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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.
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.
What do you need?
- Familiarity with other programming languages
- Familiarity with Unix shell to read command line examples
- Install the Nix package manager to run the examples
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:
1 + 2
3
Use nix-instantiate --eval to evaluate the expression in a Nix file.
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.
echo 1 + 2 > default.nix
nix-instantiate --eval
3
:::
Use nix repl to evaluate Nix expressions interactively (by typing them on the command line):
nix repl
Welcome to Nix 2.5.1. Type :? for help.
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 sets
Attribute sets are unordered collections of name-value-pairs.
Together with primitive data types and lists, they work like in JSON and look very similar.
Nix language:
{
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
}
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
}
}
:::{note}
Recursive attribute sets
You will sometimes see attribute sets declared with rec prepended.
This allows access to attributes from within the set.
Example:
rec {
one = 1;
two = one + 1;
three = two + 1;
}
{ one = 1; three = 3; two = 2; }
Counter-example:
{
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 the <ref-rec-expression> and to use the let expression instead.
let expression
Also known as “let binding” or “let ... in ...”.
let expressions allow assigning names to values for repeated use.
Example:
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:
let
b = a + 1
a = 1;
in
a + b
3
Only the expressions in the let expression can access the newly declared names.
We say: the bindings have local scope.
Counter-example:
{
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| }
Accessing attributes
Attributes in a set can be accessed with a dot (.) and the attribute name.
Example:
let
attrset = { x = 1; };
in
attrset.x
1
Accessing nested attributes works the same way.
Example:
let
attrset = { a = { b = { c = 2; }; }; };
in
attrset.a.b.c
2
with
with allows access to attributes without repeatedly referencing their attribute set.
Example:
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:
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
One can assign attributes from variables that have the same name with inherit.
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:
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.
Arguments
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.
Applying a function to an argument means writing the argument after the function.
Example
(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 like any to other value.
Example:
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:
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.
let
f = x: y: x + y;
in
f 1 2
3
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:
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:
let
f = {a, b ? 0}: a + b
in
f { a = 1; }
1
Example:
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:
let
f = {a, b, ...}: a + b
in
f { a = 1; b = 2; c = 3; }
3
Named keyword arguments
Also known as “@ syntax” or “‘at’ syntax”:
{a, b, ...}@args: a + b + args.c
or
args@{a, b, ...}: a + b + args.c
where additional attributes are subsumed under a name.
Example:
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
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 environment variable.
In practice, <nixpkgs> points to the file system path of some revision of the Nix package collection.
For example, <nixpkgs/lib> points to the subdirectory lib of that file system path.
Character strings
String interpolation
let
name = "Nix";
in
"hello ${name}"
"hello Nix"
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:
''
one
two
three
''
"one\n two\n three\n"
See escaping rules.
Summary
As a programming language, Nix is
-
declarative
It has no notion of executing sequential steps. Dependencies between operations are established only through data.
-
purely functional
Pure means: Nix does not change the value of declarations during computation – there are no variables, only names for immutable values.
Functional means: In Nix, functions are like any other value. Functions can be assigned to names, taken as arguments, or returned by functions.
-
lazy
It will only evaluate expressions when their result is needed.1
-
dynamically typed
Type errors are only detected when operations are actually evaluated.2
-
purpose-built
The Nix language only exists for the Nix package manager. It is not intended for general purpose use.
-
Details: Nix manual - attribute naming rules ↩︎
-
Details: Nix manual - lists ↩︎
-
Details: Nix manual - primitive data types ↩︎