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manual: remove "Writing Nix Expressions" chapter

Browse source 
it is out of date, all over the place in level of detail, is really
about `nixpkgs`, and in general instructions should not be part of
a reference manual.

also:
- update redirects and internal links
- use "Nix language" consistently
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# Advanced Attributes
Derivations can declare some infrequently used optional attributes.
- [`allowedReferences`]{#adv-attr-allowedReferences}\
The optional attribute `allowedReferences` specifies a list of legal
references (dependencies) of the output of the builder. For example,
```nix
allowedReferences = [];
```
enforces that the output of a derivation cannot have any runtime
dependencies on its inputs. To allow an output to have a runtime
dependency on itself, use `"out"` as a list item. This is used in
NixOS to check that generated files such as initial ramdisks for
booting Linux don’t have accidental dependencies on other paths in
the Nix store.
- [`allowedRequisites`]{#adv-attr-allowedRequisites}\
This attribute is similar to `allowedReferences`, but it specifies
the legal requisites of the whole closure, so all the dependencies
recursively. For example,
```nix
allowedRequisites = [ foobar ];
```
enforces that the output of a derivation cannot have any other
runtime dependency than `foobar`, and in addition it enforces that
`foobar` itself doesn't introduce any other dependency itself.
- [`disallowedReferences`]{#adv-attr-disallowedReferences}\
The optional attribute `disallowedReferences` specifies a list of
illegal references (dependencies) of the output of the builder. For
example,
```nix
disallowedReferences = [ foo ];
```
enforces that the output of a derivation cannot have a direct
runtime dependencies on the derivation `foo`.
- [`disallowedRequisites`]{#adv-attr-disallowedRequisites}\
This attribute is similar to `disallowedReferences`, but it
specifies illegal requisites for the whole closure, so all the
dependencies recursively. For example,
```nix
disallowedRequisites = [ foobar ];
```
enforces that the output of a derivation cannot have any runtime
dependency on `foobar` or any other derivation depending recursively
on `foobar`.
- [`exportReferencesGraph`]{#adv-attr-exportReferencesGraph}\
This attribute allows builders access to the references graph of
their inputs. The attribute is a list of inputs in the Nix store
whose references graph the builder needs to know. The value of
this attribute should be a list of pairs `[ name1 path1 name2
path2 ... ]`. The references graph of each *pathN* will be stored
in a text file *nameN* in the temporary build directory. The text
files have the format used by `nix-store --register-validity`
(with the deriver fields left empty). For example, when the
following derivation is built:
```nix
derivation {
...
exportReferencesGraph = [ "libfoo-graph" libfoo ];
};
```
the references graph of `libfoo` is placed in the file
`libfoo-graph` in the temporary build directory.
`exportReferencesGraph` is useful for builders that want to do
something with the closure of a store path. Examples include the
builders in NixOS that generate the initial ramdisk for booting
Linux (a `cpio` archive containing the closure of the boot script)
and the ISO-9660 image for the installation CD (which is populated
with a Nix store containing the closure of a bootable NixOS
configuration).
- [`impureEnvVars`]{#adv-attr-impureEnvVars}\
This attribute allows you to specify a list of environment variables
that should be passed from the environment of the calling user to
the builder. Usually, the environment is cleared completely when the
builder is executed, but with this attribute you can allow specific
environment variables to be passed unmodified. For example,
`fetchurl` in Nixpkgs has the line
```nix
impureEnvVars = [ "http_proxy" "https_proxy" ... ];
```
to make it use the proxy server configuration specified by the user
in the environment variables `http_proxy` and friends.
This attribute is only allowed in *fixed-output derivations* (see
below), where impurities such as these are okay since (the hash
of) the output is known in advance. It is ignored for all other
derivations.
> **Warning**
>
> `impureEnvVars` implementation takes environment variables from
> the current builder process. When a daemon is building its
> environmental variables are used. Without the daemon, the
> environmental variables come from the environment of the
> `nix-build`.
