flake-utils/README.md

# flake-utils

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**STATUS: stable**

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

### `system -> (<system> -> <system>)`

A map from system to system built from `allSystems`. It's mainly useful to
detect typos and auto-complete if you use
[rnix-lsp](https://github.com/nix-community/rnix-lsp).

Eg: instead of typing `"x86_64-linux"`, use `system.x86_64-linux`

### `allSystems -> [<system>]`

A list of all systems defined in nixpkgs. For a smaller list see `defaultSystems`

### `defaultSystems -> [<system>]`

The list of systems supported by nixpkgs and built by hydra.
Useful if you want add additional platforms:

```nix
eachSystem (defaultSystems ++ [system.armv7l-linux]) (system: { hello = 42; })
```

### `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 [ system.x86_64-linux ] (system: { hello = 42; })
# => { hello = { x86_64-linux = 42; }; }
eachSystem allSystems (system: { hello = 42; })
# => {
   hello.aarch64-darwin = 42,
   hello.aarch64-genode = 42,
   hello.aarch64-linux = 42,
   ...
   hello.x86_64-redox = 42,
   hello.x86_64-solaris = 42,
   hello.x86_64-windows = 42
}
```

### `eachDefaultSystem -> (<system> -> attrs)`

`eachSystem` pre-populated with `defaultSystems`.

#### Example

[$ examples/each-system/flake.nix](examples/each-system/flake.nix) as nix
```nix
{
  description = "Flake utils demo";

  inputs.flake-utils.url = "github:numtide/flake-utils";

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let pkgs = nixpkgs.legacyPackages.${system}; in
      rec {
        packages = flake-utils.lib.flattenTree {
          hello = pkgs.hello;
          gitAndTools = pkgs.gitAndTools;
        };
        defaultPackage = packages.hello;
        apps.hello = flake-utils.lib.mkApp { drv = packages.hello; };
        defaultApp = apps.hello;
      }
    );
}
```

### `mkApp { drv, name ? drv.pname or drv.name, exePath ? drv.passthru.exePath or "/bin/${name}"`

A small utility that builds the structure expected by the special `apps` and `defaultApp` prefixes.


### `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»;
  # ...
}
```

### `simpleFlake -> attrs -> attrs`

This function should be useful for most common use-cases where you have a
simple flake that builds a package. It takes nixpkgs and a bunch of other
parameters and outputs a value that is compatible as a flake output.

Input:
```nix
{
  # pass an instance of self
  self
, # pass an instance of the nixpkgs flake
  nixpkgs
, # we assume that the name maps to the project name, and also that the
  # overlay has an attribute with the `name` prefix that contains all of the
  # project's packages.
  name
, # nixpkgs config
  config ? { }
, # pass either a function or a file
  overlay ? null
, # use this to load other flakes overlays to supplement nixpkgs
  preOverlays ? [ ]
, # maps to the devShell output. Pass in a shell.nix file or function.
  shell ? null
, # pass the list of supported systems
  systems ? [ system.x86_64-linux ]
}: null
```

#### Example

Here is how it looks like in practice:

[$ examples/simple-flake/flake.nix](examples/simple-flake/flake.nix) as nix
```nix
{
  description = "Flake utils demo";

  inputs.flake-utils.url = "github:numtide/flake-utils";

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.simpleFlake {
      inherit self nixpkgs;
      name = "simple-flake";
      overlay = ./overlay.nix;
      shell = ./shell.nix;
    };
}
```

## 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.