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document context-dependent keywords

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Documents some of the weirdness of __curPos and the or keyword.
This does not fit well into any existing section for either of
them, though the use of or as a quasi-operator is mentioned in
the section on operators.

Addresses lix-project/lix#353

Change-Id: I7c906c8368843dca6944e8b22573b6d201cd9a76
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terru - 2024-05-28 15:58:13 +02:00
parent 71b32bb87c
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11
doc/manual/src/language/builtin-constants.md
View file

@ -7,3 +7,14 @@ These constants are built into the Nix language evaluator:
{{#include @generated@/language/builtin-constants.md}}
</dl>
## Things which might be mistaken for constants
<dl>
<dt><code>__curPos</code></dt>
<dd>
This is not a constant but a [context-dependent keyword](@docroot@/language/constructs.md#keywords-__curPos)
</dd>
</dl>

67
doc/manual/src/language/constructs.md
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@ -380,3 +380,70 @@ let a = 1; in let a = 2; in let a = 3; in let a = 4; in ...
Comments can be single-line, started with a `#` character, or
inline/multi-line, enclosed within `/* ... */`.
## Context-dependent keywords
<dl>
<dt id="keywords-__curPos">
<a href="#keywords-__curPos"><code>__curPos</code></a>
</dt>
<dd>
A quasi-constant which will be replaced with an attribute set describing
the location where `__curPos` was used, with attributes `file`, `line`,
and `column`. For example, `import ./file.nix` will result in
```nix
{
column = 1;
file = "/path/to/some/file.nix";
line = 1;
}
```
assuming `file.nix` contains nothing but `__curPos`.
In context without a source file (such as `nix-repl`), it will always
be replaced with `null`:
```nix-repl
nix-repl> __curPos
null
```
While it may vaguely look like a builtin, this is a very different beast
that is handled directly by the parser. It thus cannot be shadowed,
bound to a different name, and is also not available under
[`builtins`](@docroot@/language/builtin-constants.md#builtins-builtins).
```nix-repl
nix-repl> let __curPos = "no"; in __curPos
null
```
Despite this `__curPos`, much like `or`, may still be used as an identifier,
it is only treated specially when it appears as an unqualified name:
```nix-repl
nix-repl> { __curPos = 1; }.__curPos
1
```
</dd>
<dt id="keywords-or">
<a href="#keywords-or"><code>or</code></a>
</dt>
<dd>
`or` is used in [Attribute selection](@docroot@/language/operators.html#attribute-selection),
where it is a keyword.
However, it is not a keyword in some other contexts, and can be used as
a binding name in attribute sets, let-bindings, non-initial function
application position, and as a label in attribute paths.
Its use as anything other than a keyword is discouraged.
</dd>
</dl>