uses string-to-int (should be string-to-number on recent emacsen)
Is buggy: on my system (skeletor-hs--cabal-sandboxes-supported?) returns t
immanuel@prometheus:~/nvme/haskell/gp-zurihac-2020$ cabal --version
cabal-install version 3.2.0.0
compiled using version 3.2.0.0 of the Cabal library
immanuel@prometheus:~/nvme/haskell/gp-zurihac-2020$ cabal sandbox init
cabal: unrecognised command: sandbox (try --help)
Thanks for the report. If you'd like to see this fixed you're welcome to get your hands dirty and submit a PR. I'm intentionally lazy with supporting my older OSS projects these days :)