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24
doc/manual/src/SUMMARY.md.in
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@ -26,21 +26,14 @@
- [Copying Closures via SSH](package-management/copy-closure.md)
- [Serving a Nix store via SSH](package-management/ssh-substituter.md)
- [Serving a Nix store via S3](package-management/s3-substituter.md)
- [Writing Nix Expressions](expressions/writing-nix-expressions.md)
- [A Simple Nix Expression](expressions/simple-expression.md)
- [Expression Syntax](expressions/expression-syntax.md)
- [Build Script](expressions/build-script.md)
- [Arguments and Variables](expressions/arguments-variables.md)
- [Building and Testing](expressions/simple-building-testing.md)
- [Generic Builder Syntax](expressions/generic-builder.md)
- [Writing Nix Expressions](expressions/expression-language.md)
- [Values](expressions/language-values.md)
- [Language Constructs](expressions/language-constructs.md)
- [Operators](expressions/language-operators.md)
- [Derivations](expressions/derivations.md)
- [Advanced Attributes](expressions/advanced-attributes.md)
- [Built-in Constants](expressions/builtin-constants.md)
- [Built-in Functions](expressions/builtins.md)
- [Nix Language](language/index.md)
- [Data Types](language/values.md)
- [Language Constructs](language/constructs.md)
- [Operators](language/operators.md)
- [Derivations](language/derivations.md)
- [Advanced Attributes](language/advanced-attributes.md)
- [Built-in Constants](language/builtin-constants.md)
- [Built-in Functions](language/builtins.md)
- [Advanced Topics](advanced-topics/advanced-topics.md)
- [Remote Builds](advanced-topics/distributed-builds.md)
- [Tuning Cores and Jobs](advanced-topics/cores-vs-jobs.md)
@ -72,6 +65,7 @@
- [CLI guideline](contributing/cli-guideline.md)
- [Release Notes](release-notes/release-notes.md)
- [Release X.Y (202?-??-??)](release-notes/rl-next.md)
- [Release 2.11 (2022-08-25)](release-notes/rl-2.11.md)
- [Release 2.10 (2022-07-11)](release-notes/rl-2.10.md)
- [Release 2.9 (2022-05-30)](release-notes/rl-2.9.md)
- [Release 2.8 (2022-04-19)](release-notes/rl-2.8.md)

4
doc/manual/src/advanced-topics/distributed-builds.md
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@ -12,14 +12,14 @@ machine is accessible via SSH and that it has Nix installed. You can
test whether connecting to the remote Nix instance works, e.g.
```console
$ nix ping-store --store ssh://mac
$ nix store ping --store ssh://mac
```
will try to connect to the machine named `mac`. It is possible to
specify an SSH identity file as part of the remote store URI, e.g.
```console
$ nix ping-store --store ssh://mac?ssh-key=/home/alice/my-key
$ nix store ping --store ssh://mac?ssh-key=/home/alice/my-key
```
Since builds should be non-interactive, the key should not have a

4
doc/manual/src/command-ref/nix-copy-closure.md
View file

@ -30,8 +30,8 @@ Since `nix-copy-closure` calls `ssh`, you may be asked to type in the
appropriate password or passphrase. In fact, you may be asked _twice_
because `nix-copy-closure` currently connects twice to the remote
machine, first to get the set of paths missing on the target machine,
and second to send the dump of those paths. If this bothers you, use
`ssh-agent`.
and second to send the dump of those paths. When using public key
authentication, you can avoid typing the passphrase with `ssh-agent`.
# Options

2
doc/manual/src/command-ref/nix-env.md
View file

@ -198,7 +198,7 @@ a number of possible ways:
another.
- If `--from-expression` is given, *args* are Nix
[functions](../expressions/language-constructs.md#functions)
[functions](../language/constructs.md#functions)
that are called with the active Nix expression as their single
argument. The derivations returned by those function calls are
installed. This allows derivations to be specified in an

4
doc/manual/src/command-ref/nix-instantiate.md
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@ -51,7 +51,7 @@ standard input.
- `--strict`\
When used with `--eval`, recursively evaluate list elements and
attributes. Normally, such sub-expressions are left unevaluated
(since the Nix expression language is lazy).
(since the Nix language is lazy).
> **Warning**
>
@ -66,7 +66,7 @@ standard input.
When used with `--eval`, print the resulting value as an XML
representation of the abstract syntax tree rather than as an ATerm.
The schema is the same as that used by the [`toXML`
built-in](../expressions/builtins.md).
built-in](../language/builtins.md).
- `--read-write-mode`\
When used with `--eval`, perform evaluation in read/write mode so

