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+---
+title: Agda
+author: Alex Rice (alexarice)
+date: 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](#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](https://agda.readthedocs.io/en/v2.6.1/tools/package-system.html).
+
+## 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](https://github.com/agda/agda/issues/4613).