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reword documentation on nix-path config option (#7772)

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* docs: unify documentation on search paths

- put all the information on search path semantics into `builtins.findFile`
- put all the information on determining the value of `builtins.nixPath` into the
  `nix-path` setting

  maybe `builtins.nixPath` is a better place for this, but those bits
  can still be moved around now that it's all next to each other.
- link to the syntax page for lookup paths from all places that are
  concerned with it
- add or clarify examples
- add a test verifying a claim from documentation
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Valentin Gagarin 2024-07-31 21:41:26 +02:00 • committed by GitHub
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148
src/libexpr/primops.cc
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@ -1843,34 +1843,6 @@ static RegisterPrimOp primop_findFile(PrimOp {
.doc = R"(
Find *lookup-path* in *search-path*.
A search path is represented list of [attribute sets](./types.md#attribute-set) with two attributes:
- `prefix` is a relative path.
- `path` denotes a file system location
The exact syntax depends on the command line interface.
Examples of search path attribute sets:
- ```
{
prefix = "nixos-config";
path = "/etc/nixos/configuration.nix";
}
```
- ```
{
prefix = "";
path = "/nix/var/nix/profiles/per-user/root/channels";
}
```
The lookup algorithm checks each entry until a match is found, returning a [path value](@docroot@/language/types.md#type-path) of the match:
- If *lookup-path* matches `prefix`, then the remainder of *lookup-path* (the "suffix") is searched for within the directory denoted by `path`.
Note that the `path` may need to be downloaded at this point to look inside.
- If the suffix is found inside that directory, then the entry is a match.
The combined absolute path of the directory (now downloaded if need be) and the suffix is returned.
[Lookup path](@docroot@/language/constructs/lookup-path.md) expressions are [desugared](https://en.wikipedia.org/wiki/Syntactic_sugar) using this and [`builtins.nixPath`](#builtins-nixPath):
```nix
@ -1882,6 +1854,119 @@ static RegisterPrimOp primop_findFile(PrimOp {
```nix
builtins.findFile builtins.nixPath "nixpkgs"
```
A search path is represented as a list of [attribute sets](./types.md#attribute-set) with two attributes:
- `prefix` is a relative path.
- `path` denotes a file system location
Examples of search path attribute sets:
- ```
{
prefix = "";
path = "/nix/var/nix/profiles/per-user/root/channels";
}
```
- ```
{
prefix = "nixos-config";
path = "/etc/nixos/configuration.nix";
}
```
- ```
{
prefix = "nixpkgs";
path = "https://github.com/NixOS/nixpkgs/tarballs/master";
}
```
- ```
{
prefix = "nixpkgs";
path = "channel:nixpkgs-unstable";
}
```
- ```
{
prefix = "flake-compat";
path = "flake:github:edolstra/flake-compat";
}
```
The lookup algorithm checks each entry until a match is found, returning a [path value](@docroot@/language/types.md#type-path) of the match:
- If a prefix of `lookup-path` matches `prefix`, then the remainder of *lookup-path* (the "suffix") is searched for within the directory denoted by `path`.
The contents of `path` may need to be downloaded at this point to look inside.
- If the suffix is found inside that directory, then the entry is a match.
The combined absolute path of the directory (now downloaded if need be) and the suffix is returned.
> **Example**
>
> A *search-path* value
>
> ```
> [
> {
> prefix = "";
> path = "/home/eelco/Dev";
> }
> {
> prefix = "nixos-config";
> path = "/etc/nixos";
> }
> ]
> ```
>
> and a *lookup-path* value `"nixos-config"` will cause Nix to try `/home/eelco/Dev/nixos-config` and `/etc/nixos` in that order and return the first path that exists.
If `path` starts with `http://` or `https://`, it is interpreted as the URL of a tarball that will be downloaded and unpacked to a temporary location.
The tarball must consist of a single top-level directory.
The URLs of the tarballs from the official `nixos.org` channels can be abbreviated as `channel:<channel-name>`.
See [documentation on `nix-channel`](@docroot@/command-ref/nix-channel.md) for details about channels.
> **Example**
>
> These two search path entries are equivalent:
>
> - ```
> {
> prefix = "nixpkgs";
> path = "channel:nixpkgs-unstable";
> }
> ```
> - ```
> {
> prefix = "nixpkgs";
> path = "https://nixos.org/channels/nixos-unstable/nixexprs.tar.xz";
> }
> ```
Search paths can also point to source trees using [flake URLs](@docroot@/command-ref/new-cli/nix3-flake.md#url-like-syntax).
> **Example**
>
> The search path entry
>
> ```
> {
> prefix = "nixpkgs";
> path = "flake:nixpkgs";
> }
> ```
> specifies that the prefix `nixpkgs` shall refer to the source tree downloaded from the `nixpkgs` entry in the flake registry.
>
> Similarly
>
> ```
> {
> prefix = "nixpkgs";
> path = "flake:github:nixos/nixpkgs/nixos-22.05";
> }
> ```
>
> makes `<nixpkgs>` refer to a particular branch of the `NixOS/nixpkgs` repository on GitHub.
)",
.fun = prim_findFile,
});
@ -4731,6 +4816,13 @@ void EvalState::createBaseEnv()
.doc = R"(
The value of the [`nix-path` configuration setting](@docroot@/command-ref/conf-file.md#conf-nix-path): a list of search path entries used to resolve [lookup paths](@docroot@/language/constructs/lookup-path.md).
> **Example**
>
> ```bash
> $ NIX_PATH= nix-instantiate --eval --expr "builtins.nixPath" -I foo=bar --no-pure-eval
> [ { path = "bar"; prefix = "foo"; } ]
> ```
Lookup path expressions are [desugared](https://en.wikipedia.org/wiki/Syntactic_sugar) using this and
[`builtins.findFile`](./builtins.html#builtins-findFile):