2
doc/manual/src/command-ref/nix-store.md
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@ -121,7 +121,7 @@ Special exit codes:
- `102`\
Hash mismatch, the build output was rejected because it does not
match the [`outputHash` attribute of the
derivation](../expressions/advanced-attributes.md).
derivation](../language/advanced-attributes.md).
- `104`\
Not deterministic, the build succeeded in check mode but the

4
doc/manual/src/command-ref/opt-common.md
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@ -145,7 +145,7 @@ Most Nix commands accept the following command-line options:
expression evaluator will automatically try to call functions that
it encounters. It can automatically call functions for which every
argument has a [default
value](../expressions/language-constructs.md#functions) (e.g.,
value](../language/constructs.md#functions) (e.g.,
`{ argName ? defaultValue }: ...`). With `--arg`, you can also
call functions that have arguments without a default value (or
override a default value). That is, if the evaluator encounters a
@ -164,7 +164,7 @@ Most Nix commands accept the following command-line options:
So if you call this Nix expression (e.g., when you do `nix-env -iA
pkgname`), the function will be called automatically using the
value [`builtins.currentSystem`](../expressions/builtins.md) for
value [`builtins.currentSystem`](../language/builtins.md) for
the `system` argument. You can override this using `--arg`, e.g.,
`nix-env -iA pkgname --arg system \"i686-freebsd\"`. (Note that
since the argument is a Nix string literal, you have to escape the

