(reading-nix-language)=

# Reading the Nix language without fear

The Nix language is used to declare packages and configurations for the Nix package manager.

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.

Using the Nix language in practice entails multiple things:

- language: syntax and semantics
- standard libraries: `builtins` and `nixpkgs/lib`
- generic build mechanisms: `stdenv`, trivial builders, ...
- composition and configuration mechanisms: `override`, `overrideAttrs`, overlays, `callPackage`, ...
- ecosystem-specific packaging mechanisms: `buildGoModule`, `buildPythonApplication`, ...
- NixOS module system: `config`, `option`, ...

This guide *only* covers syntax and semantics, and will guide you to resources for learning the other components.

## What will you learn?

This guide should enable you to read typical Nix language code and understand its structure.

It shows the most common and distingushing patterns in the Nix language:

- assigning names and accessing values
- declaring and calling functions
- referencing file system paths
- working with character strings
- using built-in functions and the standard library
- declaring build inputs and build outputs

It *does not* explain all Nix language features in detail.
See the [Nix manual][manual-language] for a full language reference.

[manual-language]: https://nixos.org/manual/nix/stable/expressions/expression-language.html

## What do you need?

- Familiarity with other programming languages
- Familiarity with Unix shell to read command line examples <!-- TODO: link to yet-to-be instructions on "how to read command line examples" -->
- 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

All examples in this guide are valid Nix files that you can run yourself.

The following example is a Nix expression adding two numbers:

```nix
1 + 2
```

    3

Use `nix-instantiate --eval` to evaluate the expression in a Nix file.

```console
echo 1 + 2 > file.nix

nix-instantiate --eval file.nix
```

    3

:::{note}
`nix-instantiate --eval` will evaluate `default.nix` if no file name is specified.

```console
echo 1 + 2 > default.nix

nix-instantiate --eval
```

    3
:::

Use `nix repl` to evaluate Nix expressions interactively (by typing them on the command line):

```console
nix repl
```

    Welcome to Nix 2.5.1. Type :? for help.

```console
nix-repl> 1 + 2
```

    3

# Names and values

There are two ways to assign names to values in Nix: attribute sets and `let` expressions.

Assignments are denoted by a single equal sign (`=`).

## Attribute set `{ ... }`

An attribute set is an unordered collection of name-value-pairs.

Together with primitive data types and lists, attribute sets work like objects in JSON and look very similar.

<table>
<tr>
  <th>Nix language</th>
  <th>JSON</th>
</tr>
<tr>
<td>

```nix
{
  string = "hello";
  integer = 1;
  float = 3.141;
  bool = true;
  null = null;
  list = [ 1 "two" false ];
  attribute-set = {
    a = "hello";
    b = 2;
    c = 2.718;
    d = false;
  }; # comments are supported
}
```

</td>
<td>

```json
{
  "string": "hello",
  "integer": 1,
  "float": 3.141,
  "bool": true,
  "null": null,
  "list": [1, "two", false],
  "set": {
    "a": "hello",
    "b": 1,
    "c": 2.718,
    "d": false
  }
}
```

</td>
</tr>
</table>

:::{note}
- Attribute names usually do not need quotes.[^1]
- List elements are separated by white space.[^2]
:::

[^1]: Details: Nix manual - attribute naming rules <!-- TODO: create and link manual section -->
[^2]: Details: [Nix manual - lists][manual-lists]

[manual-lists]: https://nixos.org/manual/nix/stable/expressions/language-values.html#lists

### Recursive attribute set `rec { ... }`

You will sometimes see attribute sets declared with `rec` prepended.
This allows access to attributes from within the set.

Example:

```nix
rec {
  one = 1;
  two = one + 1;
  three = two + 1;
}
```

    { one = 1; three = 3; two = 2; }

Counter-example:

```nix
{
  one = 1;
  two = one + 1;
  three = two + 1;
}
```

    error: undefined variable 'one'

           at «string»:3:9:

                2|   one = 1;
                3|   two = one + 1;
                 |         ^
                4|   three = two + 1;

{ref}`We recommend to avoid recursive sets <rec-expression>` and to use the `let` expression instead.

## `let ... in ...`

Also known as “`let` expression” or “`let` binding”

`let` expressions allow assigning names to values for repeated use.

Example:

```nix
let
  a = 1;
in
a + a
```

    2

As in attribute sets, names can be assigned in any order.
In contrast to attribute sets, the expressions on the right of the assignment can refer to other assigned names.

Example:

```nix
let
  b = a + 1
  a = 1;
in
a + b
```

    3

Only expressions within the `let` expression itself can access the newly declared names.
We say: the bindings have local scope.

Counter-example:

```nix
{
  a = let x = 1; in x;
  b = x;
}
```

    error: undefined variable 'x'

           at «string»:3:7:

                2|   a = let x = 1; in x;
                3|   b = x;
                 |       ^
                4| }


<!-- TODO: exercise - use let to reuse a value in an attribute set -->

## Attribute access

Attributes in a set can be accessed with a dot (`.`) and the attribute name.

Example:

```nix
let
  attrset = { x = 1; };
in
attrset.x
```

    1

Accessing nested attributes works the same way.

