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flake-utils/README.md

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# flake-utils
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**STATUS: alpha**
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Pure Nix flake utility functions.
The goal of this project is to build a collection of pure Nix functions that don't
depend on nixpkgs, and that are useful in the context of writing other Nix
flakes.
## Usage
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### `defaultSystems -> [<system>]`
A list of all the systems supported by the nixpkgs project.
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Useful if you want add additional platforms:
```nix
eachSystem (defaultSystems ++ ["armv7l-linux"]) (system: { hello = 42; })
```
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### `eachSystem -> [<system>] -> (<system> -> attrs)`
A common case is to build the same structure for each system. Instead of
building the hierarchy manually or per prefix, iterate over each systems and
then re-build the hierarchy.
Eg:
```nix
eachSystem ["x86_64-linux"] (system: { hello = 42; })
# => { hello.x86_64-linux.hello = 42; }
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```
### `eachDefaultSystem -> (<system> -> attrs)`
`eachSystem` pre-populated with `defaultSystems`.
### `mkApp { drv, name ? drv.pname or drv.name, execPath ? drv.passthru.execPath or "/bin/${name}"`
A small utility that builds the structure expected by the special `apps` and `defaultApp` prefixes.
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### `flattenTree -> attrs -> attrs`
Nix flakes insists on having a flat attribute set of derivations in
various places like the `packages` and `checks` attributes.
This function traverses a tree of attributes (by respecting
recurseIntoAttrs) and only returns their derivations, with a flattened
key-space.
Eg:
```nix
flattenTree { hello = pkgs.hello; gitAndTools = pkgs.gitAndTools }
```
Returns:
```nix
{
hello = «derivation»;
gitAndTools_git = «derivation»;
gitAndTools_hub = «derivation»;
# ...
}
```
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## Example
Here is how it looks like in practice:
[$ example/flake.nix](example/flake.nix) as nix
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```nix
{
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description = "Flake utils demo";
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inputs.flake-utils.url = "github:numtide/flake-utils";
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outputs = { self, nixpkgs, flake-utils }:
flake-utils.lib.eachDefaultSystem (system:
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let pkgs = nixpkgs.legacyPackages.${system}; in
rec {
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packages.hello = pkgs.hello;
defaultPackage = packages.hello;
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apps.hello = flake-utils.lib.mkApp { drv = packages.hello; };
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defaultApp = apps.hello;
}
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);
}
```
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## Known issues
```
$ nix flake check
warning: unknown flake output 'lib'
```
nixpkgs is currently having the same issue so I assume that it will be
eventually standardized.