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feat: overlays

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Ryan Yin 2024-02-12 17:40:36 +08:00
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docs/nixpkgs/multiple-nixpkgs.md
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# The Ingenious Uses of Multiple nixpkgs Instances
In the section [Downgrade or Upgrade Packages](/nixos-with-flakes/downgrade-or-upgrade-packages.md), we have seen how to instantiate multiple distinct nixpkgs instances using the method `import nixpkgs {...}`. There are numerous applications for this technique, some common ones include:
In the section [Downgrade or Upgrade Packages](../nixos-with-flakes/downgrade-or-upgrade-packages.md), we have seen how to instantiate multiple distinct nixpkgs instances using the method `import nixpkgs {...}`. There are numerous applications for this technique, some common ones include:
1. Instantiate nixpkgs instances with different commit IDs to install various versions of software packages. This approach was used in the previous section [Downgrade or Upgrade Packages](/nixos-with-flakes/downgrade-or-upgrade-packages.md).

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docs/nixpkgs/overlays.md
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# Overlays
In the previous section, we learned about overriding derivations using the `override` function.
However, this approach will generate a new derivation and doesn't modify the original derivation in `pkgs` instance.
To globally modify derivations in the detault `pkgs` instance, Nix provides a feature called "overlays".
In the previous section, we learned about overriding derivations by `pkgs.xxx.override { ... }` or `pkgs.xxx.overrideAttrs (finalAttrs: previousAttrs: { ... });`.
However, this approach will generate a new derivation and doesn't modify the original derivation in `pkgs` instance.
If the derivation you want to override is also used by other Nix packages, they will still use the unmodified derivation.
To globally modify derivations in the detault nixpkgs instance, Nix provides a feature called "overlays".
In traditional Nix environments, overlays can be configured globally using the `~/.config/nixpkgs/overlays.nix` or `~/.config/nixpkgs/overlays/*.nix` files.
However, with Flakes feature, to ensure system reproducibility, overlays cannot rely on configurations outside of the Git repository.

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docs/zh/nixpkgs/overlays.md
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# Overlays
前面介绍的 override 函数会生成一个新的 Derivation，因此它不会修改 pkgs 实例中原有的 Derivation，只适合作为局部参数使用。
前面介绍的 `pkgs.xxx.override { ... }` 跟 `pkgs.xxx.overrideAttrs (finalAttrs: previousAttrs: { ... });` 都不会修改 pkgs 实例中原有的 Derivation，而是返回一个新的 Derivation，因此它们只适合作为局部参数使用。
但如果你需要覆写的 Derivation 还被其他 Nix 包所依赖，那其他 Nix 包使用的仍然会是未被修改的 Derivation.
为了解决这个问题，Nix 提供了 overlays 这个功能。