lix/doc/manual/src/language/values.md

Data Types

Primitives

• String

  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

  "--with-freetype2-library=" + freetype + "/lib"
  

  (where freetype is a derivation), you can instead write the more natural

  "--with-freetype2-library=${freetype}/lib"
  

  The latter is automatically translated to the former. A more complicated example (from the Nix expression for Qt):

  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:

  ''
    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

  "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:

  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 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.

• Number

  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.

• Path

  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.

• Boolean

  Booleans with values true and false.

• Null

  The null value, denoted as null.

List

Lists are formed by enclosing a whitespace-separated list of values between square brackets. For example,

[ 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.,

[ 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:

{
  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,

{ 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,

{ 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:

{ "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:

{ 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:

{ ${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.,

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.