Groking the NixOS Langauge

By Mike Levin

Understanding the Nix System Configuration Language Basics in Haskell

This is an exchange between myself and Perplexity.AI (on the free Claude model at the time of its occurrence, with me trying to gain some intuitive understanding of the strange Haskell-inspired Nix system configuration language. I really don’t like taking up additional languages, but there’s a category of usually imperative languages like SQL and HTML you’re never going to get away from. The Nix system configuration language is shaping up to be one of those for me, and this is coming to grips with my Pythonic gripes.

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Me: Explain to me philosophically the NixOS declarative system building language. It doesn’t make sense to me, but the appeal of what it’s promising is overwhelming. I need to learn how to think about it.

Perplexity: The philosophical appeal of NixOS’s declarative system building language lies in its emphasis on purity, determinism, and reproducibility, which are key principles in functional programming and system design.

Declarative Approach

NixOS uses a declarative approach, meaning you describe the desired state of your system rather than the steps to achieve it. This is akin to stating what you want rather than how to get it, allowing for more predictable and consistent system configurations[1][2].

Functional Language

Nix expressions are written in a pure, lazy functional language. Purity ensures that expressions have no side effects and always yield the same result given the same input, which contributes to system reliability and predictability[2][4]. Laziness allows for efficient evaluation by computing results only when needed, which can optimize resource usage[2].

Reproducibility

Nix ensures that builds are isolated and reproducible. This means that if a package works on one machine, it will work the same way on another, reducing “it works on my machine” problems[1]. The Nix store, where packages are stored, uses cryptographic hashes to ensure that builds are based on exact inputs, further enhancing reproducibility[2].

System Configuration

NixOS requires a single configuration file, making system management straightforward. This file defines the entire system state, allowing for easy rollbacks and consistent deployments across different environments[2].

Understanding NixOS philosophically involves appreciating these principles and how they contribute to a more reliable and manageable system.

Sources

[1] Nix & NixOS | Declarative builds and deployments https://nixos.org
[2] NixOS and the Nix expression language | stace.dev https://stace.dev/nixos-and-the-nix-expression-language/
[3] Declarative Systems - General - Universal Blue - Discourse https://universal-blue.discourse.group/t/declarative-systems/2285
[4] Nix language basics — nix.dev documentation https://nix.dev/tutorials/nix-language.html
[5] Alternative language - NixOS Discourse https://discourse.nixos.org/t/alternative-language/5218
[6] NixOS: Declarative Builds and Deployments - Hacker News https://news.ycombinator.com/item?id=38995068
[7] NixOS is the best linux back-end ever designed. But it can't become ... https://discourse.nixos.org/t/nixos-is-the-best-linux-back-end-ever-designed-but-it-cant-become-mainstream-without-a-front-end/46526
[8] Do you think Nix language is confusing? : r/NixOS - Reddit https://www.reddit.com/r/NixOS/comments/1dat919/do_you_think_nix_language_is_confusing/

Me: Okay, I get what it’s promising. Now get into the nitty gritty bit in a baby steps build it up from hello world barebones examples. I know is it starts at an /etc/nixos/configuration.nix file, but that’s where I’m lost. I’m a Python person with a long history with scripting languages, but not functional languages like Haskell which nix looks based on. Build up maybe with 3 steps from a non-functional system to one that boots to a command-line interface on generic hardware. Point out the key principles of the language, particularly issues of syntax, scope, parameter padding, closure and such.

Perplexity: To get started with NixOS and understand its declarative system, let’s break down the process into three steps, focusing on creating a simple system configuration that can boot to a command-line interface.

Step 1: Basic Configuration

Start with the /etc/nixos/configuration.nix file. This file is the heart of NixOS configuration, where you declare the system’s desired state.

{ config, pkgs, ... }:

{
  imports = [ ];

  boot.loader.grub.device = "/dev/sda"; # Adjust according to your disk
  fileSystems."/" = {
    device = "/dev/sda1"; # Adjust according to your partition
    fsType = "ext4";
  };

  services.sshd.enable = true; # Enable SSH for remote access
}

• Syntax: Nix uses a functional language with attribute sets, similar to dictionaries in Python.
• Scope: Variables are scoped within the braces {} and can be referenced using dot notation.

Step 2: Adding Packages

Next, enhance your configuration by adding essential packages and enabling a simple service.

{ config, pkgs, ... }:

{
  imports = [ ];

  environment.systemPackages = with pkgs; [ vim git ];

  services.sshd.enable = true;

  boot.loader.grub.device = "/dev/sda";
  fileSystems."/" = {
    device = "/dev/sda1";
    fsType = "ext4";
  };
}

• Packages: Use environment.systemPackages to declare packages. The with keyword helps include multiple packages concisely.
• Closures: Nix ensures that all dependencies for these packages are included, forming a closure.

Step 3: Boot to CLI

Finally, ensure the system boots to a command-line interface by setting the appropriate display manager.

