flake-utils/README.md
2022-01-20 18:42:55 +01:00

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# 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.