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

title	author	date
Agda	Alex Rice (alexarice)	2020-01-06

Agda

How to use Agda

Agda can be installed from agda:

$ nix-env -iA agda

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).

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 library in your home directory (for instance if it is a development version) then typecheck it manually (using agda.withPackages if necessary) and then override the src attribute of the package to point to your local repository.

Agda will not by default use these libraries. To tell agda to use the 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: .
depends: 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 with the --compile flag. A version of ghc with ieee 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.

The build phase for agdaPackages.mkDerivation simply 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 (or 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. The install phase simply copies all .agda, .agdai and .agda-lib files to the output directory. Again, this can be overridden.

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 }:

and mkDerivation should be called instead of agdaPackages.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.