{ config, pkgs, ... }:

{
  imports = [ ];

  environment.systemPackages = with pkgs; [ vim git ];

  services.sshd.enable = true;

  boot.loader.grub.device = "/dev/sda";
  fileSystems."/" = {
    device = "/dev/sda1";
    fsType = "ext4";
  };

  services.xserver.enable = false; # Disable graphical interface
}

• Declarative: This configuration ensures that the system state is as declared, without imperative commands.
• Reproducibility: You can replicate this setup on any machine with the same configuration file.

By following these steps, you can build a basic NixOS system from scratch, understanding key principles like syntax, scope, and closures.

Sources

[1] Hello world project templates for getting started quickly with Nix https://github.com/jwiegley/hello
[2] Nix language basics https://nix.dev/tutorials/nix-language.html
[3] How to build nix hello world - Stack Overflow https://stackoverflow.com/questions/48470942/how-to-build-nix-hello-world
[4] Nix by example - NixOS Wiki https://nixos.wiki/wiki/Nix_by_example
[5] zetavg/nix-hello-world - GitHub https://github.com/zetavg/nix-hello-world
[6] Declarative Systems https://universal-blue.discourse.group/t/declarative-systems/2285
[7] Writing a (Nix-Friendly) Hello World Web App in Haskell with Scotty https://webbureaucrat.gitlab.io/articles/writing-a-hello-world-web-app-in-haskell-with-scotty/
[8] NixOS and the Nix expression language https://stace.dev/nixos-and-the-nix-expression-language/

Me: Okay I notice a few things here that strike me as unusual. Usually when looking at something like this, one’s mind goes to XML or JSON and desires seeing a root element. An I correct in assuming there is no requirement for an all-encompassing singleton root element, and instead we are immediately hit with “pairs” in the spirit of key/value or name/value pairs, each side of the pair contained in curly braces. But the curly brace pairs can be nested as fileSystems shows, an example where the name or key doesn’t need curly braces. What is the overarching rules here? What sort of abstract data structures am I looking at, and where can I go see the data structure, nesting and termination rules as one would for JSON or XML?

Perplexity: You’re correct in observing that NixOS configuration files do not require a single root element like XML or JSON. Instead, they use a functional language with a syntax that might appear unusual at first glance. Let’s break down the structure and rules:

Attribute Sets

Nix expressions are built around attribute sets, which are similar to dictionaries in Python or objects in JSON. They consist of key-value pairs enclosed in curly braces {}. Here are some key points:

• Key-Value Pairs: Each entry in an attribute set is a key-value pair. The key is a name (identifier), and the value can be any valid Nix expression, including other attribute sets.
• Nesting: Attribute sets can be nested, allowing for hierarchical structures. For example, fileSystems."/" is a nested attribute set where "/" is the key.
• Dot Notation: You can access nested attributes using dot notation, similar to accessing properties in JavaScript objects.

Syntax Rules

• Keys: Keys can be unquoted if they are valid identifiers (alphanumeric and underscores). If a key contains special characters (like /), it must be quoted.
• Values: Values can be strings, numbers, booleans, lists, or other attribute sets.
• Lists: Lists are enclosed in square brackets [] and can contain any Nix expressions.

Abstract Data Structures

The primary data structure in Nix is the attribute set. Here’s a simple breakdown:

• Attribute Set: { key1 = value1; key2 = value2; }
• List: [ value1 value2 value3 ]
• String: "string value"

Resources for Learning

To understand the data structure, nesting, and termination rules better, here are some resources you can explore:

1. Nix Manual: The official Nix manual provides detailed explanations of the language syntax and semantics. It is a comprehensive resource for understanding Nix expressions.
   • Nix Manual
2. Nix Pills: This is a series of tutorials that introduce Nix concepts incrementally, making it easier to grasp the language’s unique features.
   • Nix Pills
3. NixOS Wiki: The community-maintained wiki contains articles and examples on various NixOS topics, including configuration.
   • NixOS Wiki

By exploring these resources, you’ll gain a deeper understanding of the Nix language’s structure and how to effectively use it to manage system configurations.

Me: If this is true:

“Nix expressions are built around attribute sets, which are similar to dictionaries in Python or objects in JSON. They consist of key-value pairs enclosed in curly braces {}. “

…then why does the example open with:

{ config, pkgs, … }:

That appear to be the key component of a key/value data structure but it closes before the expression is complete. There’s no assignment in those curly braces. So what gives? Is this the left-hand side of an assignment using curly braces differently (already) than attribute sets? So the top use of nix opens with an exception to the rule? I find this very confusing.

Perplexity: Great observation! The syntax { config, pkgs, ... }: at the beginning of a NixOS configuration file can indeed be confusing at first. This is not an attribute set but rather a function argument pattern. Let me explain:

Function Argument Pattern

In Nix, functions are first-class citizens, and the configuration file is essentially a function. The line { config, pkgs, ... }: is a pattern that matches the arguments passed to this function. Here’s a breakdown:

• Curly Braces: In this context, the curly braces are used for pattern matching, not for defining an attribute set.
• Arguments: config, pkgs, and ... are arguments that the function receives. These are provided by the NixOS system when it evaluates the configuration.
  • config: Represents the current configuration state.
  • pkgs: Provides access to the package set.
  • ...: A shorthand for “other arguments” that might be passed, allowing for flexibility.

