Language Constructs

This section covers syntax and semantics of the Nix language.

The most common way is to enclose the string between double quotes, e.g., "foo bar". Strings can span multiple lines. The results of other expressions can be included into a string by enclosing them in ${ }, a feature known as string interpolation.

The following must be escaped to represent them within a string, by prefixing with a backslash (\):

Double quote (")
Example
```
"\""
```
"""
Backslash (\)
Example
```
"\\"
```
"\"
Dollar sign followed by an opening curly bracket (${) – "dollar-curly"
Example
```
"\${"
```
"${"

The newline, carriage return, and tab characters can be written as \n, \r and \t, respectively.

A "double-dollar-curly" ($${) can be written literally.

Example
"$${"
"$${"

String values are output on the terminal with Nix-specific escaping. Strings written to files will contain the characters encoded by the escaping.

The second way to write string literals is as an indented string, which is enclosed between pairs of double single-quotes (''), like so:

''
  This is the first line.
  This is the second line.
    This is the third line.
''

This kind of string literal intelligently strips indentation from the start of each line. To be precise, it strips from each line a number of spaces equal to the minimal indentation of the string as a whole (disregarding the indentation of empty lines). For instance, the first and second line are indented two spaces, while the third line is indented four spaces. Thus, two spaces are stripped from each line, so the resulting string is

"This is the first line.\nThis is the second line.\n  This is the third line.\n"

Note

Whitespace and newline following the opening '' is ignored if there is no non-whitespace text on the initial line.

Warning

Prefixed tab characters are not stripped.
Example

The following indented string is prefixed with tabs:

'' all: @echo hello ''
"\tall:\n\t\t@echo hello\n"

Indented strings support string interpolation.

The following must be escaped to represent them in an indented string:

$ is escaped by prefixing it with two single quotes ('')
Example
```
''
  ''$
''
```
```
"$\n"
```
'' is escaped by prefixing it with one single quote (')
Example
```
''
  '''
''
```
```
"''\n"
```

These special characters are escaped as follows:

Linefeed (\n): ''\n
Carriage return (\r): ''\r
Tab (\t): ''\t

''\ escapes any other character.

A "double-dollar-curly" ($${) can be written literally.

Example

''
  $${
''

"$\${\n"

Indented strings are primarily useful in that they allow multi-line string literals to follow the indentation of the enclosing Nix expression, and that less escaping is typically necessary for strings representing languages such as shell scripts and configuration files because '' is much less common than ". Example:

stdenv.mkDerivation {
  ...
  postInstall =
    ''
      mkdir $out/bin $out/etc
      cp foo $out/bin
      echo "Hello World" > $out/etc/foo.conf
      ${if enableBar then "cp bar $out/bin" else ""}
    '';
  ...
}

Finally, as a convenience, URIs as defined in appendix B of RFC 2396 can be written as is, without quotes. For instance, the string "http://example.org/foo.tar.bz2" can also be written as http://example.org/foo.tar.bz2.

Number

Numbers, which can be integers (like 123) or floating point (like 123.43 or .27e13).

See arithmetic and comparison operators for semantics.

Path

Paths can be expressed by path literals such as ./builder.sh.

A path literal must contain at least one slash to be recognised as such. For instance, builder.sh is not a path: it's parsed as an expression that selects the attribute sh from the variable builder.

Path literals are resolved relative to their base directory. Path literals may also refer to absolute paths by starting with a slash.

Note

Absolute paths make expressions less portable. In the case where a function translates a path literal into an absolute path string for a configuration file, it is recommended to write a string literal instead. This avoids some confusion about whether files at that location will be used during evaluation. It also avoids unintentional situations where some function might try to copy everything at the location into the store.

If the first component of a path is a ~, it is interpreted such that the rest of the path were relative to the user's home directory. For example, ~/foo would be equivalent to /home/edolstra/foo for a user whose home directory is /home/edolstra. Path literals that start with ~ are not allowed in pure evaluation.

Path literals can also include string interpolation, besides being interpolated into other expressions.

At least one slash (/) must appear before any interpolated expression for the result to be recognized as a path.

a.${foo}/b.${bar} is a syntactically valid number division operation. ./a.${foo}/b.${bar} is a path.

