Composing devenv.yaml

[domenkozar]

Please leave a thumbs up if you'd like to see this happen.

[domenkozar]

This one is blocked until NixOS/nix#7340 is merged

[majidaldo]

i'm coming from a conda-centric data science experience. i really want this to work!
https://github.com/pnnl/hydraconda/blob/master/project/readme.md

i've been looking for ways to create composable environments.