Package PineAPPL for Nix

.envrc

@@ -0,0 +1,11 @@
+if ! has nix_direnv_version || ! nix_direnv_version 2.2.1; then
+  source_url "https://raw.githubusercontent.com/nix-community/nix-direnv/2.2.1/direnvrc" "sha256-zelF0vLbEl5uaqrfIzbgNzJWGmLzCmYAkInj/LNxvKs="
+fi
+
+watch_file devenv.nix
+watch_file devenv.lock
+watch_file devenv.yaml
+if ! use flake . --impure
+then
+  echo "devenv could not be built. The devenv environment was not loaded. Make the necessary changes to devenv.nix and hit enter to try again." >&2
+fi

.gitignore

@@ -1 +1,4 @@
 /target
+
+# Devenv files, to handle the development shell defined by the flake
+.devenv/

docs/maintainers-guide.md

@@ -33,6 +33,7 @@
   fastNLO tables to PineAPPL grids
 - `pineappl_py`: the crate that builds PineAPPL's Python interface
 - `xtask`: crate for [cargo-xtask] commands
+- `.envrc`: [direnv] hooks (mainly enable Nix shell)
 - `.gitignore`: PineAPPL's Git ignore rules
 - `.readthedocs.yml`: configuration for PineAPPL's [Read-the-Docs] Python
   interface documentation
@@ -44,6 +45,9 @@
 - `README.md`: PineAPPL's repository README file. This file also serves as the
   homepage of [PineAPPL's website]
 - `_config.yml`: configuration file for PineAPPL's Github-pages website
+- `flake.lock`: Nix [flake lock file]
+- `flake.nix`: Nix [flake], which tracks cross-language dependencies to define a
+  reproducible build for the PineAPPL packages, and a suitable development shell
 - `install-capi.sh`: POSIX-compliant shell script to download and install
   PineAPPL's pre-built CAPI
 
@@ -57,6 +61,9 @@
 [lock file]: https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
 [license v3]: https://www.gnu.org/licenses/gpl-3.0.en.html
 [PineAPPL's website]: https://nnpdf.github.io/pineappl/
+[flake]: https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#flake-format
+[flake lock file]: https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#lock-files
+[direnv]: https://direnv.net/
 
 ## Abbreviations and terminology