[Nix-dev] How can we make import-from-derivation more useful?

[Daniel Peebles]

A few days ago, I proposed importing from a derivation
<https://github.com/NixOS/nixpkgs/pull/11319#issuecomment-167144900> to
save us from having to manually manage autogenerated firefox/thunderbird
fixed-output derivaiton hash files and junk up the nixpkgs repository with
them. In response, Vladimír Čunát pointed out that nix-env would currently
force a download at evaluation time as a result of that change, and that's
undesirable.

I see it as inevitable that we'll have to start importing from derivations
to make external package ecosystems more manageable. Currently,
haskellPackages is already eating up a large chunk of the overall
repository size, and as we start adopting similar automated processes to
manage other ecosystems, I see no way to keep the repo size manageable. It
also feels like a bit of an abuse of a VCS to be putting autogenerated
files in it, as we do today. Especially when nix is generally so good at
doing that sort of thing for us.

It seems like the main obstacle standing in the way of this pattern is
nix-env, given its habit (also unsustainable) of forcing the evaluation of
a large chunk of the packages expression. People have expressed a desire to
deprecate that sort of functionality, but I don't know what sort of
timeline such a change could be made on, so I'm looking for ways to make it
more manageable in the meantime.

Does anyone have ideas on how to improve this? Or am I wrong about things
getting out of control if we don't? Are there other options?

Thanks,
Dan
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