Language Constructs
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 thelet
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
andz
. 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 acceptsy
andz
. -
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 asa
, usingargs.a
, which was given as an additional attribute to the function.Warning
args@
binds the nameargs
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 totrue
but thedb4
argument set tonull
, 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 ...
Comments
Comments can be single-line, started with a #
character, or
inline/multi-line, enclosed within /* ... */
.