Lookup path literals such as <nixpkgs> also resolve to path values.

List

Lists are formed by enclosing a whitespace-separated list of values between square brackets. For example,

[ 123 ./foo.nix "abc" (f { x = y; }) ]

defines a list of four elements, the last being the result of a call to the function f. Note that function calls have to be enclosed in parentheses. If they had been omitted, e.g.,

[ 123 ./foo.nix "abc" f { x = y; } ]

the result would be a list of five elements, the fourth one being a function and the fifth being a set.

Note that lists are only lazy in values, and they are strict in length.

Elements in a list can be accessed using builtins.elemAt.

Attribute Set

An attribute set is a collection of name-value-pairs called attributes.

Attribute sets are written enclosed in curly brackets ({ }). Attribute names and attribute values are separated by an equal sign (=). Each value can be an arbitrary expression, terminated by a semicolon (;)

An attribute name is a string without context, and is denoted by a name (an identifier or string literal).

Syntax

attrset → { { name = expr ; } }

Attributes can appear in any order. An attribute name may only occur once in each attribute set.

Example

This defines an attribute set with attributes named:

x with the value 123, an integer

text with the value "Hello", a string

y where the value is the result of applying the function f to the attribute set { bla = 456; }
{
  x = 123;
  text = "Hello";
  y = f { bla = 456; };
}

Attributes in nested attribute sets can be written using attribute paths.

Syntax

attrset → { { attrpath = expr ; } }

An attribute path is a dot-separated list of names.

Syntax

attrpath = name { . name }

Example

{ a.b.c = 1; a.b.d = 2; }

{
  a = {
    b = {
      c = 1;
      d = 2;
    };
  };
}

Attribute names can also be set implicitly by using the inherit keyword.

Example

{ inherit (builtins) true; }

{ true = true; }

Attributes can be accessed with the . operator.

Example:

{ a = "Foo"; b = "Bar"; }.a

This evaluates to "Foo".

It is possible to provide a default value in an attribute selection using the or keyword.

Example:

{ a = "Foo"; b = "Bar"; }.c or "Xyzzy"

{ a = "Foo"; b = "Bar"; }.c.d.e.f.g or "Xyzzy"

will both evaluate to "Xyzzy" because there is no c attribute in the set.

You can use arbitrary double-quoted strings as attribute names:

{ "$!@#?" = 123; }."$!@#?"

let bar = "bar"; in
{ "foo ${bar}" = 123; }."foo ${bar}"

Both will evaluate to 123.

Attribute names support string interpolation:

let bar = "foo"; in
{ foo = 123; }.${bar}

let bar = "foo"; in
{ ${bar} = 123; }.foo

Both will evaluate to 123.

In the special case where an attribute name inside of a set declaration evaluates to null (which is normally an error, as null cannot be coerced to a string), that attribute is simply not added to the set:

{ ${if foo then "bar" else null} = true; }

This will evaluate to {} if foo evaluates to false.

A set that has a __functor attribute whose value is callable (i.e. is itself a function or a set with a __functor attribute whose value is callable) can be applied as if it were a function, with the set itself passed in first , e.g.,

let add = { __functor = self: x: x + self.x; };
    inc = add // { x = 1; };
in inc 1

evaluates to 2. This can be used to attach metadata to a function without the caller needing to treat it specially, or to implement a form of object-oriented programming, for example.

Recursive sets

Recursive sets are like normal attribute sets, but the attributes can refer to each other.

rec-attrset = rec { [ name = expr ; ]... }

Example:

rec {
  x = y;
  y = 123;
}.x

This evaluates to 123.

Note that without rec the binding x = y; would refer to the variable y in the surrounding scope, if one exists, and would be invalid if no such variable exists. That is, in a normal (non-recursive) set, attributes are not added to the lexical scope; in a recursive set, they are.

Recursive sets of course introduce the danger of infinite recursion. For example, the expression

rec {
  x = y;
  y = x;
}.x

will crash with an infinite recursion encountered error message.