Example:

```nix
let
  attrset = { a = { b = { c = 1; }; }; };
in
attrset.a.b.c
```

    1

The dot (`.`) notation also works when assigning attributes.

Example:

```nix
let
  attrset = { a.b.c = 1; };
in
attrset
```

    { a = { b = { c = 1; }; }; }

## `with ...; ...`

The `with` expression allows access to attributes without repeatedly referencing their attribute set.

Example:

```nix
let
  a = {
    x = 1;
    y = 2;
    z = 3;
  };
in
with a; [ x y z ]
```

    [ 1 2 3 ]

The expression

    with a; [ x y z ]

is equivalent to

    [ a.x a.y a.z ]

Attributes made available through `with` are only in scope of the expression following the semicolon (`;`).

Counter-example:

```nix
let
  a = {
    x = 1;
    y = 2;
    z = 3;
  };
in
{
  b = with a; [ x y z ];
  c = x;
}
```

    error: undefined variable 'x'

           at «string»:10:7:

                9|   b = with a; [ x y z ];
               10|   c = x;
                 |       ^
               11| }

## `inherit ...`

With `inherit` one can assign existing names to attributes of the same name.
It is for convenience to avoid repeating the same name multiple times.

Example:

```
let
  x = 1;
  y = 2;
in
{
  inherit x y;
}
```

    { x = 1; y = 2; }

The fragment

    inherit x y;

is equivalent to

    x = x; y = y;

It is also possible to `inherit` attributes from another set with parentheses (`inherit ( ... ) ...`).

Example:

```nix
let
  a = { x = 1; y = 2; };
in
{
  inherit (a) x y;
}
```

    { x = 1; y = 2; }

The fragment

    inherit (a) x y;

is equivalent to

    x = a.x; y = a.y;

# Functions

Functions are everywhere in the Nix language.

## Argument

Nix functions take exactly one argument.

    x: x + 1

Argument and function body are separated by a colon (`:`).

Wherever you see a colon (`:`) in Nix language code:
- on its left is the function argument
- on its right is the function body.

Calling a function with an operand means writing the operand after the function.

Example

```nix
(x: x + 1) 1
```

    2

Nix functions have no name when declared.
We say they are anonymous, or call such a function a *lambda*.

We can assign functions a name as to any other value.

Example:

```nix
let
  f = x: x + 1;
in
f 1
```

    2

Arguments can be chained.

    x: y: x + y

This can be used like a function that takes two arguments, but offers additional flexibility.

The above function takes one argument and returns a function `y: x + y` with `x` set to the passed value.

Example:

```nix
let
  f = x: y: x + y;
in
f 1
```

    <LAMBDA>

The `<LAMBDA>` indicates the resulting value is an anonymous function.

Applying that to another argument yields the inner body `x + y`, which can now be fully evaluated.

```nix
let
  f = x: y: x + y;
in
f 1 2
```

    3

<!-- TODO: exercise - assign the lambda a name and do something with it -->

## Attribute set argument

Also known as “keyword arguments”.

Nix functions can explicitly take an attribute set as argument.

    {a, b}: a + b

The argument defines the exact attributes that have to be in that set.
Leaving out or passing additional attributes is an error.

Example:

```nix
let
  f = {a, b}: a + b
in
f { a = 1; b = 2; }
```

    3

## Default attributes

Also known as “default arguments”.

Arguments can have default values for attributes, denoted with a question mark (`?`).

    {a, b ? 0}: a + b

Attributes in the argument are not required if they have a default value.

Example:

```nix
let
  f = {a, b ? 0}: a + b
in
f { a = 1; }
```

    1

Example:

```nix
let
  f = {a ? 0, b ? 0}: a + b
in
f { } # empty attribute set
```

    0

## Additional attributes

Additional attributes are allowed with an ellipsis (`...`):

    {a, b, ...}: a + b

Example:

```nix
let
  f = {a, b, ...}: a + b
in
f { a = 1; b = 2; c = 3; }
```

    3

## Named attribute argument

Also known as “@ pattern”, “@ syntax”, or “‘at’ syntax”:

    {a, b, ...}@args: a + b + args.c

or

    args@{a, b, ...}: a + b + args.c

where the passed attribute set is given a name, and some of its attributes are required.

Example:

```nix
let
  f = {a, b, ...}@args: a + b + args.c
in
f { a = 1; b = 2; c = 3; }
```

    6

This can be useful if the passed attribute set also needs to be processed as a whole.

# File system paths

Nix language offers additional convenience for file system paths.[^3]

Absolute paths always start with a slash (`/`):

    /absolute/path

Paths are relative when they contain at least one slash (`/`) but to not start with one.
They are relative to the file containing the expression:

    ./relative

    relative/path

One dot (`.`) as the last path element is ignored.
It can be used to denote the current directory:

    ./.

[^3]: Details: [Nix manual - primitive data types][manual-primitives]

## Search path

Also known as “angle bracket syntax”.

    <nixpkgs>

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].
For example, `<nixpkgs/lib>` points to the subdirectory `lib` of that file system path.

