pyrox.dev
1# Ruby {#sec-language-ruby}
2
3## Using Ruby {#using-ruby}
4
5Several versions of Ruby interpreters are available on Nix, as well as over 250 gems and many applications written in Ruby. The attribute `ruby` refers to the default Ruby interpreter, which is currently MRI 2.6. It's also possible to refer to specific versions, e.g. `ruby_2_y`, `jruby`, or `mruby`.
6
7In the Nixpkgs tree, Ruby packages can be found throughout, depending on what they do, and are called from the main package set. Ruby gems, however are separate sets, and there's one default set for each interpreter (currently MRI only).
8
9There are two main approaches for using Ruby with gems. One is to use a specifically locked `Gemfile` for an application that has very strict dependencies. The other is to depend on the common gems, which we'll explain further down, and rely on them being updated regularly.
10
11The interpreters have common attributes, namely `gems`, and `withPackages`. So you can refer to `ruby.gems.nokogiri`, or `ruby_2_7.gems.nokogiri` to get the Nokogiri gem already compiled and ready to use.
12
13Since not all gems have executables like `nokogiri`, it's usually more convenient to use the `withPackages` function like this: `ruby.withPackages (p: with p; [ nokogiri ])`. This will also make sure that the Ruby in your environment will be able to find the gem and it can be used in your Ruby code (for example via `ruby` or `irb` executables) via `require "nokogiri"` as usual.
14
15### Temporary Ruby environment with `nix-shell` {#temporary-ruby-environment-with-nix-shell}
16
17Rather than having a single Ruby environment shared by all Ruby development projects on a system, Nix allows you to create separate environments per project. `nix-shell` gives you the possibility to temporarily load another environment akin to a combined `chruby` or `rvm` and `bundle exec`.
18
19There are two methods for loading a shell with Ruby packages. The first and recommended method is to create an environment with `ruby.withPackages` and load that.
20
21```ShellSession
22$ nix-shell -p "ruby.withPackages (ps: with ps; [ nokogiri pry ])"
23```
24
25The other method, which is not recommended, is to create an environment and list all the packages directly.
26
27```ShellSession
28$ nix-shell -p ruby.gems.nokogiri ruby.gems.pry
29```
30
31Again, it's possible to launch the interpreter from the shell. The Ruby interpreter has the attribute `gems` which contains all Ruby gems for that specific interpreter.
32
33#### Load Ruby environment from `.nix` expression {#load-ruby-environment-from-.nix-expression}
34
35As explained [in the `nix-shell` section](https://nixos.org/manual/nix/stable/command-ref/nix-shell) of the Nix manual, `nix-shell` can also load an expression from a `.nix` file.
36Say we want to have Ruby 2.6, `nokogori`, and `pry`. Consider a `shell.nix` file with:
37
38```nix
39with import <nixpkgs> {};
40ruby.withPackages (ps: with ps; [ nokogiri pry ])
41```
42
43What's happening here?
44
451. We begin with importing the Nix Packages collections. `import <nixpkgs>` imports the `<nixpkgs>` function, `{}` calls it and the `with` statement brings all attributes of `nixpkgs` in the local scope. These attributes form the main package set.
462. Then we create a Ruby environment with the `withPackages` function.
473. The `withPackages` function expects us to provide a function as an argument that takes the set of all ruby gems and returns a list of packages to include in the environment. Here, we select the packages `nokogiri` and `pry` from the package set.
48
49#### Execute command with `--run` {#execute-command-with---run}
50
51A convenient flag for `nix-shell` is `--run`. It executes a command in the `nix-shell`. We can e.g. directly open a `pry` REPL:
52
53```ShellSession
54$ nix-shell -p "ruby.withPackages (ps: with ps; [ nokogiri pry ])" --run "pry"
55```
56
57Or immediately require `nokogiri` in pry:
58
59```ShellSession
60$ nix-shell -p "ruby.withPackages (ps: with ps; [ nokogiri pry ])" --run "pry -rnokogiri"
61```
62
63Or run a script using this environment:
64
65```ShellSession
66$ nix-shell -p "ruby.withPackages (ps: with ps; [ nokogiri pry ])" --run "ruby example.rb"
67```
68
69#### Using `nix-shell` as shebang {#using-nix-shell-as-shebang}
70
71In fact, for the last case, there is a more convenient method. You can add a [shebang](<https://en.wikipedia.org/wiki/Shebang_(Unix)>) to your script specifying which dependencies `nix-shell` needs. With the following shebang, you can just execute `./example.rb`, and it will run with all dependencies.
