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docs: add language/string-literals.md
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,190 @@ | ||
| # String literals | ||
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| A *string literal* represents a [string](types.md#type-string) value. | ||
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| > **Syntax** | ||
| > | ||
| > *expression* → *string* | ||
| > | ||
| > *string* → `"` ( *string_char*\* [*interpolation_element*][string interpolation] )* *string_char*\* `"` | ||
| > | ||
| > *string* → `''` ( *indented_string_char*\* [*interpolation_element*][string interpolation] )* *indented_string_char*\* `''` | ||
| > | ||
| > *string* → *uri* | ||
| > | ||
| > *string_char* ~ `[^"$\\]|\$(?!\{)|\\.` | ||
| > | ||
| > *indented_string_char* ~ `[^$']|\$\$|\$(?!\{)|''[$']|''\\.|'(?!')` | ||
| > | ||
| > *uri* ~ `[A-Za-z][+\-.0-9A-Za-z]*:[!$%&'*+,\-./0-9:=?@A-Z_a-z~]+` | ||
| Strings can be written in three ways. | ||
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| The most common way is to enclose the string between double quotes, e.g., `"foo bar"`. | ||
| Strings can span multiple lines. | ||
| The results of other expressions can be included into a string by enclosing them in `${ }`, a feature known as [string interpolation]. | ||
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| [string interpolation]: ./string-interpolation.md | ||
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| The following must be escaped to represent them within a string, by prefixing with a backslash (`\`): | ||
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| - Double quote (`"`) | ||
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| > **Example** | ||
| > | ||
| > ```nix | ||
| > "\"" | ||
| > ``` | ||
| > | ||
| > "\"" | ||
| - Backslash (`\`) | ||
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| > **Example** | ||
| > | ||
| > ```nix | ||
| > "\\" | ||
| > ``` | ||
| > | ||
| > "\\" | ||
| - Dollar sign followed by an opening curly bracket (`${`) – "dollar-curly" | ||
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| > **Example** | ||
| > | ||
| > ```nix | ||
| > "\${" | ||
| > ``` | ||
| > | ||
| > "\${" | ||
| The newline, carriage return, and tab characters can be written as `\n`, `\r` and `\t`, respectively. | ||
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| A "double-dollar-curly" (`$${`) can be written literally. | ||
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| > **Example** | ||
| > | ||
| > ```nix | ||
| > "$${" | ||
| > ``` | ||
| > | ||
| > "$\${" | ||
| String values are output on the terminal with Nix-specific escaping. | ||
| Strings written to files will contain the characters encoded by the escaping. | ||
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| The second way to write string literals is as an *indented string*, which is enclosed between pairs of *double single-quotes* (`''`), like so: | ||
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| ```nix | ||
| '' | ||
| This is the first line. | ||
| This is the second line. | ||
| This is the third line. | ||
| '' | ||
| ``` | ||
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| 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 | ||
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| ```nix | ||
| "This is the first line.\nThis is the second line.\n This is the third line.\n" | ||
| ``` | ||
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| > **Note** | ||
| > | ||
| > Whitespace and newline following the opening `''` is ignored if there is no non-whitespace text on the initial line. | ||
| > **Warning** | ||
| > | ||
| > Prefixed tab characters are not stripped. | ||
| > | ||
| > > **Example** | ||
| > > | ||
| > > The following indented string is prefixed with tabs: | ||
| > > | ||
| > > <pre><code class="nohighlight">'' | ||
| > > all: | ||
| > > @echo hello | ||
| > > '' | ||
| > > </code></pre> | ||
| > > | ||
| > > "\tall:\n\t\t@echo hello\n" | ||
| Indented strings support [string interpolation]. | ||
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| The following must be escaped to represent them in an indented string: | ||
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| - `$` is escaped by prefixing it with two single quotes (`''`) | ||
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| > **Example** | ||
| > | ||
| > ```nix | ||
| > '' | ||
| > ''$ | ||
| > '' | ||
| > ``` | ||
| > | ||
| > "$\n" | ||
| - `''` is escaped by prefixing it with one single quote (`'`) | ||
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| > **Example** | ||
| > | ||
| > ```nix | ||
| > '' | ||
| > ''' | ||
| > '' | ||
| > ``` | ||
| > | ||
| > "''\n" | ||
| These special characters are escaped as follows: | ||
| - Linefeed (`\n`): `''\n` | ||
| - Carriage return (`\r`): `''\r` | ||
| - Tab (`\t`): `''\t` | ||
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| `''\` escapes any other character. | ||
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| A "double-dollar-curly" (`$${`) can be written literally. | ||
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| > **Example** | ||
| > | ||
| > ```nix | ||
| > '' | ||
| > $${ | ||
| > '' | ||
| > ``` | ||
| > | ||
| > "$\${\n" | ||
| 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: | ||
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| ```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 ""} | ||
| ''; | ||
| ... | ||
| } | ||
| ``` | ||
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| 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`. |
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