[NIX_PATH]: https://nixos.org/manual/nix/unstable/command-ref/env-common.html?highlight=nix_path#env-NIX_PATH
[nixpkgs]: https://github.org/NixOS/nixpkgs
[manual-primitives]: https://nixos.org/manual/nix/stable/expressions/language-values.html#primitives

# Character strings

<!-- TODO: introduction -->

## String interpolation

Also known as “antiquotiation”.

<!-- TODO: details, especially how this relates to derivations and store paths -->

```nix
let
  name = "Nix";
in
"hello ${name}"
```

    "hello Nix"

<!-- TODO: difference between ${foo} and $foo in build scripts -->

## Indented strings

    ''
      multi
      line
      string
    ''

You will recognize indented strings by *double single quotes*.
Equal amounts of prepended white space are trimmed from the result.

Example:

```nix
''
  one
   two
    three
''
```

    "one\n two\n  three\n"

<!-- TODO: See [escaping rules](). -->

# Using existing functions

There are two widely used libraries that *together* can be considered standard for the Nix language.
You need to know about both to understand and navigate Nix language code.

We recommend to at least skim them to familiarise yourself with what is available.

## `builtins`

Nix comes with many functions that are built into the language.

:::{note}
The Nix manual lists all [Built-in Functions][nix-builtins] and shows how to use them.
:::

These functions are available under the `builtins` constant. Example:

    builtins.toString

Most of them are implemented in the Nix language interpreter itself, which means they usually execute faster than their equivalents implemented in the Nix language.

[nix-builtins]: https://nixos.org/manual/nix/stable/expressions/builtins.html

## `pkgs.lib`

The Nix package collection [`nixpkgs`][nixpkgs] contains an attribute set called `lib`, which provides a large number of useful functions.

:::{note}
The `nixpkgs` manual lists all [Nixpkgs library functions][nixpkgs-functions].
:::

These functions are accessed through `pkgs.lib`. Example:

    pkgs.lib.strings.toUpper

[nixpkgs-functions]: https://nixos.org/manual/nixpkgs/stable/#sec-functions-library

# Building software using side effects

So far we have only covered what we call *pure expressions*:
declaring data and transforming it with functions.

Building software requires interaction with the outside world, which we call *side effects*.

There are two main side effects in the Nix language that are relevant here:
1. Reading files from the file system as build inputs
2. Writing files to the file system as build outputs

<!-- TODO: side effects - fetchers and derivations -->



See the [Nix Pills][nix-pills] series for a detailed explanation on how Nix the package manager builds software using library functions and packages from the Nix package collection.

[nix-pills]: https://nixos.org/guides/nix-pills/

# Summary

You should now be able to read Nix language code for simple packages and configurations, and come up with similiar explanations of the following examples.

Example:

```nix
{ pkgs ? import <nixpkgs> {}, ... }:
pkgs.mkShell {
  # ...
}
```

Explanation:

This expression is a function that takes an attribute set as an argument.
If the argument has the attribute `pkgs`, it will be used in the function body.
Otherwise, the default value of importing from the search path `<nixpkgs>` and calling the resulting function with an empty attribute set will be used.
The attribute `mkShell` of the `pkgs` set is a function that is passed an attribute set as argument.
Its return value is also the result of the outer function.

(This example declares a shell environment.)

Example:

```nix
{ config, pkgs, ... }: {
  imports = [ ./hardware-configuration.nix ];
  environment.systemPackages = with pkgs; [ git ];
  # ...
}
```

Explanation:

This expression is also a function that takes an attribute set as an argument.
It returns an attribute set.
The argument must at least have the attributes `config` and `pkgs`.
The returned attribute set contains an attribute `imports` and a nested attribute set `environment` with an attribute `systemPackages`.
`imports` is a list with one element: a path to a file next to this Nix file, called `hardware-configuration.nix`.
(Note: it is not the impure built-in `import`, but a regular attribute name!)
`systemPackages` will evaluate to a list with one element: the `git` attribute of the `pkgs` set.
The `config` argument is not used.

(This example is a NixOS configuration.)

Example:

```nix
{ lib, stdenv }:

stdenv.mkDerivation rec {

  pname = "hello";

  version = "2.12";

  src = builtins.fetchTarball {
    url = "mirror://gnu/hello/hello-${version}.tar.gz";
    sha256 = "1ayhp9v4m4rdhjmnl2bq3cibrbqqkgjbl3s7yk2nhlh8vj3ay16g";
  };

  meta = with lib; {
    license = licenses.gpl3Plus;
  };
}
```

Explanation:

This expression is a function that takes an attribute set which must have exactly the attributes `lib` and `stdenv`.
It returns the result of evaluating the function `mkDerivaion`, which is an attribute of `stdenv`, applied to a recursive set.
The recursive set passed to `mkDerivation` uses its own `version` attribute in the argument to the built-in function `fetchTarball`.
The `meta` attribute is itself an attribute set, where the `license` attribute has the value that was assigned to the nested attribute `lib.licenses.gpl3Plus`.

(This example is a (simplified) package declaration from `nixpkgs`.)

# Next steps