72
73```ruby
74#! /usr/bin/env nix-shell
75#! nix-shell -i ruby -p "ruby.withPackages (ps: with ps; [ nokogiri rest-client ])"
76
77require 'nokogiri'
78require 'rest-client'
79
80body = RestClient.get('http://example.com').body
81puts Nokogiri::HTML(body).at('h1').text
82```
83
84## Developing with Ruby {#developing-with-ruby}
85
86### Using an existing Gemfile {#using-an-existing-gemfile}
87
88In most cases, you'll already have a `Gemfile.lock` listing all your dependencies. This can be used to generate a `gemset.nix` which is used to fetch the gems and combine them into a single environment. The reason why you need to have a separate file for this, is that Nix requires you to have a checksum for each input to your build. Since the `Gemfile.lock` that `bundler` generates doesn't provide us with checksums, we have to first download each gem, calculate its SHA256, and store it in this separate file.
89
90So the steps from having just a `Gemfile` to a `gemset.nix` are:
91
92```ShellSession
93$ bundle lock
94$ bundix
95```
96
97If you already have a `Gemfile.lock`, you can run `bundix` and it will work the same.
98
99To update the gems in your `Gemfile.lock`, you may use the `bundix -l` flag, which will create a new `Gemfile.lock` in case the `Gemfile` has a more recent time of modification.
100
101Once the `gemset.nix` is generated, it can be used in a `bundlerEnv` derivation. Here is an example you could use for your `shell.nix`:
102
103```nix
104# ...
105let
106 gems = bundlerEnv {
107 name = "gems-for-some-project";
108 gemdir = ./.;
109 };
110in mkShell { packages = [ gems gems.wrappedRuby ]; }
111```
112
113With this file in your directory, you can run `nix-shell` to build and use the gems. The important parts here are `bundlerEnv` and `wrappedRuby`.
114
115The `bundlerEnv` is a wrapper over all the gems in your gemset. This means that all the `/lib` and `/bin` directories will be available, and the executables of all gems (even of indirect dependencies) will end up in your `$PATH`. The `wrappedRuby` provides you with all executables that come with Ruby itself, but wrapped so they can easily find the gems in your gemset.
116
117One common issue that you might have is that you have Ruby 2.6, but also `bundler` in your gemset. That leads to a conflict for `/bin/bundle` and `/bin/bundler`. You can resolve this by wrapping either your Ruby or your gems in a `lowPrio` call. So in order to give the `bundler` from your gemset priority, it would be used like this:
118
119```nix
120# ...
121mkShell { buildInputs = [ gems (lowPrio gems.wrappedRuby) ]; }
122```
123
124Sometimes a Gemfile references other files. Such as `.ruby-version` or vendored gems. When copying the Gemfile to the nix store we need to copy those files alongside. This can be done using `extraConfigPaths`. For example:
125
126```nix
127 gems = bundlerEnv {
128 name = "gems-for-some-project";
129 gemdir = ./.;
130 extraConfigPaths = [ "${./.}/.ruby-version" ];
131 };
132```
133
134### Gem-specific configurations and workarounds {#gem-specific-configurations-and-workarounds}
135
136In some cases, especially if the gem has native extensions, you might need to modify the way the gem is built.
137
138This is done via a common configuration file that includes all of the workarounds for each gem.
139
140This file lives at `/pkgs/development/ruby-modules/gem-config/default.nix`, since it already contains a lot of entries, it should be pretty easy to add the modifications you need for your needs.
141
142In the meanwhile, or if the modification is for a private gem, you can also add the configuration to only your own environment.
143
144Two places that allow this modification are the `ruby` derivation, or `bundlerEnv`.
145
146Here's the `ruby` one:
147
148```nix
149{ pg_version ? "10", pkgs ? import <nixpkgs> { } }:
150let
151 myRuby = pkgs.ruby.override {
152 defaultGemConfig = pkgs.defaultGemConfig // {
153 pg = attrs: {
154 buildFlags =
155 [ "--with-pg-config=${pkgs."postgresql_${pg_version}"}/bin/pg_config" ];
156 };
157 };
158 };
159in myRuby.withPackages (ps: with ps; [ pg ])
160```
161
162And an example with `bundlerEnv`:
163
164```nix
165{ pg_version ? "10", pkgs ? import <nixpkgs> { } }:
166let
167 gems = pkgs.bundlerEnv {
168 name = "gems-for-some-project";
169 gemdir = ./.;
170 gemConfig = pkgs.defaultGemConfig // {
171 pg = attrs: {
172 buildFlags =
173 [ "--with-pg-config=${pkgs."postgresql_${pg_version}"}/bin/pg_config" ];
174 };
175 };
176 };
177in mkShell { buildInputs = [ gems gems.wrappedRuby ]; }
178```
179
180And finally via overlays:
181
182```nix
183{ pg_version ? "10" }:
184let
185 pkgs = import <nixpkgs> {
186 overlays = [
187 (self: super: {
188 defaultGemConfig = super.defaultGemConfig // {
189 pg = attrs: {
190 buildFlags = [
191 "--with-pg-config=${
192 pkgs."postgresql_${pg_version}"
193 }/bin/pg_config"
194 ];
195 };
196 };
197 })
198 ];
199 };
200in pkgs.ruby.withPackages (ps: with ps; [ pg ])
201```
202
203Then we can get whichever postgresql version we desire and the `pg` gem will always reference it correctly:
204
205```ShellSession
206$ nix-shell --argstr pg_version 9_4 --run 'ruby -rpg -e "puts PG.library_version"'
20790421
208
209$ nix-shell --run 'ruby -rpg -e "puts PG.library_version"'
210100007
211```
212
213Of course for this use-case one could also use overlays since the configuration for `pg` depends on the `postgresql` alias, but for demonstration purposes this has to suffice.
