Title: How to use cpan or pip packages on Nix and NixOS

	Title: How to use cpan or pip packages on Nix and NixOS
	Author: Solène
	Date: 18 September 2021
	Tags: nixos nix perl python
	Description:

	# Introduction

	When using Nix/NixOS and requiring some development libraries available
	in pip (for python) or cpan (for perl) but not available as package, it
	can be extremely complicated to get those on your system because the
	usual way won't work.

	# Nix-shell

	The command nix-shell will be our friend here, we will define a new
	environment in which we will have to create the package for the
	libraries we need. If you really think this library is useful, it may
	be time to contribute to nixpkgs so everyone can enjoy it :)

	The simple way to invoke nix-shell is to use packages, for example the
	command ` nix-shell -p python38Packages.pyyaml` will give you access to
	the python library pyyaml for Python 3.8 as long as you run python from
	this current shell.

	The same way for Perl, we can start a shell with some packages
	available for databases access, multiples packages can be passed to
	"nix-shell -p" like this: `nix-shell -p perl532Packages.DBI
	perl532Packages.DBDSQLite`.

	# Defining a nix-shell

	Reading the explanations found on a blog and help received on Mastodon,
	I've been able to understand how to use a simple nix-shell definition
	file to declare new cpan or pip packages.

	Mattia Gheda's blog: Introduction to nix-shell
	Mastodon toot from @[email protected] how to declare a python package on the …

	What we want is to create a file that will define the state of the
	shell, it will contain new packages needed but also the list of
	packages.

	# Skeleton

	Create a file with the nix extension (or really, whatever the file name
	you want), special file name "shell.nix" will be automatically picked
	up when using "nix-shell" instead of passing the file name as
	parameter.

	```nix configuration file
	with (import <nixpkgs> {});
	let
	# we will declare new packages here
	in
	mkShell {
	buildInputs = [ ]; # we will declare package list here
	}
	```

	Now we will see how to declare a python or perl library.

	## Python

	For python, we need to know the package name on pypi.org and its
	version. Reusing the previous template, the code would look like this
	for the package Crossplane

	```nix configuration file
	with (import <nixpkgs> {}).pkgs;
	let
	crossplane = python37.pkgs.buildPythonPackage rec {
	pname = "crossplane";
	version = "0.5.7";
	src = python37.pkgs.fetchPypi {
	inherit pname version;
	sha256 = "a3d3ee1776bcccebf7a58cefeb365775374ab38bd544408117717ccd9f264f6…
	};

	meta = { };
	};


	in
	mkShell {
	buildInputs = [ crossplane python37 ];
	}
	```

	If you need another library, replace crossplane variable name but also
	pname value by the new name, don't forget to update that name in
	buildInputs at the end of the file. Use the correct version value too.

	There are two references to python37 here, this implies we need python
	3.7, adapt to the version you want.

	The only tricky part is the sha256 value, the only way I found to find
	it easily is the following.

	1. declare the package with a random sha256 value (like echo hello \|
	sha256)
	2. run nix-shell on the file, see it complaining about the wrong
	checksum
	3. get the url of the file, download it and run sha256 on it
	4. update the file with the new value

	## Perl

	For perl, it is required to use a script available in the official git
	repository when packages are made. We will only download the latest
	checkout because it's quite huge.

	In this example I will generate a package for Data::Traverse.

	```shell instructions
	$ git clone --depth 1 https://github.com/nixos/nixpkgs
	$ cd nixpkgs/maintainers/scripts
	$ nix-shell -p perlPackages.{CPANPLUS,perl,GetoptLongDescriptive,LogLog4perl,Re…
	$ ./nix-generate-from-cpan.pl Data::Traverse
	attribute name: DataTraverse
	module: Data::Traverse
	version: 0.03
	package: Data-Traverse-0.03.tar.gz (Data-Traverse-0.03, DataTraverse)
	path: authors/id/F/FR/FRIEDO
	downloaded to: /home/solene/.cpanplus/authors/id/F/FR/FRIEDO/Data-Traverse-0.03…
	sha-256: dd992ad968bcf698acf9fd397601ef23d73c59068a6227ba5d3055fd186af16f
	unpacked to: /home/solene/.cpanplus/5.34.0/build/EB15LXwI8e/Data-Traverse-0.03
	runtime deps:
	build deps:
	description: Unknown
	license: unknown
	License 'unknown' is ambiguous, please verify
	RSS feed: https://metacpan.org/feed/distribution/Data-Traverse
	===
	DataTraverse = buildPerlPackage {
	pname = "Data-Traverse";
	version = "0.03";
	src = fetchurl {
	url = "mirror://cpan/authors/id/F/FR/FRIEDO/Data-Traverse-0.03.tar.gz";
	sha256 = "dd992ad968bcf698acf9fd397601ef23d73c59068a6227ba5d3055fd186af16…
	};
	meta = {
	};
	};
	```

	We will only reuse the part after the ===, this is nix code that
	defines a package named DataTraverse.

	The shell definition will look like this:

	```nix shell definition code
	with (import <nixpkgs> {});
	let
	DataTraverse = buildPerlPackage {
	pname = "Data-Traverse";
	version = "0.03";
	src = fetchurl {
	url = "mirror://cpan/authors/id/F/FR/FRIEDO/Data-Traverse-0.03.tar.gz";
	sha256 = "dd992ad968bcf698acf9fd397601ef23d73c59068a6227ba5d3055fd186af16…
	};
	meta = { };
	};

	in
	mkShell {
	buildInputs = [ DataTraverse perl ];
	# putting perl here is only required when not using NixOS, this tell you want…
	}
	```

	Then, run "nix-shell myfile.nix" and run you perl script using
	Data::Traverse, it should work!

	# Conclusion

	Using not packaged libraries is not that bad once you understand the
	logic of declaring it properly as a new package that you keep locally
	and then hook it to your current shell session.

	Finding the syntax, the logic and the method when you are not a Nix
	guru made me despair. I've been struggling a lot with this, trying to
	install from cpan or pip (even if it wouldn't work after next update of
	my system and I didn't even got it to work.