- [`outputHash`]{#adv-attr-outputHash}; [`outputHashAlgo`]{#adv-attr-outputHashAlgo}; [`outputHashMode`]{#adv-attr-outputHashMode}\
These attributes declare that the derivation is a so-called
*fixed-output derivation*, which means that a cryptographic hash of
the output is already known in advance. When the build of a
fixed-output derivation finishes, Nix computes the cryptographic
hash of the output and compares it to the hash declared with these
attributes. If there is a mismatch, the build fails.
The rationale for fixed-output derivations is derivations such as
those produced by the `fetchurl` function. This function downloads a
file from a given URL. To ensure that the downloaded file has not
been modified, the caller must also specify a cryptographic hash of
the file. For example,
```nix
fetchurl {
url = "http://ftp.gnu.org/pub/gnu/hello/hello-2.1.1.tar.gz";
sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
}
```
It sometimes happens that the URL of the file changes, e.g., because
servers are reorganised or no longer available. We then must update
the call to `fetchurl`, e.g.,
```nix
fetchurl {
url = "ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz";
sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
}
```
If a `fetchurl` derivation was treated like a normal derivation, the
output paths of the derivation and *all derivations depending on it*
would change. For instance, if we were to change the URL of the
Glibc source distribution in Nixpkgs (a package on which almost all
other packages depend) massive rebuilds would be needed. This is
unfortunate for a change which we know cannot have a real effect as
it propagates upwards through the dependency graph.
For fixed-output derivations, on the other hand, the name of the
output path only depends on the `outputHash*` and `name` attributes,
while all other attributes are ignored for the purpose of computing
the output path. (The `name` attribute is included because it is
part of the path.)
As an example, here is the (simplified) Nix expression for
`fetchurl`:
```nix
{ stdenv, curl }: # The curl program is used for downloading.
{ url, sha256 }:
stdenv.mkDerivation {
name = baseNameOf (toString url);
builder = ./builder.sh;
buildInputs = [ curl ];
# This is a fixed-output derivation; the output must be a regular
# file with SHA256 hash sha256.
outputHashMode = "flat";
outputHashAlgo = "sha256";
outputHash = sha256;
inherit url;
}
```
The `outputHashAlgo` attribute specifies the hash algorithm used to
compute the hash. It can currently be `"sha1"`, `"sha256"` or
`"sha512"`.
The `outputHashMode` attribute determines how the hash is computed.
It must be one of the following two values:
- `"flat"`\
The output must be a non-executable regular file. If it isn’t,
the build fails. The hash is simply computed over the contents
of that file (so it’s equal to what Unix commands like
`sha256sum` or `sha1sum` produce).
This is the default.
- `"recursive"`\
The hash is computed over the NAR archive dump of the output
(i.e., the result of [`nix-store
--dump`](../command-ref/nix-store.md#operation---dump)). In
this case, the output can be anything, including a directory
tree.
The `outputHash` attribute, finally, must be a string containing
the hash in either hexadecimal or base-32 notation. (See the
[`nix-hash` command](../command-ref/nix-hash.md) for information
about converting to and from base-32 notation.)
- [`__contentAddressed`]{#adv-attr-__contentAddressed}
If this **experimental** attribute is set to true, then the derivation
outputs will be stored in a content-addressed location rather than the
traditional input-addressed one.
This only has an effect if the `ca-derivation` experimental feature is enabled.
Setting this attribute also requires setting `outputHashMode` and `outputHashAlgo` like for *fixed-output derivations* (see above).
- [`passAsFile`]{#adv-attr-passAsFile}\
A list of names of attributes that should be passed via files rather
than environment variables. For example, if you have
```nix
passAsFile = ["big"];
big = "a very long string";
```
then when the builder runs, the environment variable `bigPath`
will contain the absolute path to a temporary file containing `a
very long string`. That is, for any attribute *x* listed in
`passAsFile`, Nix will pass an environment variable `xPath`
holding the path of the file containing the value of attribute
*x*. This is useful when you need to pass large strings to a
builder, since most operating systems impose a limit on the size
of the environment (typically, a few hundred kilobyte).
