev. I will make a PR to fix it, I just want to address this for long-term clarity :)