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doc/manual/src/expressions/language-values.md
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Nix has the following basic data types: Nix has the following basic data types:
- *Strings* can be written in three ways. ### Strings
The most common way is to enclose the string between double quotes, *Strings* can be written in three ways.
e.g., `"foo bar"`. Strings can span multiple lines. The special
characters `"` and `\` and the character sequence `${` must be The most common way is to enclose the string between double quotes,
escaped by prefixing them with a backslash (`\`). Newlines, carriage e.g., `"foo bar"`. Strings can span multiple lines. The special
returns and tabs can be written as `\n`, `\r` and `\t`, characters `"` and `\` and the character sequence `${` must be
respectively. escaped by prefixing them with a backslash (`\`). Newlines, carriage
returns and tabs can be written as `\n`, `\r` and `\t`,
You can include the result of an expression into a string by respectively.
enclosing it in `${...}`, a feature known as *antiquotation*. The
enclosed expression must evaluate to something that can be coerced You can include the result of an expression into a string by
into a string (meaning that it must be a string, a path, or a enclosing it in `${...}`, a feature known as *antiquotation*. The
derivation). For instance, rather than writing enclosed expression must evaluate to something that can be coerced
into a string (meaning that it must be a string, a path, or a
```nix derivation). For instance, rather than writing
"--with-freetype2-library=" + freetype + "/lib"
``` ```nix
"--with-freetype2-library=" + freetype + "/lib"
(where `freetype` is a derivation), you can instead write the more ```
natural
(where `freetype` is a derivation), you can instead write the more
```nix natural
"--with-freetype2-library=${freetype}/lib"
``` ```nix
"--with-freetype2-library=${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)): The latter is automatically translated to the former. A more
complicated example (from the Nix expression for
```nix [Qt](http://www.trolltech.com/products/qt)):
configureFlags = "
-system-zlib -system-libpng -system-libjpeg ```nix
${if openglSupport then "-dlopen-opengl configureFlags = "
-L${mesa}/lib -I${mesa}/include -system-zlib -system-libpng -system-libjpeg
-L${libXmu}/lib -I${libXmu}/include" else ""} ${if openglSupport then "-dlopen-opengl
${if threadSupport then "-thread" else "-no-thread"} -L${mesa}/lib -I${mesa}/include
"; -L${libXmu}/lib -I${libXmu}/include" else ""}
``` ${if threadSupport then "-thread" else "-no-thread"}
";
Note that Nix expressions and strings can be arbitrarily nested; in ```
this case the outer string contains various antiquotations that
themselves contain strings (e.g., `"-thread"`), some of which in Note that Nix expressions and strings can be arbitrarily nested; in
turn contain expressions (e.g., `${mesa}`). this case the outer string contains various antiquotations that
themselves contain strings (e.g., `"-thread"`), some of which in
The second way to write string literals is as an *indented string*, turn contain expressions (e.g., `${mesa}`).
which is enclosed between pairs of *double single-quotes*, like so:
The second way to write string literals is as an *indented string*,
```nix which is enclosed between pairs of *double single-quotes*, like so:
```nix
''
This is the first line.
This is the second line.
This is the third line.
''
```
This kind of string literal intelligently strips indentation from
the start of each line. To be precise, it strips from each line a
number of spaces equal to the minimal indentation of the string as a
whole (disregarding the indentation of empty lines). For instance,
the first and second line are indented two spaces, while the third
line is indented four spaces. Thus, two spaces are stripped from
each line, so the resulting string is
```nix
"This is the first line.\nThis is the second line.\n This is the third line.\n"
```
Note that the whitespace and newline following the opening `''` is
ignored if there is no non-whitespace text on the initial line.
Antiquotation (`${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 =
'' ''
This is the first line. mkdir $out/bin $out/etc
This is the second line. cp foo $out/bin
This is the third line. echo "Hello World" > $out/etc/foo.conf
'' ${if enableBar then "cp bar $out/bin" else ""}
``` '';
...
This kind of string literal intelligently strips indentation from }
the start of each line. To be precise, it strips from each line a ```
number of spaces equal to the minimal indentation of the string as a
whole (disregarding the indentation of empty lines). For instance,
the first and second line are indented two spaces, while the third
line is indented four spaces. Thus, two spaces are stripped from
each line, so the resulting string is
```nix
"This is the first line.\nThis is the second line.\n This is the third line.\n"
```
Note that the whitespace and newline following the opening `''` is
ignored if there is no non-whitespace text on the initial line.
Antiquotation (`${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* Finally, as a convenience, *URIs* as defined in appendix B of
(like `123.43` or `.27e13`). [RFC 2396](http://www.ietf.org/rfc/rfc2396.txt) can be written *as
is*, without quotes. For instance, the string
Numbers are type-compatible: pure integer operations will always `"http://example.org/foo.tar.bz2"` can also be written as
return integers, whereas any operation involving at least one `http://example.org/foo.tar.bz2`.
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 ### Numbers
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. Numbers, which can be *integers* (like `123`) or *floating point*
`./${foo}-${bar}.nix` is a more convenient way of writing (like `123.43` or `.27e13`).
`./. + "/" + foo + "-" + bar + ".nix"` or `./. + "/${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.${bar}` is a path.
- *Booleans* with values `true` and `false`. Numbers are type-compatible: pure integer operations will always
return integers, whereas any operation involving at least one
floating point number will have a floating point number as a result.
- The null value, denoted as `null`. ### Paths
*Paths*, e.g., `/bin/sh` or `./builder.sh`. A path must contain at
least one slash to be recognised as such. For instance, `builder.sh`
is not a path: it's parsed as an expression that selects the
attribute `sh` from the variable `builder`. If the file name is
relative, i.e., if it does not begin with a slash, it is made
absolute at parse time relative to the directory of the Nix
expression that contained it. For instance, if a Nix expression in
`/foo/bar/bla.nix` refers to `../xyzzy/fnord.nix`, the absolute path
is `/foo/xyzzy/fnord.nix`.
If the first component of a path is a `~`, it is interpreted as if
the rest of the path were relative to the user's home directory.
e.g. `~/foo` would be equivalent to `/home/edolstra/foo` for a user
whose home directory is `/home/edolstra`.
Paths can also be specified between angle brackets, e.g.
`<nixpkgs>`. This means that the directories listed in the
environment variable `NIX_PATH` will be searched for the given file
or directory name.
Antiquotation is supported in any paths except those in angle brackets.
`./${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