bug#32339: Nix import

[Andreas Enge <andreas@enge.fr>]

So, I just discussed with a Nix expert, Profpatsch.

Apparently something changed in nixpkgs a while ago, so that the following
code does not work any more:
$ export NIX_REMOTE=daemon
$ nix-instantiate --eval --strict -A hello nixpkgs/
trace: `types.list` is deprecated; use `types.listOf` instead
trace: types.optionSet is deprecated; use types.submodule instead
trace: Warning: `showVal` is deprecated and will be removed in the next release, please use `traceSeqN`
trace: `lib.nixpkgsVersion` is deprecated, use `lib.version` instead!
trace: lib.zip is deprecated, use lib.zipAttrsWith instead
trace: `mkStrict' is obsolete; use `mkOverride 0' instead.
trace: warning: `stdenv.isArm` is deprecated after 18.03. Please use `stdenv.isAarch32` instead
trace: warning: `stdenv.isArm` is deprecated after 18.03. Please use `stdenv.isAarch32` instead
trace: warning: `stdenv.isArm` is deprecated after 18.03. Please use `stdenv.isAarch32` instead
trace: warning: `stdenv.isArm` is deprecated after 18.03. Please use `stdenv.isAarch32` instead
error: overrideDerivation not yet supported for functors
(use '--show-trace' to show detailed location information)

We should have checked the return value...

Omitting the "--strict" flag is not a solution, since then lazy evaluation
leads to most interesting fields of the result being empty (the return string
value is "<CODE>").

A potential solution is evaluating for different package fields separately,
but this would require more or less a complete rewrite of the importer.
I wonder if the solution would not be to drop it completely... Interestingly
enough, the programmatic nature of Nix makes it much more difficult to import
packages, since we cannot simply use the abstract syntax tree of the
textual representation of a package.

Here are a few possible commands to run:
$ nix-instantiate --eval --strict -E "with import $PATH_TO_NIXPKGS {}; singular.version"
"4.1.1p2"

$ nix-instantiate --eval --strict -E "with import $PATH_TO_NIXPKGS {}; map (drv: drv.name) singular.buildInputs"
[ "gmp-6.1.2" "ncurses-6.1-20190112" "readline-6.3p08" "ntl-11.3.2" "flint-2.5.2" "cddlib-0.94j" ]

$ nix-instantiate --eval --strict -E "with import $PATH_TO_NIXPKGS {}; map (drv: drv.name) singular.nativeBuildInputs"
[ "bison-3.3.2" "perl-5.28.2" "pkg-config-0.29.2" "hook" ]
(notice that there is now a difference between build and native build
inputs, and even more types)

$ nix-instantiate --eval --strict -E "with import $PATH_TO_NIXPKGS {}; singular.src"
[error message]

$ nix-instantiate --eval --strict -E "with import $PATH_TO_NIXPKGS {}; (singular.override { fetchurl = lib.id; }).src"
{ sha256 = "07x9kri8vl4galik7lr6pscq3c51n8570pyw64i7gbj0m706f7wf"; url = "http://www.mathematik.uni-kl.de/ftp/pub/Math/Singular/SOURCES/4-1-1/singular-4.1.1p2.tar.gz"; }

$ nix-instantiate --eval --strict -E "with import $PATH_TO_NIXPKGS {}; (linbox.override { fetchurl = lib.id; }).src"
[error message]

$ nix-instantiate --eval --strict -E "with import $PATH_TO_NIXPKGS {}; (linbox.override { fetchFromGitHub = lib.id; }).src"
{ owner = "linbox-team"; repo = "linbox"; rev = "v1.6.3"; sha256 = "10j6dspbsq7d2l4q3y0c1l1xwmaqqba2fxg59q5bhgk9h5d7q571"; }

So for the latter two cases, one would need to try both and catch errors.

Andreas