78
doc/manual/src/contributing/hacking.md
View file

@ -42,7 +42,7 @@ $ nix develop
```
To get a shell with a different compilation environment (e.g. stdenv,
gccStdenv, clangStdenv, clang11Stdenv):
gccStdenv, clangStdenv, clang11Stdenv, ccacheStdenv):
```console
$ nix-shell -A devShells.x86_64-linux.clang11StdenvPackages
@ -54,6 +54,9 @@ or if you have a flake-enabled nix:
$ nix develop .#clang11StdenvPackages
```
Note: you can use `ccacheStdenv` to drastically improve rebuild
time. By default, ccache keeps artifacts in `~/.cache/ccache/`.
To build Nix itself in this shell:
```console
@ -83,9 +86,7 @@ by:
$ nix develop
```
## Testing
Nix comes with three different flavors of tests: unit, functional and integration.
## Running tests
### Unit-tests
@ -108,3 +109,72 @@ These tests include everything that needs to interact with external services or
Because these tests are expensive and require more than what the standard github-actions setup provides, they only run on the master branch (on <https://hydra.nixos.org/jobset/nix/master>).
You can run them manually with `nix build .#hydraJobs.tests.{testName}` or `nix-build -A hydraJobs.tests.{testName}`
### Installer tests
After a one-time setup, the Nix repository's GitHub Actions continuous integration (CI) workflow can test the installer each time you push to a branch.
Creating a Cachix cache for your installer tests and adding its authorization token to GitHub enables [two installer-specific jobs in the CI workflow](https://github.com/NixOS/nix/blob/88a45d6149c0e304f6eb2efcc2d7a4d0d569f8af/.github/workflows/ci.yml#L50-L91):
- The `installer` job generates installers for the platforms below and uploads them to your Cachix cache:
- `x86_64-linux`
- `armv6l-linux`
- `armv7l-linux`
- `x86_64-darwin`
- The `installer_test` job (which runs on `ubuntu-latest` and `macos-latest`) will try to install Nix with the cached installer and run a trivial Nix command.
#### One-time setup
1. Have a GitHub account with a fork of the [Nix repository](https://github.com/NixOS/nix).
2. At cachix.org:
- Create or log in to an account.
- Create a Cachix cache using the format `<github-username>-nix-install-tests`.
- Navigate to the new cache > Settings > Auth Tokens.
- Generate a new Cachix auth token and copy the generated value.
3. At github.com:
- Navigate to your Nix fork > Settings > Secrets > Actions > New repository secret.
- Name the secret `CACHIX_AUTH_TOKEN`.
- Paste the copied value of the Cachix cache auth token.
#### Using the CI-generated installer for manual testing
After the CI run completes, you can check the output to extract the installer URL:
1. Click into the detailed view of the CI run.
2. Click into any `installer_test` run (the URL you're here to extract will be the same in all of them).
3. Click into the `Run cachix/install-nix-action@v...` step and click the detail triangle next to the first log line (it will also be `Run cachix/install-nix-action@v...`)
4. Copy the value of `install_url`
5. To generate an install command, plug this `install_url` and your GitHub username into this template:
```console
sh <(curl -L <install_url>) --tarball-url-prefix https://<github-username>-nix-install-tests.cachix.org/serve
```
<!-- #### Manually generating test installers
There's obviously a manual way to do this, and it's still the only way for
platforms that lack GA runners.
I did do this back in Fall 2020 (before the GA approach encouraged here). I'll
sketch what I recall in case it encourages someone to fill in detail, but: I
didn't know what I was doing at the time and had to fumble/ask around a lot--
so I don't want to uphold any of it as "right". It may have been dumb or
the _hard_ way from the getgo. Fundamentals may have changed since.
Here's the build command I used to do this on and for x86_64-darwin:
nix build --out-link /tmp/foo ".#checks.x86_64-darwin.binaryTarball"
I used the stable out-link to make it easier to script the next steps:
link=$(readlink /tmp/foo)
cp $link/*-darwin.tar.xz ~/somewheres
I've lost the last steps and am just going from memory:
From here, I think I had to extract and modify the `install` script to point
it at this tarball (which I scped to my own site, but it might make more sense
to just share them locally). I extracted this script once and then just
search/replaced in it for each new build.
The installer now supports a `--tarball-url-prefix` flag which _may_ have
solved this need?
-->

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doc/manual/src/expressions/arguments-variables.md
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@ -1,80 +0,0 @@
# Arguments and Variables
The [Nix expression for GNU Hello](expression-syntax.md) is a
function; it is missing some arguments that have to be filled in
somewhere. In the Nix Packages collection this is done in the file
`pkgs/top-level/all-packages.nix`, where all Nix expressions for
packages are imported and called with the appropriate arguments. Here
are some fragments of `all-packages.nix`, with annotations of what
they mean:
```nix
...
rec { ①
hello = import ../applications/misc/hello/ex-1 ② { ③
inherit fetchurl stdenv perl;
};
perl = import ../development/interpreters/perl { ④
inherit fetchurl stdenv;
};
fetchurl = import ../build-support/fetchurl {
inherit stdenv; ...
};
stdenv = ...;
}
```
1. This file defines a set of attributes, all of which are concrete
derivations (i.e., not functions). In fact, we define a *mutually
recursive* set of attributes. That is, the attributes can refer to
each other. This is precisely what we want since we want to “plug”
the various packages into each other.
2. Here we *import* the Nix expression for GNU Hello. The import
operation just loads and returns the specified Nix expression. In
fact, we could just have put the contents of the Nix expression
for GNU Hello in `all-packages.nix` at this point. That would be
completely equivalent, but it would make `all-packages.nix` rather
bulky.
Note that we refer to `../applications/misc/hello/ex-1`, not
`../applications/misc/hello/ex-1/default.nix`. When you try to
import a directory, Nix automatically appends `/default.nix` to the
file name.
3. This is where the actual composition takes place. Here we *call* the
function imported from `../applications/misc/hello/ex-1` with a set
containing the things that the function expects, namely `fetchurl`,
`stdenv`, and `perl`. We use inherit again to use the attributes
defined in the surrounding scope (we could also have written
`fetchurl = fetchurl;`, etc.).
The result of this function call is an actual derivation that can be