- [`preferLocalBuild`]{#adv-attr-preferLocalBuild}\
If this attribute is set to `true` and [distributed building is
enabled](../advanced-topics/distributed-builds.md), then, if
possible, the derivation will be built locally instead of forwarded
to a remote machine. This is appropriate for trivial builders
where the cost of doing a download or remote build would exceed
the cost of building locally.
- [`allowSubstitutes`]{#adv-attr-allowSubstitutes}\
If this attribute is set to `false`, then Nix will always build this
derivation; it will not try to substitute its outputs. This is
useful for very trivial derivations (such as `writeText` in Nixpkgs)
that are cheaper to build than to substitute from a binary cache.
> **Note**
>
> You need to have a builder configured which satisfies the
> derivation’s `system` attribute, since the derivation cannot be
> substituted. Thus it is usually a good idea to align `system` with
> `builtins.currentSystem` when setting `allowSubstitutes` to
> `false`. For most trivial derivations this should be the case.

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# Built-in Constants
Here are the constants built into the Nix expression evaluator:
- `builtins`\
The set `builtins` contains all the built-in functions and values.
You can use `builtins` to test for the availability of features in
the Nix installation, e.g.,
```nix
if builtins ? getEnv then builtins.getEnv "PATH" else ""
```
This allows a Nix expression to fall back gracefully on older Nix
installations that don’t have the desired built-in function.
- [`builtins.currentSystem`]{#builtins-currentSystem}\
The built-in value `currentSystem` evaluates to the Nix platform
identifier for the Nix installation on which the expression is being
evaluated, such as `"i686-linux"` or `"x86_64-darwin"`.

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# Built-in Functions
This section lists the functions built into the Nix expression
evaluator. (The built-in function `derivation` is discussed above.)
Some built-ins, such as `derivation`, are always in scope of every Nix
expression; you can just access them right away. But to prevent
polluting the namespace too much, most built-ins are not in
scope. Instead, you can access them through the `builtins` built-in
value, which is a set that contains all built-in functions and values.
For instance, `derivation` is also available as `builtins.derivation`.
<dl>
<dt><code>derivation <var>attrs</var></code>;
<code>builtins.derivation <var>attrs</var></code></dt>
<dd><p><var>derivation</var> is described in
<a href="derivations.md">its own section</a>.</p></dd>

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</dl>

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# Language Constructs
## Recursive sets
Recursive sets are just normal sets, but the attributes can refer to
each other. For example,
```nix
rec {
x = y;
y = 123;
}.x
```
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
```nix
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.
For instance,
```nix
let
x = "foo";
y = "bar";
in x + y
```
evaluates to `"foobar"`.
## Inheriting attributes
When defining a 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. For instance,
```nix
let x = 123; in
{ inherit x;
y = 456;
}
```
is equivalent to
```nix
let x = 123; in
{ x = x;
y = 456;
}
```
and both evaluate to `{ x = 123; y = 456; }`. (Note that this works
because `x` is added to the lexical scope by the `let` construct.) It is
also possible to inherit attributes from another set. For instance, in
this fragment from `all-packages.nix`,
```nix
graphviz = (import ../tools/graphics/graphviz) {
inherit fetchurl stdenv libpng libjpeg expat x11 yacc;
inherit (xlibs) libXaw;
};
xlibs = {
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 `xlibs` (X11 client-side libraries) set.
Summarizing the fragment
```nix
...
inherit x y z;
inherit (src-set) a b c;
...
```
is equivalent to
```nix
...
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.
## Functions
Functions have the following form:
```nix
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:
```nix
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.,
```nix
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
```nix
{ 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 (`...`):
```nix
{ 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,
```nix
{ 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:
```nix
args@{ x, y, z, ... }: z + y + x + args.a
```
but can also be written as:
```nix
{ x, y, z, ... } @ args: z + y + x + args.a
```
Here `args` is bound to the entire argument, which is further
matched against the pattern `{ x, y, z,
... }`. `@`-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**
>
> The `args@` expression is bound to the argument passed to the
> function which means that attributes with defaults that aren't
> explicitly specified in the function call won't cause an
> evaluation error, but won't exist in `args`.
