+ Novice Nix: Flake Templates +

Flakes are very handy to setup entirely pure, project-specific dependencies (not just dependencies, but build steps, shell environments and more) in a declarative way. Writing Flake expressions can get repetitive though, oftentimes, you’d much rather start off with a skeleton. Luckily, nix already supports templates!

You might already be familiar with nix flake init, that drops a “default” flake expression into your current working directory. If you head over to the manpage:

nix flake init --help

You will read that nix flake init creates a flake using the “default template”. Additionally, you can create a flake from a specific template by passing the -t flag. Where does this default originate from?

Flake Registries

Quick detour into registries! Registries are a way to alias popular flakes using identifiers:

# list a few predefined registries
+$ nix registry list
+. . . 
+global flake:nixpkgs github:NixOS/nixpkgs
+global flake:patchelf github:NixOS/patchelf
+global flake:nix-serve github:edolstra/nix-serve
+global flake:templates github:NixOS/templates
+global flake:nickel github:tweag/nickel
+. . .
+
+# you can do 
+$ nix flake show nickel
+
+# instead of 
+$ nix flake show github:tweag/nickel
+
+# which is short for
+$ nix flake show git+https://github.com/tweag/nickel

You might notice a registry called templates aliased to github:NixOS/templates. Take a peek with nix flake show:

$ nix flake show templates
+github:NixOS/templates/79f48a7b822f35c068c5e235da2e9fbd154cecee
+├───defaultTemplate: template: A very basic flake
+└───templates
+    ├───bash-hello: template: An over-engineered Hello World in bash
+    ├───c-hello: template: An over-engineered Hello World in C
+    ├───rust-web-server: template: A Rust web server including a NixOS module
+    ├───simpleContainer: template: A NixOS container running apache-httpd
+    └───trivial: template: A very basic flake

Aha! There is a flake output called defaultTemplate. This is the template being sourced when you run nix flake init. Astute readers may conclude the following:

$ nix flake init
+
+# is equivalent to
+$ nix flake init -t templates#defaultTemplate
+
+# is equivalent to
+$ nix flake init -t github:NixOS/templates#defaultTemplate
+
+# which is short for
+$ nix flake init -t git+https://NixOS/templates#defaultTemplate

Similarly, the other templates can be accessed via:

$ nix flake init -t templates#c-hello
+$ nix flake init -t templates#simpleContainer
+# I think you get the drift ...

Rolling your own templates

Alright, so all we need to do is:

create a flake with a templates output
populate our template directories with content
(optionally) alias our custom templates flake to an identifier using registries, for easier access

Start off by creating a directory to store your templates in (we will be converting this to a registry later):

$ mkdir ~/mytemplates

A flake that exposes a “template” as its output looks something like this:

# inside ~/mytemplates/flake.nix
+
+{
+  description = "Pepper's flake templates";
+
+  outputs = { self, ... }: {
+    templates = {
+      latex-report = {
+        path = ./latex-report-template;
+        description = "A latex whitepaper project";
+      };
+      rust-hello = {
+        path = ./rust-hello-template;
+        description = "Simple Hello World in Rust";
+      };
+    };
+  };
+}

The path attribute to each template is what gets copied over when you initialize a flake. Running nix flake init -t .#latex-report will initialize the current directory with the contents of ./latex-report-template (we are yet to populate these directories).

The output of nix flake show should be something like:

$ nix flake show
+path:/home/np/code/nix-stuff/template-tests?narHash=sha256-{...}
+└───templates
+    ├───latex-report: template: A latex whitepaper project
+    └───rust-hello: template: Simple Hello World in Rust

Populate your template directories with content, here are my template directories for example:

$ tree mytemplates
+mytemplates/
+├── flake.nix
+├── latex-report-template
+│   ├── flake.nix
+│   ├── makefile
+│   └── src
+│       ├── meta.sty
+│       └── report.tex
+└── rust-hello-template
+    ├── Cargo.toml
+    ├── flake.nix
+    └── src
+        └── main.rs

And that’s it! Start using your templates with:

$ nix flake init -t ~/mytemplates#rust-hello
+$ tree .
+.
+├── Cargo.toml
+├── flake.nix
+└── src
+    └── main.rs

To avoid writing ~/mytemplates each time, simply alias it to a registry:

# alias it to `biscuits`
+$ nix registry add biscuits ~/mytemplates
+
+# you will see it listed under `user` registries
+$ nix registry list
+. . .
+user   flake:biscuits path:/home/np/template-tests
+. . .
+
+$ nix flake init -t biscuits#latex-report

Extending the official templates

I personally, would like the biscuits registry to include not just my homemade templates, but also the templates from NixOS/templates (and maybe a couple of other repositories in the wild):

 {
+   description = "Pepper's flake templates";
+ 
++  inputs = {
++    official-templates.url = github:NixOS/templates;
++    other-templates.url = github:some-other/templates;
++  };
+ 
+   outputs = { self, official-templates, other-templates ... }: {
+ 
+     templates = {
+       latex-report = {
+         path = ./latex-report-template;
+         description = "A latex whitepaper project";
+       };
+       rust-hello = {
+         path = ./rust-hello-template;
+         description = "Simple Hello World in Rust, with overloaded Rust toolchain";
+       };
+     }
++    // official-templates.templates
++    // other-templates.templates;
+ 
+   };
+ }

Running nix flake show biscuits will now list templates from the biscuits registry as well as the ones from NixOS/templates. Ensure that the names don’t collide though.

+ +

+ Hi. +

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I'm Akshay, I go by nerd or nerdypepper on the internet.

+ I am a compsci undergrad, Rust programmer and an enthusiastic Vimmer. + I write open-source stuff to pass time. + I also design fonts: + scientifica, + curie. +

Send me a mail at nerdy@peppe.rs or a message at nerdypepper@irc.rizon.net.

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