built by Nix (since when we fill in the arguments of the function,
what we get is its body, which is the call to `stdenv.mkDerivation`
in the [Nix expression for GNU Hello](expression-syntax.md)).
> **Note**
>
> Nixpkgs has a convenience function `callPackage` that imports and
> calls a function, filling in any missing arguments by passing the
> corresponding attribute from the Nixpkgs set, like this:
>
> ```nix
> hello = callPackage ../applications/misc/hello/ex-1 { };
> ```
>
> If necessary, you can set or override arguments:
>
> ```nix
> hello = callPackage ../applications/misc/hello/ex-1 { stdenv = myStdenv; };
> ```
4. Likewise, we have to instantiate Perl, `fetchurl`, and the standard
environment.

70
doc/manual/src/expressions/build-script.md
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@ -1,70 +0,0 @@
# Build Script
Here is the builder referenced from Hello's Nix expression (stored in
`pkgs/applications/misc/hello/ex-1/builder.sh`):
```bash
source $stdenv/setup ①
PATH=$perl/bin:$PATH ②
tar xvfz $src ③
cd hello-*
./configure --prefix=$out ④
make ⑤
make install
```
The builder can actually be made a lot shorter by using the *generic
builder* functions provided by `stdenv`, but here we write out the build
steps to elucidate what a builder does. It performs the following steps:
1. When Nix runs a builder, it initially completely clears the
environment (except for the attributes declared in the derivation).
This is done to prevent undeclared inputs from being used in the
build process. If for example the `PATH` contained `/usr/bin`, then
you might accidentally use `/usr/bin/gcc`.
So the first step is to set up the environment. This is done by
calling the `setup` script of the standard environment. The
environment variable `stdenv` points to the location of the
standard environment being used. (It wasn't specified explicitly
as an attribute in Hello's Nix expression, but `mkDerivation` adds
it automatically.)
2. Since Hello needs Perl, we have to make sure that Perl is in the
`PATH`. The `perl` environment variable points to the location of
the Perl package (since it was passed in as an attribute to the
derivation), so `$perl/bin` is the directory containing the Perl
interpreter.
3. Now we have to unpack the sources. The `src` attribute was bound to
the result of fetching the Hello source tarball from the network, so
the `src` environment variable points to the location in the Nix
store to which the tarball was downloaded. After unpacking, we `cd`
to the resulting source directory.
The whole build is performed in a temporary directory created in
`/tmp`, by the way. This directory is removed after the builder
finishes, so there is no need to clean up the sources afterwards.
Also, the temporary directory is always newly created, so you don't
have to worry about files from previous builds interfering with the
current build.
4. GNU Hello is a typical Autoconf-based package, so we first have to
run its `configure` script. In Nix every package is stored in a
separate location in the Nix store, for instance
`/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1`. Nix
computes this path by cryptographically hashing all attributes of
the derivation. The path is passed to the builder through the `out`
environment variable. So here we give `configure` the parameter
`--prefix=$out` to cause Hello to be installed in the expected
location.
5. Finally we build Hello (`make`) and install it into the location
specified by `out` (`make install`).
If you are wondering about the absence of error checking on the result
of various commands called in the builder: this is because the shell
script is evaluated with Bash's `-e` option, which causes the script to
be aborted if any command fails without an error check.

12
doc/manual/src/expressions/expression-language.md
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@ -1,12 +0,0 @@
# Nix Expression Language
The Nix expression 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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doc/manual/src/expressions/expression-syntax.md
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@ -1,93 +0,0 @@
# Expression Syntax
Here is a Nix expression for GNU Hello:
```nix
{ stdenv, fetchurl, perl }: ①
stdenv.mkDerivation { ②
name = "hello-2.1.1"; ③
builder = ./builder.sh; ④
src = fetchurl { ⑤
url = "ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz";
sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
};
inherit perl; ⑥
}
```
This file is actually already in the Nix Packages collection in
`pkgs/applications/misc/hello/ex-1/default.nix`. It is customary to
place each package in a separate directory and call the single Nix
expression in that directory `default.nix`. The file has the following
elements (referenced from the figure by number):
1. This states that the expression is a *function* that expects to be
called with three arguments: `stdenv`, `fetchurl`, and `perl`. They
are needed to build Hello, but we don't know how to build them here;
that's why they are function arguments. `stdenv` is a package that
is used by almost all Nix Packages; it provides a
“standard” environment consisting of the things you would expect
in a basic Unix environment: a C/C++ compiler (GCC, to be precise),
the Bash shell, fundamental Unix tools such as `cp`, `grep`, `tar`,
etc. `fetchurl` is a function that downloads files. `perl` is the
Perl interpreter.
Nix functions generally have the form `{ x, y, ..., z }: e` where
`x`, `y`, etc. are the names of the expected arguments, and where
*e* is the body of the function. So here, the entire remainder of
the file is the body of the function; when given the required
arguments, the body should describe how to build an instance of
the Hello package.
2. So we have to build a package. Building something from other stuff
is called a *derivation* in Nix (as opposed to sources, which are
built by humans instead of computers). We perform a derivation by
calling `stdenv.mkDerivation`. `mkDerivation` is a function
provided by `stdenv` that builds a package from a set of
*attributes*. A set is just a list of key/value pairs where each
key is a string and each value is an arbitrary Nix
expression. They take the general form `{ name1 = expr1; ...
nameN = exprN; }`.
3. The attribute `name` specifies the symbolic name and version of
the package. Nix doesn't really care about these things, but they
are used by for instance `nix-env -q` to show a “human-readable”
name for packages. This attribute is required by `mkDerivation`.
