depot/doc/languages-frameworks/agda.section.md

Agda

How to use Agda

Agda is available as the agda package.

The agda package installs an Agda-wrapper, which calls agda with --library-file set to a generated library-file within the nix store, this means your library-file in $HOME/.agda/libraries will be ignored. By default the agda package installs Agda with no libraries, i.e. the generated library-file is empty. To use Agda with libraries, the agda.withPackages function can be used. This function either takes:

• A list of packages,
• or a function which returns a list of packages when given the agdaPackages attribute set,
• or an attribute set containing a list of packages and a GHC derivation for compilation (see below).
• or an attribute set containing a function which returns a list of packages when given the agdaPackages attribute set and a GHC derivation for compilation (see below).

For example, suppose we wanted a version of Agda which has access to the standard library. This can be obtained with the expressions:

agda.withPackages [ agdaPackages.standard-library ]

or

agda.withPackages (p: [ p.standard-library ])

or can be called as in the Compiling Agda section.

If you want to use a different version of a library (for instance a development version) override the src attribute of the package to point to your local repository

agda.withPackages (p: [
  (p.standard-library.overrideAttrs (oldAttrs: {
    version = "local version";
    src = /path/to/local/repo/agda-stdlib;
  }))
])

You can also reference a GitHub repository

agda.withPackages (p: [
  (p.standard-library.overrideAttrs (oldAttrs: {
    version = "1.5";
    src =  fetchFromGitHub {
      repo = "agda-stdlib";
      owner = "agda";
      rev = "v1.5";
      hash = "sha256-nEyxYGSWIDNJqBfGpRDLiOAnlHJKEKAOMnIaqfVZzJk=";
    };
  }))
])

If you want to use a library not added to Nixpkgs, you can add a dependency to a local library by calling agdaPackages.mkDerivation.

agda.withPackages (p: [
  (p.mkDerivation {
    pname = "your-agda-lib";
    version = "1.0.0";
    src = /path/to/your-agda-lib;
  })
])

Again you can reference GitHub

agda.withPackages (p: [
  (p.mkDerivation {
    pname = "your-agda-lib";
    version = "1.0.0";
    src = fetchFromGitHub {
      repo = "repo";
      owner = "owner";
      version = "...";
      rev = "...";
      hash = "...";
    };
  })
])

See Building Agda Packages for more information on mkDerivation.

Agda will not by default use these libraries. To tell Agda to use a library we have some options:

• Call agda with the library flag:

  $ agda -l standard-library -i . MyFile.agda
  

• Write a my-library.agda-lib file for the project you are working on which may look like:

  name: my-library
  include: .
  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.

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:

agda.withPackages {
  pkgs = [ /* ... */ ];
  ghc = haskell.compiler.ghcHEAD;
}

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

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

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

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

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 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

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:

{ mkDerivation, standard-library, fetchFromGitHub }:
{}

Note that the derivation function is called with mkDerivation set to agdaPackages.mkDerivation, therefore you could use a similar set as in your default.nix from Writing Agda Packages with agdaPackages.mkDerivation replaced with mkDerivation.

Here is an example skeleton derivation for iowa-stdlib:

mkDerivation {
  version = "1.5.0";
  pname = "iowa-stdlib";

  src = <...>;

  libraryFile = "";
  libraryName = "IAL-1.3";

  buildPhase = ''
    patchShebangs find-deps.sh
    make
  '';
}

This library has a file called .agda-lib, and so we give an empty string to libraryFile as nothing precedes .agda-lib in the filename. This file contains name: IAL-1.3, and so we let libraryName = "IAL-1.3". This library does not use an Everything.agda file and instead has a Makefile, so there is no need to set everythingFile and we set a custom buildPhase.

When writing an Agda package it is essential to make sure that no .agda-lib file gets added to the store as a single file (for example by using writeText). This causes Agda to think that the nix store is a Agda library and it will attempt to write to it whenever it typechecks something. See https://github.com/agda/agda/issues/4613.

In the pull request adding this library, you can test whether it builds correctly by writing in a comment:

@ofborg build agdaPackages.iowa-stdlib

Maintaining Agda packages

As mentioned before, the aim is to have a compatible, and up-to-date package set. These two conditions sometimes exclude each other: For example, if we update agdaPackages.standard-library because there was an upstream release, this will typically break many reverse dependencies, i.e. downstream Agda libraries that depend on the standard library. In nixpkgs we are typically among the first to notice this, since we have build tests in place to check this.

In a pull request updating e.g. the standard library, you should write the following comment:

@ofborg build agdaPackages.standard-library.passthru.tests

This will build all reverse dependencies of the standard library, for example agdaPackages.agda-categories, or agdaPackages.generic.

In some cases it is useful to build all Agda packages. This can be done with the following Github comment:

@ofborg build agda.passthru.tests.allPackages

Sometimes, the builds of the reverse dependencies fail because they have not yet been updated and released. You should drop the maintainers a quick issue notifying them of the breakage, citing the build error (which you can get from the ofborg logs). If you are motivated, you might even send a pull request that fixes it. Usually, the maintainers will answer within a week or two with a new release. Bumping the version of that reverse dependency should be a further commit on your PR.

In the rare case that a new release is not to be expected within an acceptable time, mark the broken package as broken by setting meta.broken = true;. This will exclude it from the build test. It can be added later when it is fixed, and does not hinder the advancement of the whole package set in the meantime.