>
> For instance
>
> ```nix
> let
> function = args@{ a ? 23, ... }: args;
> in
> function {}
> ````
>
> will evaluate to an empty attribute set.
Note that functions do not have names. If you want to give them a name,
you can bind them to an attribute, e.g.,
```nix
let concat = { x, y }: x + y;
in concat { x = "foo"; y = "bar"; }
```
## Conditionals
Conditionals look like this:
```nix
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:
```nix
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:.
```nix
{ 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:
1. 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](https://en.wikipedia.org/wiki/Truth_table#Logical_implication)
Boolean operation.
2. 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.
3. 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.)
4. 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*,
```nix
with e1; e2
```
introduces the set *e1* into the lexical scope of the expression *e2*.
For instance,
```nix
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.,
```nix
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.
```nix
let a = 3; in with { a = 1; }; let a = 4; in with { a = 2; }; ...
```
establishes the same scope as
```nix
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 `/* ... */`.

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# Derivations
The most important built-in function is `derivation`, which is used to
describe a single derivation (a build action). It takes as input a set,
the attributes of which specify the inputs of the build.
- There must be an attribute named [`system`]{#attr-system} whose value must be a
string specifying a Nix system type, such as `"i686-linux"` or
`"x86_64-darwin"`. (To figure out your system type, run `nix -vv
--version`.) The build can only be performed on a machine and
operating system matching the system type. (Nix can automatically
[forward builds for other
platforms](../advanced-topics/distributed-builds.md) by forwarding
them to other machines.)
- There must be an attribute named `name` whose value must be a
string. This is used as a symbolic name for the package by
`nix-env`, and it is appended to the output paths of the derivation.
- There must be an attribute named `builder` that identifies the
program that is executed to perform the build. It can be either a
derivation or a source (a local file reference, e.g.,
`./builder.sh`).
- Every attribute is passed as an environment variable to the builder.
Attribute values are translated to environment variables as follows:
- Strings and numbers are just passed verbatim.
- A *path* (e.g., `../foo/sources.tar`) causes the referenced file
to be copied to the store; its location in the store is put in
the environment variable. The idea is that all sources should
reside in the Nix store, since all inputs to a derivation should
reside in the Nix store.
- A *derivation* causes that derivation to be built prior to the
present derivation; its default output path is put in the
environment variable.
- Lists of the previous types are also allowed. They are simply
concatenated, separated by spaces.
- `true` is passed as the string `1`, `false` and `null` are
passed as an empty string.
- The optional attribute `args` specifies command-line arguments to be
passed to the builder. It should be a list.
- The optional attribute `outputs` specifies a list of symbolic
outputs of the derivation. By default, a derivation produces a
single output path, denoted as `out`. However, derivations can
produce multiple output paths. This is useful because it allows
outputs to be downloaded or garbage-collected separately. For
instance, imagine a library package that provides a dynamic library,
header files, and documentation. A program that links against the
library doesn’t need the header files and documentation at runtime,
and it doesn’t need the documentation at build time. Thus, the
library package could specify:
```nix
outputs = [ "lib" "headers" "doc" ];
```
This will cause Nix to pass environment variables `lib`, `headers`
and `doc` to the builder containing the intended store paths of each
output. The builder would typically do something like
```bash
./configure \
--libdir=$lib/lib \
--includedir=$headers/include \
--docdir=$doc/share/doc
```
for an Autoconf-style package. You can refer to each output of a
derivation by selecting it as an attribute, e.g.
```nix
buildInputs = [ pkg.lib pkg.headers ];
```
The first element of `outputs` determines the *default output*.
Thus, you could also write
```nix
buildInputs = [ pkg pkg.headers ];
```
since `pkg` is equivalent to `pkg.lib`.
The function `mkDerivation` in the Nixpkgs standard environment is a
wrapper around `derivation` that adds a default value for `system` and
always uses Bash as the builder, to which the supplied builder is passed
as a command-line argument. See the Nixpkgs manual for details.