Let-expressions

A let-expression allows you to define local variables for an expression.

let-in = let [ identifier = expr ]... in expr

Example:

let
  x = "foo";
  y = "bar";
in x + y

This evaluates to "foobar".

Inheriting attributes

When defining an attribute set or in a let-expression it is often convenient to copy variables from the surrounding lexical scope (e.g., when you want to propagate attributes). This can be shortened using the inherit keyword.

Example:

let x = 123; in
{
  inherit x;
  y = 456;
}

is equivalent to

let x = 123; in
{
  x = x;
  y = 456;
}

and both evaluate to { x = 123; y = 456; }.

Note

This works because x is added to the lexical scope by the let construct.

It is also possible to inherit attributes from another attribute set.

Example:

In this fragment from all-packages.nix,

graphviz = (import ../tools/graphics/graphviz) {
  inherit fetchurl stdenv libpng libjpeg expat x11 yacc;
  inherit (xorg) libXaw;
};

xorg = {
  libX11 = ...;
  libXaw = ...;
  ...
}

libpng = ...;
libjpg = ...;
...

the set used in the function call to the function defined in ../tools/graphics/graphviz inherits a number of variables from the surrounding scope (fetchurl ... yacc), but also inherits libXaw (the X Athena Widgets) from the xorg set.

Summarizing the fragment

...
inherit x y z;
inherit (src-set) a b c;
...

is equivalent to

...
x = x; y = y; z = z;
a = src-set.a; b = src-set.b; c = src-set.c;
...

when used while defining local variables in a let-expression or while defining a set.

In a let expression, inherit can be used to selectively bring specific attributes of a set into scope. For example

let
  x = { a = 1; b = 2; };
  inherit (builtins) attrNames;
in
{
  names = attrNames x;
}

is equivalent to

let
  x = { a = 1; b = 2; };
in
{
  names = builtins.attrNames x;
}

both evaluate to { names = [ "a" "b" ]; }.

Functions

Functions have the following form:

pattern: body

The pattern specifies what the argument of the function must look like, and binds variables in the body to (parts of) the argument. There are three kinds of patterns:

If a pattern is a single identifier, then the function matches any argument. Example:
```
let negate = x: !x;
    concat = x: y: x + y;
in if negate true then concat "foo" "bar" else ""
```
Note that concat is a function that takes one argument and returns a function that takes another argument. This allows partial parameterisation (i.e., only filling some of the arguments of a function); e.g.,
```
map (concat "foo") [ "bar" "bla" "abc" ]
```
evaluates to [ "foobar" "foobla" "fooabc" ].
A set pattern of the form { name1, name2, …, nameN } matches a set containing the listed attributes, and binds the values of those attributes to variables in the function body. For example, the function
```
{ x, y, z }: z + y + x
```
can only be called with a set containing exactly the attributes x, y and z. No other attributes are allowed. If you want to allow additional arguments, you can use an ellipsis (...):
```
{ x, y, z, ... }: z + y + x
```
This works on any set that contains at least the three named attributes.

It is possible to provide default values for attributes, in which case they are allowed to be missing. A default value is specified by writing name ? e, where e is an arbitrary expression. For example,
```
{ x, y ? "foo", z ? "bar" }: z + y + x
```
specifies a function that only requires an attribute named x, but optionally accepts y and z.
An @-pattern provides a means of referring to the whole value being matched:
```
args@{ x, y, z, ... }: z + y + x + args.a
```
but can also be written as:
```
{ x, y, z, ... } @ args: z + y + x + args.a
```
Here args is bound to the argument as passed, which is further matched against the pattern { x, y, z, ... }. The @-pattern makes mainly sense with an ellipsis(...) as you can access attribute names as a, using args.a, which was given as an additional attribute to the function.
Warning

args@ binds the name args to the attribute set that is passed to the function. In particular, args does not include any default values specified with ? in the function's set pattern.

For instance
```
let
  f = args@{ a ? 23, ... }: [ a args ];
in
  f {}
```
is equivalent to
```
let
  f = args @ { ... }: [ (args.a or 23) args ];
in
  f {}
```
and both expressions will evaluate to:
```
[ 23 {} ]
```

Note that functions do not have names. If you want to give them a name, you can bind them to an attribute, e.g.,

let concat = { x, y }: x + y;
in concat { x = "foo"; y = "bar"; }

Conditionals

Conditionals look like this:

if e1 then e2 else e3

where e1 is an expression that should evaluate to a Boolean value (true or false).