214
215### Platform-specific gems {#ruby-platform-specif-gems}
216
217Right now, bundix has some issues with pre-built, platform-specific gems: [bundix PR #68](https://github.com/nix-community/bundix/pull/68).
218Until this is solved, you can tell bundler to not use platform-specific gems and instead build them from source each time:
219- globally (will be set in `~/.config/.bundle/config`):
220```shell
221$ bundle config set force_ruby_platform true
222```
223- locally (will be set in `<project-root>/.bundle/config`):
224```shell
225$ bundle config set --local force_ruby_platform true
226```
227
228### Adding a gem to the default gemset {#adding-a-gem-to-the-default-gemset}
229
230Now that you know how to get a working Ruby environment with Nix, it's time to go forward and start actually developing with Ruby. We will first have a look at how Ruby gems are packaged on Nix. Then, we will look at how you can use development mode with your code.
231
232All gems in the standard set are automatically generated from a single `Gemfile`. The dependency resolution is done with `bundler` and makes it more likely that all gems are compatible to each other.
233
234In order to add a new gem to nixpkgs, you can put it into the `/pkgs/development/ruby-modules/with-packages/Gemfile` and run `./maintainers/scripts/update-ruby-packages`.
235
236To test that it works, you can then try using the gem with:
237
238```shell
239NIX_PATH=nixpkgs=$PWD nix-shell -p "ruby.withPackages (ps: with ps; [ name-of-your-gem ])"
240```
241
242### Packaging applications {#packaging-applications}
243
244A common task is to add a ruby executable to nixpkgs, popular examples would be `chef`, `jekyll`, or `sass`. A good way to do that is to use the `bundlerApp` function, that allows you to make a package that only exposes the listed executables, otherwise the package may cause conflicts through common paths like `bin/rake` or `bin/bundler` that aren't meant to be used.
245
246The absolute easiest way to do that is to write a `Gemfile` along these lines:
247
248```ruby
249source 'https://rubygems.org' do
250 gem 'mdl'
251end
252```
253
254If you want to package a specific version, you can use the standard Gemfile syntax for that, e.g. `gem 'mdl', '0.5.0'`, but if you want the latest stable version anyway, it's easier to update by running the `bundle lock` and `bundix` steps again.
255
256Now you can also make a `default.nix` that looks like this:
257
258```nix
259{ bundlerApp }:
260
261bundlerApp {
262 pname = "mdl";
263 gemdir = ./.;
264 exes = [ "mdl" ];
265}
266```
267
268All that's left to do is to generate the corresponding `Gemfile.lock` and `gemset.nix` as described above in the `Using an existing Gemfile` section.
269
270#### Packaging executables that require wrapping {#packaging-executables-that-require-wrapping}
271
272Sometimes your app will depend on other executables at runtime, and tries to find it through the `PATH` environment variable.
273
274In this case, you can provide a `postBuild` hook to `bundlerApp` that wraps the gem in another script that prefixes the `PATH`.
275
276Of course you could also make a custom `gemConfig` if you know exactly how to patch it, but it's usually much easier to maintain with a simple wrapper so the patch doesn't have to be adjusted for each version.
277
278Here's another example:
279
280```nix
281{ lib, bundlerApp, makeWrapper, git, gnutar, gzip }:
282
283bundlerApp {
284 pname = "r10k";
285 gemdir = ./.;
286 exes = [ "r10k" ];
287
288 nativeBuildInputs = [ makeWrapper ];
289
290 postBuild = ''
291 wrapProgram $out/bin/r10k --prefix PATH : ${lib.makeBinPath [ git gnutar gzip ]}
292 '';
293}
294```