4. The attribute `builder` specifies the builder. This attribute can
sometimes be omitted, in which case `mkDerivation` will fill in a
default builder (which does a `configure; make; make install`, in
essence). Hello is sufficiently simple that the default builder
would suffice, but in this case, we will show an actual builder
for educational purposes. The value `./builder.sh` refers to the
shell script shown in the [next section](build-script.md),
discussed below.
5. The builder has to know what the sources of the package are. Here,
the attribute `src` is bound to the result of a call to the
`fetchurl` function. Given a URL and a SHA-256 hash of the expected
contents of the file at that URL, this function builds a derivation
that downloads the file and checks its hash. So the sources are a
dependency that like all other dependencies is built before Hello
itself is built.
Instead of `src` any other name could have been used, and in fact
there can be any number of sources (bound to different attributes).
However, `src` is customary, and it's also expected by the default
builder (which we don't use in this example).
6. Since the derivation requires Perl, we have to pass the value of the
`perl` function argument to the builder. All attributes in the set
are actually passed as environment variables to the builder, so
declaring an attribute
```nix
perl = perl;
```
will do the trick: it binds an attribute `perl` to the function
argument which also happens to be called `perl`. However, it looks a
bit silly, so there is a shorter syntax. The `inherit` keyword
causes the specified attributes to be bound to whatever variables
with the same name happen to be in scope.

66
doc/manual/src/expressions/generic-builder.md
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@ -1,66 +0,0 @@
# Generic Builder Syntax
Recall that the [build script for GNU Hello](build-script.md) looked
something like this:
```bash
PATH=$perl/bin:$PATH
tar xvfz $src
cd hello-*
./configure --prefix=$out
make
make install
```
The builders for almost all Unix packages look like this — set up some
environment variables, unpack the sources, configure, build, and
install. For this reason the standard environment provides some Bash
functions that automate the build process. Here is what a builder using
the generic build facilities looks like:
```bash
buildInputs="$perl" ①
source $stdenv/setup ②
genericBuild ③
```
Here is what each line means:
1. The `buildInputs` variable tells `setup` to use the indicated
packages as “inputs”. This means that if a package provides a `bin`
subdirectory, it's added to `PATH`; if it has a `include`
subdirectory, it's added to GCC's header search path; and so on.
(This is implemented in a modular way: `setup` tries to source the
file `pkg/nix-support/setup-hook` of all dependencies. These “setup
hooks” can then set up whatever environment variables they want; for
instance, the setup hook for Perl sets the `PERL5LIB` environment
variable to contain the `lib/site_perl` directories of all inputs.)
2. The function `genericBuild` is defined in the file `$stdenv/setup`.
3. The final step calls the shell function `genericBuild`, which
performs the steps that were done explicitly in the previous build
script. The generic builder is smart enough to figure out whether
to unpack the sources using `gzip`, `bzip2`, etc. It can be
customised in many ways; see the Nixpkgs manual for details.
Discerning readers will note that the `buildInputs` could just as well
have been set in the Nix expression, like this:
```nix
buildInputs = [ perl ];
```
The `perl` attribute can then be removed, and the builder becomes even
shorter:
```bash
source $stdenv/setup
genericBuild
```
In fact, `mkDerivation` provides a default builder that looks exactly
like that, so it is actually possible to omit the builder for Hello
entirely.

251
doc/manual/src/expressions/language-values.md
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@ -1,251 +0,0 @@
# Values
## Simple Values
Nix has the following basic data types:
- *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 $out/bin" else ""}
'';
...
}
```
Finally, as a convenience, *URIs* as defined in appendix B of
[RFC 2396](http://www.ietf.org/rfc/rfc2396.txt) can be written *as
is*, without quotes. For instance, the string
`"http://example.org/foo.tar.bz2"` can also be written as
`http://example.org/foo.tar.bz2`.
- Numbers, which can be *integers* (like `123`) or *floating point*
(like `123.43` or `.27e13`).
Numbers are type-compatible: pure integer operations will always
return integers, whereas any operation involving at least one
floating point number will have a floating point number as a result.
- *Paths*, e.g., `/bin/sh` or `./builder.sh`. A path must contain at
least one slash to be recognised as such. For instance, `builder.sh`
is not a path: it's parsed as an expression that selects the
attribute `sh` from the variable `builder`. If the file name is
relative, i.e., if it does not begin with a slash, it is made
absolute at parse time relative to the directory of the Nix
expression that contained it. For instance, if a Nix expression in
`/foo/bar/bla.nix` refers to `../xyzzy/fnord.nix`, the absolute path
is `/foo/xyzzy/fnord.nix`.
If the first component of a path is a `~`, it is interpreted as if
the rest of the path were relative to the user's home directory.
e.g. `~/foo` would be equivalent to `/home/edolstra/foo` for a user
whose home directory is `/home/edolstra`.
Paths can also be specified between angle brackets, e.g.
`<nixpkgs>`. This means that the directories listed in the
environment variable `NIX_PATH` will be searched for the given file
or directory name.
Antiquotation is supported in any paths except those in angle brackets.
`./${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.
- *Booleans* with values `true` and `false`.
- The null value, denoted as `null`.
## Lists
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.
## Sets
Sets are really the core of the language, since ultimately the Nix
language is all about creating derivations, which are really just sets
of attributes to be passed to build scripts.
Sets are just a list of name/value pairs (called *attributes*) enclosed
in curly brackets, where each value is an arbitrary expression
terminated by a semicolon. For example:
```nix
{ x = 123;
text = "Hello";
y = f { bla = 456; };
}
```
This defines a set with attributes named `x`, `text`, `y`. The order of
the attributes is irrelevant. An attribute name may only occur once.
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.