Assertions

Assertions are generally used to check that certain requirements on or between features and dependencies hold. They look like this:

assert e1; e2

where e1 is an expression that should evaluate to a Boolean value. If it evaluates to true, e2 is returned; otherwise expression evaluation is aborted and a backtrace is printed.

Here is a Nix expression for the Subversion package that shows how assertions can be used:.

{ localServer ? false
, httpServer ? false
, sslSupport ? false
, pythonBindings ? false
, javaSwigBindings ? false
, javahlBindings ? false
, stdenv, fetchurl
, openssl ? null, httpd ? null, db4 ? null, expat, swig ? null, j2sdk ? null
}:

assert localServer -> db4 != null; ①
assert httpServer -> httpd != null && httpd.expat == expat; ②
assert sslSupport -> openssl != null && (httpServer -> httpd.openssl == openssl); ③
assert pythonBindings -> swig != null && swig.pythonSupport;
assert javaSwigBindings -> swig != null && swig.javaSupport;
assert javahlBindings -> j2sdk != null;

stdenv.mkDerivation {
  name = "subversion-1.1.1";
  ...
  openssl = if sslSupport then openssl else null; ④
  ...
}

The points of interest are:

This assertion states that if Subversion is to have support for local repositories, then Berkeley DB is needed. So if the Subversion function is called with the localServer argument set to true but the db4 argument set to null, then the evaluation fails.

Note that -> is the logical implication Boolean operation.
This is a more subtle condition: if Subversion is built with Apache (httpServer) support, then the Expat library (an XML library) used by Subversion should be same as the one used by Apache. This is because in this configuration Subversion code ends up being linked with Apache code, and if the Expat libraries do not match, a build- or runtime link error or incompatibility might occur.
This assertion says that in order for Subversion to have SSL support (so that it can access https URLs), an OpenSSL library must be passed. Additionally, it says that if Apache support is enabled, then Apache's OpenSSL should match Subversion's. (Note that if Apache support is not enabled, we don't care about Apache's OpenSSL.)
The conditional here is not really related to assertions, but is worth pointing out: it ensures that if SSL support is disabled, then the Subversion derivation is not dependent on OpenSSL, even if a non-null value was passed. This prevents an unnecessary rebuild of Subversion if OpenSSL changes.

With-expressions

A with-expression,

with e1; e2

introduces the set e1 into the lexical scope of the expression e2. For instance,

let as = { x = "foo"; y = "bar"; };
in with as; x + y

evaluates to "foobar" since the with adds the x and y attributes of as to the lexical scope in the expression x + y. The most common use of with is in conjunction with the import function. E.g.,

with (import ./definitions.nix); ...

makes all attributes defined in the file definitions.nix available as if they were defined locally in a let-expression.

The bindings introduced by with do not shadow bindings introduced by other means, e.g.

let a = 3; in with { a = 1; }; let a = 4; in with { a = 2; }; ...

establishes the same scope as

let a = 1; in let a = 2; in let a = 3; in let a = 4; in ...

Variables coming from outer with expressions are shadowed:

with { a = "outer"; };
with { a = "inner"; };
a

Does evaluate to "inner".

Comments

Inline comments start with # and run until the end of the line.
Example
```
# A number
2 # Equals 1 + 1
```
```
2
```

Block comments start with /* and run until the next occurrence of */.

Example

/*
Block comments
can span multiple lines.
*/ "hello"

"hello"

This means that block comments cannot be nested.

Example

/* /* nope */ */ 1

error: syntax error, unexpected '*'

       at «string»:1:15:

            1| /* /* nope */ *
             |               ^

Consider escaping nested comments and unescaping them in post-processing.

Example

/* /* nested *\/ */ 1

Nix Reference Manual

Language Constructs

Basic Literals

String

Number

Path

List

Attribute Set

Recursive sets

Let-expressions

Inheriting attributes

Functions

Conditionals

Assertions

With-expressions

Comments