Alan,

> Is this backward compatible with older versions of Cabal? I am considering
> whether to migrate HaRe to use this, I would prefer not to have it then
> fail to work on older systems that are constrained not to be able to update
> Cabal.

The sandbox is a feature of cabal-install, not Cabal lib.

I carefully implemented this version not using Cabal 1.18.

--Kazu