[Nix-dev] [***SPAM***] Dynamic Dependency Management by Calling External Programs from Nix

[Eelco Dolstra]

Hi,

On 10/25/2011 01:35 AM, Shea Levy wrote:

> So I'd really like to see one of two solutions implemented (I'll do the
> implementation myself if Eelco is willing to consider accepting these solutions)
> in nix (possibly with an explicit command-line flag or nix.config setting to
> enable them, like we have with enableParallelBuilds):

You can do what you want today, since the "import" function (as opposed to
builtins.readFile) supports importing the output of a derivation.  I.e. you can do

  let drv = runCommand ... "some command that generates $out/foo.nix" ...
  in import "${drv}/foo.nix"

However, you don't want this most of the time, certainly not in Nixpkgs, because
it causes query operations like "nix-env -qa" to do a build.  Which is a bad
thing.  Actually, since "nix-env -qa" runs in read-only mode, it won't even work
(but it will work for "nix-env -i", "nix-instantiate" or "nix-build").

There are a few examples of this:

- https://nixos.org/repos/nix/nixpkgs/trunk/pkgs/build-support/vm/default.nix;
here we import the output of the functions rpmClosureGenerator and
debClosureGenerator (a derivation that builds a Nix expression containing
fetchurl calls to download the required RPMs or Debs).

- https://nixos.org/repos/nix/nixpkgs/trunk/pkgs/misc/tex/nix/default.nix; the
function to build a LaTeX document does "deps = import (pkgs.runCommand
"latex-includes" ..." to figure out the dependencies of the document (e.g.,
\input{...}) on the fly.

- https://nixos.org/repos/nix/attic/make/lib/default.nix does a similar thing to
figure out the header file dependencies of a C file.

-- 
Eelco Dolstra | http://www.st.ewi.tudelft.nl/~dolstra/