# Nix Language
The Nix language is
- *domain-specific*
It only exists for the Nix package manager:
to describe packages and configurations as well as their variants and compositions.
It is not intended for general purpose use.
- *declarative*
There is no notion of executing sequential steps.
Dependencies between operations are established only through data.
- *pure*
Values cannot change during computation.
Functions always produce the same output if their input does not change.
- *functional*
Functions are like any other value.
Functions can be assigned to names, taken as arguments, or returned by functions.
- *lazy*
Expressions are only evaluated when their value is needed.
- *dynamically typed*
Type errors are only detected when expressions are evaluated.
# Overview
This is an incomplete overview of language features, by example.
<table>
<tr>
<th>
Example
</th>
<th>
Description
</th>
</tr>
<tr>
<td>
*Basic values*
</td>
<td>
</td>
</tr>
<tr>
<td>
`"hello world"`
</td>
<td>
A string
</td>
</tr>
<tr>
<td>
```
''
multi
line
string
''
```
</td>
<td>
A multi-line string. Strips common prefixed whitespace. Evaluates to `"multi\n line\n string"`.
</td>
</tr>
<tr>
<td>
`"hello ${ { a = "world" }.a }"`
`"1 2 ${toString 3}"`
`"${pkgs.bash}/bin/sh"`
</td>
<td>
String interpolation (expands to `"hello world"`, `"1 2 3"`, `"/nix/store/<hash>-bash-<version>/bin/sh"`)
</td>
</tr>
<tr>
<td>
`true`, `false`
</td>
<td>
Booleans
</td>
</tr>
<tr>
<td>
`null`
</td>
<td>
Null value
</td>
</tr>
<tr>
<td>
`123`
</td>
<td>
An integer
</td>
</tr>
<tr>
<td>
`3.141`
</td>
<td>
A floating point number
</td>
</tr>
<tr>
<td>
`/etc`
</td>
<td>
An absolute path
</td>
</tr>
<tr>
<td>
`./foo.png`
</td>
<td>
A path relative to the file containing this Nix expression
</td>
</tr>
<tr>
<td>
`~/.config`
</td>
<td>
A home path. Evaluates to the `"<user's home directory>/.config"`.
</td>
</tr>
<tr>
<td>
<nixpkgs>
</td>
<td>
Search path. Value determined by [`$NIX_PATH` environment variable](../command-ref/env-common.md#env-NIX_PATH).
</td>
</tr>
<tr>
<td>
*Compound values*
</td>
<td>
</td>
</tr>
<tr>
<td>
`{ x = 1; y = 2; }`
</td>
<td>
A set with attributes named `x` and `y`
</td>
</tr>
<tr>
<td>
`{ foo.bar = 1; }`
</td>
<td>
A nested set, equivalent to `{ foo = { bar = 1; }; }`
</td>
</tr>
<tr>
<td>
`rec { x = "foo"; y = x + "bar"; }`
</td>
<td>
A recursive set, equivalent to `{ x = "foo"; y = "foobar"; }`
</td>
</tr>
<tr>
<td>
`[ "foo" "bar" "baz" ]`
`[ 1 2 3 ]`
`[ (f 1) { a = 1; b = 2; } [ "c" ] ]`
</td>
<td>
Lists with three elements.
</td>
</tr>
<tr>
<td>
*Operators*
</td>
<td>
</td>
</tr>
<tr>
<td>
`"foo" + "bar"`
</td>
<td>
String concatenation
</td>
</tr>
<tr>
<td>
`1 + 2`
</td>
<td>
Integer addition
</td>
</tr>
<tr>
<td>
`"foo" == "f" + "oo"`
</td>
<td>
Equality test (evaluates to `true`)
</td>
</tr>
<tr>
<td>
`"foo" != "bar"`
</td>
<td>
Inequality test (evaluates to `true`)
</td>
</tr>
<tr>
<td>
`!true`
</td>
<td>
Boolean negation
</td>
</tr>
<tr>
<td>
`{ x = 1; y = 2; }.x`
</td>
<td>
Attribute selection (evaluates to `1`)
</td>
</tr>
<tr>
<td>
`{ x = 1; y = 2; }.z or 3`
</td>
<td>
Attribute selection with default (evaluates to `3`)
</td>
</tr>
<tr>
<td>
`{ x = 1; y = 2; } // { z = 3; }`
</td>
<td>
Merge two sets (attributes in the right-hand set taking precedence)
</td>
</tr>
<tr>
<td>
*Control structures*
</td>
<td>
</td>
</tr>
<tr>
<td>
`if 1 + 1 == 2 then "yes!" else "no!"`
</td>
<td>
Conditional expression
</td>
</tr>
<tr>
<td>
`assert 1 + 1 == 2; "yes!"`
</td>
<td>
Assertion check (evaluates to `"yes!"`).
</td>
</tr>
<tr>
<td>
`let x = "foo"; y = "bar"; in x + y`
</td>
<td>
Variable definition
</td>
</tr>
<tr>
<td>
`with builtins; head [ 1 2 3 ]`
</td>
<td>
Add all attributes from the given set to the scope (evaluates to `1`)
</td>
</tr>
<tr>
<td>
*Functions (lambdas)*
</td>
<td>
</td>
</tr>
<tr>
<td>
`x: x + 1`
</td>
<td>
A function that expects an integer and returns it increased by 1
</td>
</tr>
<tr>
<td>
`x: y: x + y`
</td>
<td>
Curried function, equivalent to `x: (y: x + y)`. Can be used like a function that takes two arguments and returns their sum.
</td>
</tr>
<tr>
<td>
`(x: x + 1) 100`
</td>
<td>
A function call (evaluates to 101)
</td>
</tr>
<tr>
<td>
`let inc = x: x + 1; in inc (inc (inc 100))`
</td>
<td>
A function bound to a variable and subsequently called by name (evaluates to 103)
</td>
</tr>
<tr>
<td>
`{ x, y }: x + y`
</td>
<td>
A function that expects a set with required attributes `x` and `y` and concatenates them
</td>
</tr>
<tr>
<td>
`{ x, y ? "bar" }: x + y`
</td>
<td>
A function that expects a set with required attribute `x` and optional `y`, using `"bar"` as default value for `y`
</td>
</tr>
<tr>
<td>
`{ x, y, ... }: x + y`
</td>
<td>
A function that expects a set with required attributes `x` and `y` and ignores any other attributes
</td>
</tr>
<tr>
<td>
`{ x, y } @ args: x + y`
`args @ { x, y }: x + y`
</td>
<td>
A function that expects a set with required attributes `x` and `y`, and binds the whole set to `args`
</td>
</tr>
<tr>
<td>
*Built-in functions*
</td>
<td>
</td>
</tr>
<tr>
<td>
`import ./foo.nix`
</td>
<td>
Load and return Nix expression in given file
</td>
</tr>
<tr>
<td>
`map (x: x + x) [ 1 2 3 ]`
</td>
<td>
Apply a function to every element of a list (evaluates to `[ 2 4 6 ]`)
</td>
</tr>
</table>