The builder is executed as follows:
- A temporary directory is created under the directory specified by
`TMPDIR` (default `/tmp`) where the build will take place. The
current directory is changed to this directory.
- The environment is cleared and set to the derivation attributes, as
specified above.
- In addition, the following variables are set:
- `NIX_BUILD_TOP` contains the path of the temporary directory for
this build.
- Also, `TMPDIR`, `TEMPDIR`, `TMP`, `TEMP` are set to point to the
temporary directory. This is to prevent the builder from
accidentally writing temporary files anywhere else. Doing so
might cause interference by other processes.
- `PATH` is set to `/path-not-set` to prevent shells from
initialising it to their built-in default value.
- `HOME` is set to `/homeless-shelter` to prevent programs from
using `/etc/passwd` or the like to find the user's home
directory, which could cause impurity. Usually, when `HOME` is
set, it is used as the location of the home directory, even if
it points to a non-existent path.
- `NIX_STORE` is set to the path of the top-level Nix store
directory (typically, `/nix/store`).
- For each output declared in `outputs`, the corresponding
environment variable is set to point to the intended path in the
Nix store for that output. Each output path is a concatenation
of the cryptographic hash of all build inputs, the `name`
attribute and the output name. (The output name is omitted if
it’s `out`.)
- If an output path already exists, it is removed. Also, locks are
acquired to prevent multiple Nix instances from performing the same
build at the same time.
- A log of the combined standard output and error is written to
`/nix/var/log/nix`.
- The builder is executed with the arguments specified by the
attribute `args`. If it exits with exit code 0, it is considered to
have succeeded.
- The temporary directory is removed (unless the `-K` option was
specified).
- If the build was successful, Nix scans each output path for
references to input paths by looking for the hash parts of the input
paths. Since these are potential runtime dependencies, Nix registers
them as dependencies of the output paths.
- After the build, Nix sets the last-modified timestamp on all files
in the build result to 1 (00:00:01 1/1/1970 UTC), sets the group to
the default group, and sets the mode of the file to 0444 or 0555
(i.e., read-only, with execute permission enabled if the file was
originally executable). Note that possible `setuid` and `setgid`
bits are cleared. Setuid and setgid programs are not currently
supported by Nix. This is because the Nix archives used in
deployment have no concept of ownership information, and because it
makes the build result dependent on the user performing the build.

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# Nix Language
The Nix language is a pure, lazy, functional language. Purity
means that operations in the language don't have side-effects (for
instance, there is no variable assignment). Laziness means that
arguments to functions are evaluated only when they are needed.
Functional means that functions are “normal” values that can be passed
around and manipulated in interesting ways. The language is not a
full-featured, general purpose language. Its main job is to describe
packages, compositions of packages, and the variability within packages.
This section presents the various features of the language.

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# Operators
The table below lists the operators in the Nix language, in
order of precedence (from strongest to weakest binding).