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@ -1,61 +0,0 @@
# Building and Testing
You can now try to build Hello. Of course, you could do `nix-env -f . -iA
hello`, but you may not want to install a possibly broken package just
yet. The best way to test the package is by using the command
`nix-build`, which builds a Nix expression and creates a symlink named
`result` in the current directory:
```console
$ nix-build -A hello
building path `/nix/store/632d2b22514d...-hello-2.1.1'
hello-2.1.1/
hello-2.1.1/intl/
hello-2.1.1/intl/ChangeLog
...
$ ls -l result
lrwxrwxrwx ... 2006-09-29 10:43 result -> /nix/store/632d2b22514d...-hello-2.1.1
$ ./result/bin/hello
Hello, world!
```
The `-A` option selects the `hello` attribute. This is faster than
using the symbolic package name specified by the `name` attribute
(which also happens to be `hello`) and is unambiguous (there can be
multiple packages with the symbolic name `hello`, but there can be
only one attribute in a set named `hello`).
`nix-build` registers the `./result` symlink as a garbage collection
root, so unless and until you delete the `./result` symlink, the output
of the build will be safely kept on your system. You can use
`nix-build`’s `-o` switch to give the symlink another name.
Nix has transactional semantics. Once a build finishes successfully, Nix
makes a note of this in its database: it registers that the path denoted
by `out` is now “valid”. If you try to build the derivation again, Nix
will see that the path is already valid and finish immediately. If a
build fails, either because it returns a non-zero exit code, because Nix
or the builder are killed, or because the machine crashes, then the
output paths will not be registered as valid. If you try to build the
derivation again, Nix will remove the output paths if they exist (e.g.,
because the builder died half-way through `make
install`) and try again. Note that there is no “negative caching”: Nix
doesn't remember that a build failed, and so a failed build can always
be repeated. This is because Nix cannot distinguish between permanent
failures (e.g., a compiler error due to a syntax error in the source)
and transient failures (e.g., a disk full condition).
Nix also performs locking. If you run multiple Nix builds
simultaneously, and they try to build the same derivation, the first Nix
instance that gets there will perform the build, while the others block
(or perform other derivations if available) until the build finishes:
```console
$ nix-build -A hello
waiting for lock on `/nix/store/0h5b7hp8d4hqfrw8igvx97x1xawrjnac-hello-2.1.1x'
```
So it is always safe to run multiple instances of Nix in parallel (which
isn’t the case with, say, `make`).

