nix-super/doc/manual/src/expressions/language-values.md

# Values

## Simple Values

Nix has the following basic data types:

### Strings

*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

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

*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

*Booleans* with values `true` and `false`.

### Null

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.

## Attribute Sets

Attribute sets are collections 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.