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rework notes on search paths

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remove duplication so we can link to the respective section from the
language tutorial
This commit is contained in:
Valentin Gagarin 2022-07-29 05:07:55 +02:00
parent 41faf1b5b4
commit 42e5ab74a5
2 changed files with 30 additions and 18 deletions
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38
source/anti-patterns/language.md
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@ -89,21 +89,41 @@ buildInputs = lib.attrVals ["curl" "jq"] pkgs
## `<...>` search path ## `<...>` search path
`<...>` is syntax, commonly `<nixpkgs>` is for looking up Nix expression's path You will often see Nix language code samples that refer to `<nixpkgs>`.
specified by shell environment variable `$NIX_PATH`.
Two developers on different machines are likely to have `<nixpkgs>` point to different revisions, `<...>` is special syntax that was [introduced in 2011][1ecc97b6bd] to conveniently access values from the shell environment variable [`$NIX_PATH`][NIX_PATH].
which will lead to getting different results.
It's {ref}`possible to specify exact nixpkgs commit <ref-pinning-nixpkgs>` via `$NIX_PATH`, This means, the value of a search path depends on external system state.
but that's still problematic unless: When using search paths, the same Nix expression can produce different results.
1. You specify the commit **at one place only** and reference it else where. In most cases, `$NIX_PATH` is set to the latest channel when Nix is installed, and is therefore likely to differ from machine to machine.
2. And you can control the environment via your source code,
so that a) applies by somehow setting `$NIX_PATH` via nix-shell or NixOS options :::{note}
[Channels](https://nixos.wiki/wiki/Nix_channels) are a way of distributing Nix software.
They are being phased out, but still used by default.
:::
For example, two developers on different machines are likely to have `<nixpkgs>` point to different revisions of `nixpkgs`.
Builds may work for one and fail for the other, causing confusion.
It's <ref-pinning-nixpkgs>`possible to specify an exact \`nixpkgs\` revision ` via `$NIX_PATH`.
But that is still problematic, unless:
1. you specify the commit **at one place only**,
and
2. you control the shell environment via your source code, setting `$NIX_PATH` via `nix-shell` or NixOS options.
:::{note}
We recommend to avoid using search paths and to disable channels by permanently setting `NIX_PATH=` to be empty.
:::
See {ref}`pinning-nixpkgs` for a tutorial on how to do better. See {ref}`pinning-nixpkgs` for a tutorial on how to do better.
[1ecc97b6bd]: https://github.com/NixOS/nix/commit/1ecc97b6bdb27e56d832ca48cdafd3dbb5185a04
[NIX_PATH]: https://nixos.org/manual/nix/unstable/command-ref/env-common.html?highlight=nix_path#env-NIX_PATH
## `attr1 // attr2` merge operator ## `attr1 // attr2` merge operator
It merges two attribute sets: It merges two attribute sets:

10
source/tutorials/towards-reproducibility-pinning-nixpkgs.md
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@ -13,15 +13,7 @@ In various Nix examples, you'll often see references to [\<nixpkgs>](https://git
This is a **convenient** way to quickly demonstrate a Nix expression and get it working by importing Nix packages. This is a **convenient** way to quickly demonstrate a Nix expression and get it working by importing Nix packages.
However, the resulting Nix expression **is not fully reproducible**. The `<nixpkgs>` reference However, <ref-search-path>`the resulting Nix expression is not fully reproducible`.
is set from the **local** `$NIX_PATH` environment variable. In most cases, this is set at the time Nix is installed
to the `nixpkgs-unstable` channel, and therefore it is likely to differ from machine to machine.
:::{note}
[Channels](https://nixos.wiki/wiki/Nix_channels) are a way of distributing Nix software, but they are being phased out.
Even though they are still used by default, it is recommended to avoid channels
and `<nixpkgs>` by always setting `NIX_PATH=` to be empty.
:::
## Pinning packages with URLs inside a Nix expression ## Pinning packages with URLs inside a Nix expression