| Name | Syntax | Associativity | Description | Precedence |
| ------------------------ | ----------------------------------- | ------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------- |
| Select | *e* `.` *attrpath* \[ `or` *def* \] | none | Select attribute denoted by the attribute path *attrpath* from set *e*. (An attribute path is a dot-separated list of attribute names.) If the attribute doesn’t exist, return *def* if provided, otherwise abort evaluation. | 1 |
| Application | *e1* *e2* | left | Call function *e1* with argument *e2*. | 2 |
| Arithmetic Negation | `-` *e* | none | Arithmetic negation. | 3 |
| Has Attribute | *e* `?` *attrpath* | none | Test whether set *e* contains the attribute denoted by *attrpath*; return `true` or `false`. | 4 |
| List Concatenation | *e1* `++` *e2* | right | List concatenation. | 5 |
| Multiplication | *e1* `*` *e2*, | left | Arithmetic multiplication. | 6 |
| Division | *e1* `/` *e2* | left | Arithmetic division. | 6 |
| Addition | *e1* `+` *e2* | left | Arithmetic addition. | 7 |
| Subtraction | *e1* `-` *e2* | left | Arithmetic subtraction. | 7 |
| String Concatenation | *string1* `+` *string2* | left | String concatenation. | 7 |
| Not | `!` *e* | none | Boolean negation. | 8 |
| Update | *e1* `//` *e2* | right | Return a set consisting of the attributes in *e1* and *e2* (with the latter taking precedence over the former in case of equally named attributes). | 9 |
| Less Than | *e1* `<` *e2*, | none | Arithmetic/lexicographic comparison. | 10 |
| Less Than or Equal To | *e1* `<=` *e2* | none | Arithmetic/lexicographic comparison. | 10 |
| Greater Than | *e1* `>` *e2* | none | Arithmetic/lexicographic comparison. | 10 |
| Greater Than or Equal To | *e1* `>=` *e2* | none | Arithmetic/lexicographic comparison. | 10 |
| Equality | *e1* `==` *e2* | none | Equality. | 11 |
| Inequality | *e1* `!=` *e2* | none | Inequality. | 11 |
| Logical AND | *e1* `&&` *e2* | left | Logical AND. | 12 |
| Logical OR | *e1* <code>&#124;&#124;</code> *e2* | left | Logical OR. | 13 |
| Logical Implication | *e1* `->` *e2* | none | Logical implication (equivalent to <code>!e1 &#124;&#124; e2</code>). | 14 |

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# Data Types
## Primitives
- <a id="type-string" href="#type-string">String</a>
*Strings* can be written in three ways.
The most common way is to enclose the string between double quotes,
e.g., `"foo bar"`. Strings can span multiple lines. The special
characters `"` and `\` and the character sequence `${` must be
escaped by prefixing them with a backslash (`\`). Newlines, carriage
returns and tabs can be written as `\n`, `\r` and `\t`,
respectively.
You can include the result of an expression into a string by
enclosing it in `${...}`, a feature known as *antiquotation*. The
enclosed expression must evaluate to something that can be coerced
into a string (meaning that it must be a string, a path, or a
derivation). For instance, rather than writing
```nix
"--with-freetype2-library=" + freetype + "/lib"
```
(where `freetype` is a derivation), you can instead write the more
natural
```nix
"--with-freetype2-library=${freetype}/lib"
```
The latter is automatically translated to the former. A more
complicated example (from the Nix expression for
[Qt](http://www.trolltech.com/products/qt)):
```nix
configureFlags = "
-system-zlib -system-libpng -system-libjpeg
${if openglSupport then "-dlopen-opengl
-L${mesa}/lib -I${mesa}/include
-L${libXmu}/lib -I${libXmu}/include" else ""}
${if threadSupport then "-thread" else "-no-thread"}
";
```
Note that Nix expressions and strings can be arbitrarily nested; in
this case the outer string contains various antiquotations that
themselves contain strings (e.g., `"-thread"`), some of which in
turn contain expressions (e.g., `${mesa}`).
The second way to write string literals is as an *indented string*,
which is enclosed between pairs of *double single-quotes*, like so:
```nix
''
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
```nix
"This is the first line.\nThis is the second line.\n This is the third line.\n"
```
Note that the whitespace and newline following the opening `''` is
ignored if there is no non-whitespace text on the initial line.
Antiquotation (`${expr}`) is supported in indented strings.
Since `${` and `''` have special meaning in indented strings, you
need a way to quote them. `$` can be escaped by prefixing it with
`''` (that is, two single quotes), i.e., `''$`. `''` can be escaped
by prefixing it with `'`, i.e., `'''`. `$` removes any special
meaning from the following `$`. Linefeed, carriage-return and tab
characters can be written as `''\n`, `''\r`, `''\t`, and `''\`
escapes any other character.
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:
```nix
stdenv.mkDerivation {
...
postInstall =
''
mkdir $out/bin $out/etc
cp foo $out/bin
echo "Hello World" > $out/etc/foo.conf
${if enableBar then "cp bar $out/bin" else ""}
'';
...