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@ -1,23 +0,0 @@
# A Simple Nix Expression
This section shows how to add and test the [GNU Hello
package](http://www.gnu.org/software/hello/hello.html) to the Nix
Packages collection. Hello is a program that prints out the text “Hello,
world\!”.
To add a package to the Nix Packages collection, you generally need to
do three things:
1. Write a Nix expression for the package. This is a file that
describes all the inputs involved in building the package, such as
dependencies, sources, and so on.
2. Write a *builder*. This is a shell script that builds the package
from the inputs. (In fact, it can be written in any language, but
typically it's a `bash` shell script.)
3. Add the package to the file `pkgs/top-level/all-packages.nix`. The
Nix expression written in the first step is a *function*; it
requires other packages in order to build it. In this step you put
it all together, i.e., you call the function with the right
arguments to build the actual package.

12
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@ -1,12 +0,0 @@
This chapter shows you how to write Nix expressions, which instruct Nix
how to build packages. It starts with a simple example (a Nix expression
for GNU Hello), and then moves on to a more in-depth look at the Nix
expression language.
> **Note**
>
> This chapter is mostly about the Nix expression language. For more
> extensive information on adding packages to the Nix Packages
> collection (such as functions in the standard environment and coding
> conventions), please consult [its
> manual](http://nixos.org/nixpkgs/manual/).

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@ -3,14 +3,48 @@
- [derivation]{#gloss-derivation}\
A description of a build action. The result of a derivation is a
store object. Derivations are typically specified in Nix expressions
using the [`derivation` primitive](expressions/derivations.md). These are
using the [`derivation` primitive](language/derivations.md). These are
translated into low-level *store derivations* (implicitly by
`nix-env` and `nix-build`, or explicitly by `nix-instantiate`).
- [content-addressed derivation]{#gloss-content-addressed-derivation}\
A derivation which has the
[`__contentAddressed`](language/advanced-attributes.md#adv-attr-__contentAddressed)
attribute set to `true`.
- [fixed-output derivation]{#gloss-fixed-output-derivation}\
A derivation which includes the
[`outputHash`](language/advanced-attributes.md#adv-attr-outputHash) attribute.
- [store]{#gloss-store}\
The location in the file system where store objects live. Typically
`/nix/store`.
From the perspective of the location where Nix is
invoked, the Nix store can be referred to
as a "_local_" or a "_remote_" one:
+ A *local store* exists on the filesystem of
the machine where Nix is invoked. You can use other
local stores by passing the `--store` flag to the
`nix` command. Local stores can be used for building derivations.
+ A *remote store* exists anywhere other than the
local filesystem. One example is the `/nix/store`
directory on another machine, accessed via `ssh` or
served by the `nix-serve` Perl script.
- [chroot store]{#gloss-chroot-store}\
A local store whose canonical path is anything other than `/nix/store`.
- [binary cache]{#gloss-binary-cache}\
A *binary cache* is a Nix store which uses a different format: its
metadata and signatures are kept in `.narinfo` files rather than in a
Nix database. This different format simplifies serving store objects
over the network, but cannot host builds. Examples of binary caches
include S3 buckets and the [NixOS binary
cache](https://cache.nixos.org).
- [store path]{#gloss-store-path}\
The location in the file system of a store object, i.e., an
immediate child of the Nix store directory.
@ -22,6 +56,19 @@
derivation outputs (objects produced by running a build action), or
derivations (files describing a build action).
- [input-addressed store object]{#gloss-input-addressed-store-object}\
A store object produced by building a
non-[content-addressed](#gloss-content-addressed-derivation),
non-[fixed-output](#gloss-fixed-output-derivation)
derivation.
- [output-addressed store object]{#gloss-output-addressed-store-object}\
A store object whose store path hashes its content. This
includes derivations, the outputs of
[content-addressed derivations](#gloss-content-addressed-derivation),
and the outputs of
[fixed-output derivations](#gloss-fixed-output-derivation).
- [substitute]{#gloss-substitute}\
A substitute is a command invocation stored in the Nix database that
describes how to build a store object, bypassing the normal build
@ -29,6 +76,11 @@
store object by downloading a pre-built version of the store object
from some server.
- [substituter]{#gloss-substituter}\
A *substituter* is an additional store from which Nix will
copy store objects it doesn't have. For details, see the
[`substituters` option](command-ref/conf-file.html#conf-substituters).
- [purity]{#gloss-purity}\
The assumption that equal Nix derivations when run always produce
the same output. This cannot be guaranteed in general (e.g., a