Function Definition

The configuration file in NixOS is essentially a function that takes these arguments and returns an attribute set representing the system configuration. The actual configuration starts after the colon :.

Here’s a simplified view:

{ config, pkgs, ... }:  # Function arguments
{
  # This is the attribute set returned by the function
  environment.systemPackages = with pkgs; [ vim git ];
  services.sshd.enable = true;
}

Key Points

• Pattern Matching: This pattern matching allows the function to destructure the arguments, similar to how you might destructure objects in JavaScript or unpack tuples in Python.
• No Assignment: The line { config, pkgs, ... }: does not involve assignment. It simply defines the parameters that the function will use.

Why This Syntax?

This syntax allows NixOS configurations to be modular and reusable. By treating the configuration as a function, you can easily import and compose different configurations, passing different arguments as needed.

Understanding this distinction between pattern matching for function arguments and attribute sets is crucial for working with Nix expressions. Once you get used to this functional paradigm, it becomes a powerful tool for managing configurations.

Me: That breaks my head. I understand unpacking tuples in Python, it usually during assignment and not function calls. I understand splatting arguments in Python by way of unpacking tiples. But the parallel to what’s going on here eludes me. Is there some very basic bare bones hello world style example you can show me? Maybe one that draws the parallel between nix and Python? And what’s further, I need a follow up to distinguish that from the other use of curly braces, the Attribute Sets. That use cares are colliding in my mind.

Perplexity: I understand that this can be quite confusing, especially when coming from a Python background. Let’s break it down with a simple example and draw parallels to Python to help clarify these concepts.

Nix Function with Pattern Matching

In Nix, the pattern { config, pkgs, ... }: is used to define a function that takes a set of arguments. Let’s create a simple example to illustrate this:

# hello.nix
{ name, greeting }:  # Function arguments pattern
{
  message = "${greeting}, ${name}!";  # Attribute set returned by the function
}

• Function Arguments: { name, greeting } is a pattern that matches the arguments passed to the function. This is similar to unpacking a dictionary in Python.
• Attribute Set: The function returns an attribute set with a single key-value pair, message.

Equivalent Python Example

In Python, you might achieve something similar using a function that takes a dictionary and unpacks it:

def hello(**kwargs):
    name = kwargs.get('name')
    greeting = kwargs.get('greeting')
    return {
        'message': f"{greeting}, {name}!"
    }

# Usage
args = {'name': 'World', 'greeting': 'Hello'}
result = hello(**args)
print(result['message'])  # Output: "Hello, World!"

• Function Definition: The hello function takes keyword arguments (**kwargs) and unpacks them to access name and greeting.
• Dictionary Return: The function returns a dictionary, similar to the attribute set in Nix.

Distinguishing Between Function Arguments and Attribute Sets

1. Function Arguments: In Nix, the curly braces used in { name, greeting }: define a pattern for function arguments. This is akin to unpacking a dictionary in Python when calling a function.

2. Attribute Sets: Curly braces used elsewhere in Nix define attribute sets, similar to dictionaries in Python. For example:

   {
     key1 = "value1";
     key2 = "value2";
   }
   

   This is equivalent to a Python dictionary:

   {
     'key1': 'value1',
     'key2': 'value2'
   }
   

Key Differences

• Function Arguments: Curly braces in the function argument pattern are used for destructuring the input, not for creating an attribute set.
• Attribute Sets: Curly braces in other contexts are used to define key-value pairs, similar to dictionaries.

By understanding these distinctions, you can better grasp how Nix expressions work and how they differ from Python code. The functional paradigm in Nix allows for powerful abstractions and modular configurations, but it does require a shift in thinking from imperative programming.

The Bane of Curly Braces in Nix Programming Language

Me: Write a poem called the bane of curly braces in nix which creatively and clearly differentiates the two common uses that you will encounter right away in rapid succession and must grok, much to the chagrin of a monoglot Pythonista.

Perplexity: The Bane of Curly Braces in Nix

In the land of Nix, where syntax is king,  
Curly braces dance, and their meanings swing.  
For a Pythonista, with eyes wide and bright,  
These twisting symbols bring both awe and fright.

First, they appear in a function's embrace,  
Pattern matching arguments, setting the pace.  
"Config and pkgs," they whisper with grace,  
Destructuring inputs, finding their place.

Like Python's kwargs, they unpack with flair,  
Yet no assignment here, just patterns laid bare.  
A function's prelude, a declarative song,  
In the realm of Nix, where logic is strong.

Then, in a twist, they shift their guise,  
To form attribute sets, a new surprise.  
Key-value pairs, like dictionaries known,  
In curly confines, their structure is shown.

"Environment and services," they declare,  
Mapping the system with meticulous care.  
No longer a pattern, but a set of keys,  
Defining the state with elegant ease.

Oh, Pythonista, with monoglot heart,  
Fear not the braces that set Nix apart.  
For once you discern their duality's art,  
You'll wield them with wisdom, a functional start.

In the dance of the braces, both subtle and grand,  
You'll find your way through this declarative land.  
With patience and practice, the syntax will stick,  
And you'll master the magic of the language called Nix.