}
```
Finally, as a convenience, *URIs* as defined in appendix B of
[RFC 2396](http://www.ietf.org/rfc/rfc2396.txt) can be written *as
is*, without quotes. For instance, the string
`"http://example.org/foo.tar.bz2"` can also be written as
`http://example.org/foo.tar.bz2`.
- <a id="type-number" href="#type-number">Number</a>
Numbers, which can be *integers* (like `123`) or *floating point*
(like `123.43` or `.27e13`).
Numbers are type-compatible: pure integer operations will always
return integers, whereas any operation involving at least one
floating point number will have a floating point number as a result.
- <a id="type-path" href="#type-path">Path</a>
*Paths*, e.g., `/bin/sh` or `./builder.sh`. A path must contain at
least one slash to be recognised as such. For instance, `builder.sh`
is not a path: it's parsed as an expression that selects the
attribute `sh` from the variable `builder`. If the file name is
relative, i.e., if it does not begin with a slash, it is made
absolute at parse time relative to the directory of the Nix
expression that contained it. For instance, if a Nix expression in
`/foo/bar/bla.nix` refers to `../xyzzy/fnord.nix`, the absolute path
is `/foo/xyzzy/fnord.nix`.
If the first component of a path is a `~`, it is interpreted as if
the rest of the path were relative to the user's home directory.
e.g. `~/foo` would be equivalent to `/home/edolstra/foo` for a user
whose home directory is `/home/edolstra`.
Paths can also be specified between angle brackets, e.g.
`<nixpkgs>`. This means that the directories listed in the
environment variable `NIX_PATH` will be searched for the given file
or directory name.
Antiquotation is supported in any paths except those in angle brackets.
`./${foo}-${bar}.nix` is a more convenient way of writing
`./. + "/" + foo + "-" + bar + ".nix"` or `./. + "/${foo}-${bar}.nix"`. At
least one slash must appear *before* any antiquotations for this to be
recognized as a path. `a.${foo}/b.${bar}` is a syntactically valid division
operation. `./a.${foo}/b.${bar}` is a path.
- <a id="type-boolean" href="#type-boolean">Boolean</a>
*Booleans* with values `true` and `false`.
- <a id="type-null" href="#type-null">Null</a>
The null value, denoted as `null`.
## List
Lists are formed by enclosing a whitespace-separated list of values
between square brackets. For example,
```nix
[ 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.,
```nix
[ 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.
## Attribute Set
An attribute set is a collection of name-value-pairs (called *attributes*) enclosed in curly brackets (`{ }`).
Names and values are separated by an equal sign (`=`).
Each value is an arbitrary expression terminated by a semicolon (`;`).
Attributes can appear in any order.
An attribute name may only occur once.
Example:
```nix
{
x = 123;
text = "Hello";
y = f { bla = 456; };
}
```
This defines a set with attributes named `x`, `text`, `y`.
Attributes can be selected from a set using the `.` operator. For
instance,
```nix
{ a = "Foo"; b = "Bar"; }.a
```
evaluates to `"Foo"`. It is possible to provide a default value in an
attribute selection using the `or` keyword. For example,
```nix
{ a = "Foo"; b = "Bar"; }.c or "Xyzzy"
```
will evaluate to `"Xyzzy"` because there is no `c` attribute in the set.
You can use arbitrary double-quoted strings as attribute names:
```nix
{ "foo ${bar}" = 123; "nix-1.0" = 456; }."foo ${bar}"
```
This will evaluate to `123` (Assuming `bar` is antiquotable). In the
case where an attribute name is just a single antiquotation, the quotes
can be dropped:
```nix
{ foo = 123; }.${bar} or 456
```
This will evaluate to `123` if `bar` evaluates to `"foo"` when coerced
to a string and `456` otherwise (again assuming `bar` is antiquotable).
In the special case where an attribute name inside of a set declaration
evaluates to `null` (which is normally an error, as `null` is not
antiquotable), that attribute is simply not added to the set:
```nix
{ ${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.,
```nix
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.