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Diffstat (limited to 'doc/manual/src/language/builtin-constants.md')
| -rw-r--r-- | doc/manual/src/language/builtin-constants.md | 59 |
1 files changed, 41 insertions, 18 deletions
diff --git a/doc/manual/src/language/builtin-constants.md b/doc/manual/src/language/builtin-constants.md index 78d066a82..e6bc7e915 100644 --- a/doc/manual/src/language/builtin-constants.md +++ b/doc/manual/src/language/builtin-constants.md @@ -1,20 +1,43 @@ # Built-in Constants -Here are the constants built into the Nix expression evaluator: - - - `builtins`\ - The set `builtins` contains all the built-in functions and values. - You can use `builtins` to test for the availability of features in - the Nix installation, e.g., - - ```nix - if builtins ? getEnv then builtins.getEnv "PATH" else "" - ``` - - This allows a Nix expression to fall back gracefully on older Nix - installations that don’t have the desired built-in function. - - - [`builtins.currentSystem`]{#builtins-currentSystem}\ - 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"`. +These constants are built into the Nix language evaluator: + +- [`builtins`]{#builtins-builtins} (attribute set) + + Contains all the [built-in functions](./builtins.md) and values, in order to avoid polluting the global scope. + + 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 builtins ? getEnv then builtins.getEnv "PATH" else "" + ``` + +- [`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). |