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doc/manual: clarify documentation related to the $$ parser bug

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Due to a mistake in the grammar, a dollar character implicitly escapes a second
dollar character that immediately follows, so that it cannot start an
interpolation. Unfortunately, this behaviour has since come to be relied upon,
so it cannot be fixed. Furthermore, the documentation on regular strings did
not mention this behaviour at all, while in the case of indented strings it was
rather implicit.
Mention it explicitly in both cases, and describe how an interpolation can
follow a dollar character (namely, by escaping that). Since we have to touch
that section anyway, state that any character (other than n, r, and t; but
notably including `$` even if not succeeded by `{`) can be escaped using a
backslash in regular strings.

Change-Id: I7e5d68a9a4130eec98ce8218b485168f4b31a677
This commit is contained in:
Alois Wohlschlager 2024-07-13 16:11:07 +02:00 committed by alois31
parent 68567206f2
commit beb231784e
1 changed files with 12 additions and 9 deletions
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21
doc/manual/src/language/values.md
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@ -7,13 +7,16 @@
*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 `${` must be
escaped by prefixing them with a backslash (`\`). Newlines, carriage
returns and tabs can be written as `\n`, `\r` and `\t`,
respectively.
e.g., `"foo bar"`. Strings can span multiple lines. The backslash
(`\`) can be used to escape characters: newlines, carriage returns
and tabs may be written as `\n`, `\r` and `\t` respectively; any
other characters can be preceded by a backslash to remove any
special meaning they may have, like the special characters `"` and
`\` and the character sequence `${`.
You can include the results of other expressions into a string by enclosing them in `${ }`, a feature known as [string interpolation].
Due to a parser issue that has since come to be relied upon, the character sequence `$${` is interpreted literally and does not introduce an interpolation.
To express a `$` character immediately followed by an interpolation, the former must be escaped.
[string interpolation]: ./string-interpolation.md
@ -43,16 +46,16 @@
Note that the whitespace and newline following the opening `''` is
ignored if there is no non-whitespace text on the initial line.
Indented strings support [string interpolation].
Since `${` and `''` have special meaning in indented strings, you
need a way to quote them. `$` can be escaped by prefixing it with
`''` (that is, two single quotes), i.e., `''$`. `''` can be escaped
by prefixing it with `'`, i.e., `'''`. `$` removes any special
meaning from the following `$`. Linefeed, carriage-return and tab
by prefixing it with `'`, i.e., `'''`. Linefeed, carriage-return and tab
characters can be written as `''\n`, `''\r`, `''\t`, and `''\`
escapes any other character.
Indented strings support [string interpolation] using `${ }` the same way regular strings do.
`$${` is interpreted literally in indented strings as well, so the `$` character must be escaped if it is to be followed by an interpolation.
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