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source/tutorials/nix-language.md
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# Nix language basics
The Nix language is a domain-specific, purely functional, lazily evaluated programming language.
It is used to declare packages and configurations for the Nix package manager.
It is used to declare packages and configurations for the [Nix package manager][nix-manual].
## Reading the Nix language without fear
The purpose of Nix language is to define structured data.
It supports functions, for conveniently producing more complex data, and assigning names, for manipulating complex data as units.
You will quickly encounter Nix language expressions that may look very complicated.
Yet, the language has only few basic constructs which can be combined arbitrarily.
To that end, every valid piece of Nix language code is an *expression*.
Evaluating a *Nix expression* produces a single value.
Every *Nix file* (`.nix`) contains a single Nix expression.
:::{note}
To *evaluate* means to transform an expression according to the language rules until no further simplification is possible.
:::
[nix-manual]: https://nixos.org/manual/nix/stable/#preface
[nixpkgs-manual]: https://nixos.org/manual/nixpkgs/stable/#preface
[nixos-manual]: https://nixos.org/manual/nixos/stable/index.html#preface
[home-manager]: https://github.com/nix-community/home-manager
Notable uses of the Nix language are:
- [Nix package collection][nixpkgs] (`nixpkgs`)
It is the largest, most up-to-date software distribution in the world, and written in Nix language.
- [NixOS][nixos-manual], a Linux distribution which can be configured fully declaratively
The underlying modular configuration system is written in Nix language, and uses packages from `nixpkgs`.
The operating system environment and services it provides are configured with Nix language.
## Overview
Using the Nix language in practice entails multiple things:
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- built-in functions and the standard library
- using build inputs and build results
It *does not* explain all Nix language features in detail.
See the [Nix manual][manual-language] for a full language reference.
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- Install the Nix package manager, to run the examples
<!-- TODO: approximate amount of time, as observed with test subjects -->
## Basic Concepts
Imagine the Nix language as *JSON with functions*.
The purpose of Nix language is to define structured data.
Functions help with conveniently producing more complex data, and assigning names allows manipulating complex data as units.
To that end, every valid piece of Nix language code is an *expression*.
Evaluating a Nix expression produces a single value.
Every Nix file (`.nix`) contains a single expression.
:::{note}
To *evaluate* means to transform an expression according to the language rules until no further simplification is possible.
:::
### Running examples
### How to run the examples?
All examples in this guide are valid Nix files that you can run yourself.
@ -109,6 +119,20 @@ nix-repl> 1 + 2
3
## Reading the Nix language without fear
You will quickly encounter Nix language expressions that may look very complicated.
Yet, the language has only few basic constructs which can be combined arbitrarily.
As with any programming language, the required amount of Nix language code closely matches the complexity of the problem it is supposed to solve, and reflects how well the problem – and its solution – is understood.
Building software is a complex undertaking, and the Nix package manager both exposes and allows managing this complexity with the Nix language.
Most of the software you will want use with Nix is probably already in the [Nix package collection][nixpkgs].
Therefore you may not need to actually write any Nix language code in the beginning.
To get started, imagine the Nix language as *JSON with functions*.
## Names and values
There are two ways to assign names to values in Nix: attribute sets and `let` expressions.
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The value of a named path is a file system path that depends on the contents of the [`$NIX_PATH`][NIX_PATH] environment variable.
In practice, `<nixpkgs>` points to the file system path of some revision of the [Nix package collection][nixpkgs].
In practice, `<nixpkgs>` points to the file system path of some revision of the Nix package collection's source repository [nixpkgs][nixpkgs].
For example, `<nixpkgs/lib>` points to the subdirectory `lib` of that file system path.
While you will see many such examples, we recommend to {ref}`avoid search paths <search-path>` in practice, as they are not fully reproducible.