depend: standard-library
  ```
* Create the file `~/.agda/defaults` and add any libraries you want to use by default.

More information can be found in the [official Agda documentation on library management](https://agda.readthedocs.io/en/v2.6.1/tools/package-system.html).

## Compiling Agda {#compiling-agda}

Agda modules can be compiled using the GHC backend with the `--compile` flag. A version of `ghc` with `ieee754` is made available to the Agda program via the `--with-compiler` flag.
This can be overridden by a different version of `ghc` as follows:

```nix
agda.withPackages {
  pkgs = [
    # ...
  ];
  ghc = haskell.compiler.ghcHEAD;
}
```

To install Agda without GHC, use `ghc = null;`.

## Writing Agda packages {#writing-agda-packages}

To write a nix derivation for an Agda library, first check that the library has a `*.agda-lib` file.

A derivation can then be written using `agdaPackages.mkDerivation`. This has similar arguments to `stdenv.mkDerivation` with the following additions:

* `everythingFile` can be used to specify the location of the `Everything.agda` file, defaulting to `./Everything.agda`. If this file does not exist then either it should be patched in or the `buildPhase` should be overridden (see below).
* `libraryName` should be the name that appears in the `*.agda-lib` file, defaulting to `pname`.
* `libraryFile` should be the file name of the `*.agda-lib` file, defaulting to `${libraryName}.agda-lib`.

Here is an example `default.nix`

```nix
{
  nixpkgs ? <nixpkgs>,
}:
with (import nixpkgs { });
agdaPackages.mkDerivation {
  version = "1.0";
  pname = "my-agda-lib";
  src = ./.;
  buildInputs = [
    agdaPackages.standard-library
  ];
}
```

### Building Agda packages {#building-agda-packages}

The default build phase for `agdaPackages.mkDerivation` runs `agda` on the `Everything.agda` file.
If something else is needed to build the package (e.g. `make`) then the `buildPhase` should be overridden.
Additionally, a `preBuild` or `configurePhase` can be used if there are steps that need to be done prior to checking the `Everything.agda` file.
`agda` and the Agda libraries contained in `buildInputs` are made available during the build phase.

### Installing Agda packages {#installing-agda-packages}

The default install phase copies Agda source files, Agda interface files (`*.agdai`) and `*.agda-lib` files to the output directory.
This can be overridden.

By default, Agda sources are files ending on `.agda`, or literate Agda files ending on `.lagda`, `.lagda.tex`, `.lagda.org`, `.lagda.md`, `.lagda.rst`. The list of recognised Agda source extensions can be extended by setting the `extraExtensions` config variable.

## Maintaining the Agda package set on Nixpkgs {#maintaining-the-agda-package-set-on-nixpkgs}

We are aiming at providing all common Agda libraries as packages on `nixpkgs`,
and keeping them up to date.
Contributions and maintenance help is always appreciated,
but the maintenance effort is typically low since the Agda ecosystem is quite small.

The `nixpkgs` Agda package set tries to take up a role similar to that of [Stackage](https://www.stackage.org/) in the Haskell world.
It is a curated set of libraries that:

1. Always work together.
2. Are as up-to-date as possible.

While the Haskell ecosystem is huge, and Stackage is highly automatised,
the Agda package set is small and can (still) be maintained by hand.

### Adding Agda packages to Nixpkgs {#adding-agda-packages-to-nixpkgs}

To add an Agda package to `nixpkgs`, the derivation should be written to `pkgs/development/libraries/agda/${library-name}/` and an entry should be added to `pkgs/top-level/agda-packages.nix`. Here it is called in a scope with access to all other Agda libraries, so the top line of the `default.nix` can look like: