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diff --git a/doc/manual/src/expressions/language-values.md b/doc/manual/src/expressions/language-values.md new file mode 100644 index 000000000..ce31029cc --- /dev/null +++ b/doc/manual/src/expressions/language-values.md @@ -0,0 +1,244 @@ +# 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 space, 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. + + - *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. |