Both of those rely on having proper cabal/ghc or python/virtualenv preinstalled on the host computer; you could ship cabal/ghc and python/virtualenv and then us

Both of those rely on having proper cabal/ghc or python/virtualenv preinstalled on the host computer; you could ship cabal/ghc and python/virtualenv and then use a sandbox or virtualenv, but at that point you might as well just use separate package databases. (This at least was the original motivation for the split – I suspect you could do similar things with virtualenv/sandboxes, it's certainly not the only point in the design space.)