[Nix-dev] "IO" in Nix

[Daniel Peebles]

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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