Hi all, I've been experimenting with a pattern recently that I'd say is fairly close to a Haskell notion of IO. I have a collection of Nix-centered scripts that need to perform fairly restricted side effects:
- Build an AMI (image) for AWS - Write some disk image stuff to a raw device - Run some tests that talk to the outside world - Build some stuff that depends in a fairly constrained way on mutable external state (e.g., RPM update repos, as opposed to the deterministic base repos) - Deploy stuff These things all have the following in common: - I build as much stuff possible in "pure" Nix - I write out scripts (in the store) that all start with a ${stdenv.shell} shebang and an explicit `export PATH` clause to avoid inheriting an environment from the outside. - The scripts rarely take arguments at runtime; instead I run them with similarly to `sudo $(nix-build -A foobar)` or sometimes without `sudo`. I'm wondering if anyone else is doing something similar. It's basically (right now) analogous to a non-composable IO value in Haskell: pure code producing a "script" for some external impure interpreter to execute. I can think of a few next steps from here: start building composition operators (like bind) to chain together these impure actions without wanting to pull your hair out. I think Shea Levy's nix-exec already has a monad-flavored API for IO but I haven't seen anything like it outside of nix-exec. I'd also really like to teach Hydra how to (in restricted ways) run these IO values for me. I'd love it if my Hydra builds could produce a ton of pure work *and *upload an AMI to AWS. Then for example our nixos-unstable channel could bundle an AMI ID that's always up to date. I asked for a version of that feature in https://github.com/NixOS/hydra/issues/242 but I think this can be broader. Anyway, I was mostly wondering if anyone had feedback, ideas, suggestions. It seems like it might make sense to add a new top-level lib/ entry for producing these IO values without tearing one's hair out. I have a few ideas for combinators that make sense, and there are probably others I'm missing. Thanks, Dan
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