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Automatically document builtin constants

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This is done in roughly the same way builtin functions are documented.

Also auto-link experimental features for primops, subsuming PR #8371.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
This commit is contained in:
John Ericson 2023-05-13 13:52:45 -04:00
parent d40f0e534d
commit 22b278e011
15 changed files with 447 additions and 147 deletions
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# Built-in Constants
These constants are built into the Nix language evaluator:
<dl>

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# Built-in Constants
These constants are built into the Nix language evaluator:
- [`builtins`]{#builtins-builtins} (attribute set)
Contains all the [built-in functions](./builtins.md) and values.
Since built-in functions were added over time, [testing for attributes](./operators.md#has-attribute) in `builtins` can be used for graceful fallback on older Nix installations:
```nix
# if hasContext is not available, we assume `s` has a context
if builtins ? hasContext then builtins.hasContext s else true
```
- [`builtins.currentSystem`]{#builtins-currentSystem} (string)
The built-in value `currentSystem` evaluates to the Nix platform
identifier for the Nix installation on which the expression is being
evaluated, such as `"i686-linux"` or `"x86_64-darwin"`.
Not available in [pure evaluation mode](@docroot@/command-ref/conf-file.md#conf-pure-eval).
- [`builtins.currentTime`]{#builtins-currentTime} (integer)
Return the [Unix time](https://en.wikipedia.org/wiki/Unix_time) at first evaluation.
Repeated references to that name will re-use the initially obtained value.
Example:
```console
$ nix repl
Welcome to Nix 2.15.1 Type :? for help.
nix-repl> builtins.currentTime
1683705525
nix-repl> builtins.currentTime
1683705525
```
The [store path](@docroot@/glossary.md#gloss-store-path) of a derivation depending on `currentTime` will differ for each evaluation.
Not available in [pure evaluation mode](@docroot@/command-ref/conf-file.md#conf-pure-eval).