Convert ROOT, APPLgrid, and fastNLO tables into PineAPPL with Nix

docs/source/run.rst

@@ -1,6 +1,13 @@
 Generating a PineAPPL grid
 ==========================
 
+In the following section, we show to produced PineAPPL grids with `pinefarm`
+and how to convert pre-existing interpolation grids in different formats
+(ROOT, APPLgrid, fastNLO) into PineAPPL grids.
+
+Producing a PineAPPL grid
+-------------------------
+
 To generate a |pineappl| grid run:
 
 .. code-block:: sh
@@ -22,3 +29,95 @@ Analogously for theories:
    pinefarm list theories
 
 Recall to set the ``theories`` parameter in :ref:`pinefarm.toml <install:configure paths>`
+
+
+Converting pre-existing interpolation tables
+--------------------------------------------
+
+In some cases, when the interpolation tables are available in different format,
+it is necessary to convert them into PineAPPL grids. In order for PineAPPL to
+be able to convert those predictions, it needs to be compiled with the packages
+from which those tables were produced. Often times, however, these packages are
+hard to build because they rely on `Makefiles` that are inherently dependent on
+the platform from which they were built.
+
+An easy way to get these packages and build them with PineAPPL is to use
+`nix-shell <https://nixos.wiki/wiki/Development_environment_with_nix-shell>`_.
+`nix-shell` is a powerful tool from the `Nix <https://nixos.org/>`_ ecosystem
+that provides a reproducible development environment without modifying the
+system's global state.
+
+The interactive nix-based shell and its package manager can be easily installed
+using your OS' package manager by following the instructions on the installtion
+`page <https://nixos.org/download/#download-nix>`_.
+
+Then to use `nix-shell`, simply create a `shell.nix` file in the working directory
+with the following contents:
+
+.. code-block:: nix
+
+   with import <nixpkgs> {}; {
+     qpidEnv = stdenvNoCC.mkDerivation {
+       name = "pineappl-with-all-features";
+       buildInputs = [
+           gcc10
+           gfortran
+           root
+           lhapdf
+           cargo
+           cargo-c
+           zlib
+           fastnlo-toolkit
+       ];
+     };
+   }
+
+This will provide with all the necessary dependcies to build PineAPPL with all
+the features (ROOT, fastNLO) except APPLgrid. APPLgrid therefore needs to be
+compiled from source (see section below).
+
+To invoke the shell, simpy run:
+
+.. code-block:: sh
+
+   nix-shell
+
+One can check that everything has been set up correctly by checking (one of)
+the following commands:
+
+.. code-block:: sh
+
+   gcc --version
+   which root # Should point to nix/store/...
+   fnlo-tk-config --help # Check fastNLO
+
+We can now build APPLgrid by simply running the following commands:
+
+.. code-block:: sh
+
+   wget http://applgrid.hepforge.org/downloads/applgrid-1.6.27.tgz
+   tar -zxvf applgrid-1.6.27.tgz
+   ./configure --path=${prefix}
+   make -j
+   [sudo] make install
+   export LD_LIBRARY_PATH=${prefix}/lib:$LD_LIBRARY_PATH
+
+If everything went correctly, you should be able to run:
+
+.. code-block:: sh
+
+   applgrid-config --help
+
+With all of the dependencies installed, we can now compile PineAPPL. Inside
+the PineAPPL directory, run:
+
+.. code-block:: sh
+
+   APPL_IGRID_DIR=/path/to/applgrid-1.6.27/src cargo install --features=applgrid,fastnlo --path pineappl_cli
+
+Once this is done, we can now for example convert a ROOT file into a PineAPPL
+grid by running the following command:
+
+.. code-block:: sh
+
+   pineappl import <FILE_NAME>.root <PINEAPPL_NAME>.pineappl.lz4 NNPDF40_nnlo_as_01180