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@ -13,7 +13,7 @@ for your platform:
- multi-user on macOS
> **Notes on read-only filesystem root in macOS 10.15 Catalina +**
>
>
> - It took some time to support this cleanly. You may see posts,
> examples, and tutorials using obsolete workarounds.
> - Supporting it cleanly made macOS installs too complex to qualify
@ -31,8 +31,8 @@ $ sh <(curl -L https://nixos.org/nix/install) --no-daemon
```
This will perform a single-user installation of Nix, meaning that `/nix`
is owned by the invoking user. You should run this under your usual user
account, *not* as root. The script will invoke `sudo` to create `/nix`
is owned by the invoking user. You can run this under your usual user
account or root. The script will invoke `sudo` to create `/nix`
if it doesn’t already exist. If you don’t have `sudo`, you should
manually create `/nix` first as root, e.g.:
@ -71,11 +71,11 @@ $ sh <(curl -L https://nixos.org/nix/install) --daemon
The multi-user installation of Nix will create build users between the
user IDs 30001 and 30032, and a group with the group ID 30000. You
should run this under your usual user account, *not* as root. The script
can run this under your usual user account or root. The script
will invoke `sudo` as needed.
> **Note**
>
>
> If you need Nix to use a different group ID or user ID set, you will
> have to download the tarball manually and [edit the install
> script](#installing-from-a-binary-tarball).
@ -148,7 +148,8 @@ and `/etc/zshrc` which you may remove.
This will remove all the build users that no longer serve a purpose.
4. Edit fstab using `sudo vifs` to remove the line mounting the Nix Store
volume on `/nix`, which looks like this,
volume on `/nix`, which looks like
`UUID=<uuid> /nix apfs rw,noauto,nobrowse,suid,owners` or
`LABEL=Nix\040Store /nix apfs rw,nobrowse`. This will prevent automatic
mounting of the Nix Store volume.
@ -167,7 +168,7 @@ and `/etc/zshrc` which you may remove.
removed next.
7. Remove the Nix Store volume:
```console
sudo diskutil apfs deleteVolume /nix
```
@ -175,8 +176,20 @@ and `/etc/zshrc` which you may remove.
This will remove the Nix Store volume and everything that was added to the
store.
If the output indicates that the command couldn't remove the volume, you should
make sure you don't have an _unmounted_ Nix Store volume. Look for a
"Nix Store" volume in the output of the following command:
```console
diskutil list
```
If you _do_ see a "Nix Store" volume, delete it by re-running the diskutil
deleteVolume command, but replace `/nix` with the store volume's `diskXsY`
identifier.
> **Note**
>
>
> After you complete the steps here, you will still have an empty `/nix`
> directory. This is an expected sign of a successful uninstall. The empty
> `/nix` directory will disappear the next time you reboot.
@ -191,8 +204,7 @@ and `/etc/zshrc` which you may remove.
<!-- Note: anchors above to catch permalinks to old explanations -->
We believe we have ironed out how to cleanly support the read-only root
on modern macOS. New installs will do this automatically, and you can
also re-run a new installer to convert your existing setup.
on modern macOS. New installs will do this automatically.
This section previously detailed the situation, options, and trade-offs,
but it now only outlines what the installer does. You don't need to know

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0
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@ -0,0 +1,33 @@
# 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.

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@ -1,6 +1,6 @@
# Operators
The table below lists the operators in the Nix expression language, in
The table below lists the operators in the Nix language, in
order of precedence (from strongest to weakest binding).
| Name | Syntax | Associativity | Description | Precedence |

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@ -0,0 +1,275 @@
# 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.${bar}` is a path.
When a path appears in an antiquotation, and is thus coerced into a string,
the path is first copied into the Nix store and the resulting string is
the Nix store path. For instance `"${./foo.txt}" will cause `foo.txt` in
the current directory to be copied into the Nix store and result in the
string `"/nix/store/<HASH>-foo.txt"`.
Note that the Nix language assumes that all input files will remain
_unchanged_ during the course of the Nix expression evaluation.
If you for example antiquote a file path during a `nix repl` session, and
then later in the same session, after having changed the file contents,
evaluate the antiquotation with the file path again, then Nix will still
return the first store path. It will _not_ reread the file contents to
produce a different Nix store 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.

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@ -1,5 +1,4 @@
This chapter discusses how to do package management with Nix, i.e.,
how to obtain, install, upgrade, and erase packages. This is the
“user’s” perspective of the Nix system — people who want to *create*
packages should consult the [chapter on writing Nix
expressions](../expressions/writing-nix-expressions.md).
packages should consult the chapter on the [Nix language](../language/index.md).

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# Release 2.11 (2022-08-24)
* `nix copy` now copies the store paths in parallel as much as possible (again).
This doesn't apply for the `daemon` and `ssh-ng` stores which copy everything
in one batch to avoid latencies issues.

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# Release X.Y (202?-??-??)
* Allow explicitly selecting outputs with *store derivations* installable syntax too.
* `<nix/fetchurl.nix>` now accepts an additional argument `impure` which
defaults to `false`. If it is set to `true`, the `hash` and `sha256`
arguments will be ignored and the resulting derivation will have
`__impure` set to `true`, making it an impure derivation.
* Allow explicitly selecting outputs with *store derivations* installable syntax too.