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diff --git a/doc/languages-frameworks/agda.section.md b/doc/languages-frameworks/agda.section.md new file mode 100644 index 00000000000..775a7a1a642 --- /dev/null +++ b/doc/languages-frameworks/agda.section.md @@ -0,0 +1,256 @@ +# Agda {#agda} + +## How to use Agda {#how-to-use-agda} + +Agda is available as the [agda](https://search.nixos.org/packages?channel=unstable&show=agda&from=0&size=30&sort=relevance&query=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: + +```nix +agda.withPackages [ agdaPackages.standard-library ] +``` + +or + +```nix +agda.withPackages (p: [ p.standard-library ]) +``` + +or can be called as in the [Compiling Agda](#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 + +```nix +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 + +```nix +agda.withPackages (p: [ + (p.standard-library.overrideAttrs (oldAttrs: { + version = "1.5"; + src = fetchFromGitHub { + repo = "agda-stdlib"; + owner = "agda"; + rev = "v1.5"; + sha256 = "16fcb7ssj6kj687a042afaa2gq48rc8abihpm14k684ncihb2k4w"; + }; + })) +]) +``` + +If you want to use a library not added to Nixpkgs, you can add a +dependency to a local library by calling `agdaPackages.mkDerivation`. + +```nix +agda.withPackages (p: [ + (p.mkDerivation { + pname = "your-agda-lib"; + version = "1.0.0"; + src = /path/to/your-agda-lib; + }) +]) +``` + +Again you can reference GitHub + +```nix +agda.withPackages (p: [ + (p.mkDerivation { + pname = "your-agda-lib"; + version = "1.0.0"; + src = fetchFromGitHub { + repo = "repo"; + owner = "owner"; + version = "..."; + rev = "..."; + sha256 = "..."; + }; + }) +]) +``` + +See [Building Agda Packages](#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: + ```ShellSession + $ 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](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; +} +``` + +## 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` 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. +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: + +```nix +{ 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](#writing-agda-packages) with +`agdaPackages.mkDerivation` replaced with `mkDerivation`. + +Here is an example skeleton derivation for iowa-stdlib: + +```nix +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). + +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, +simply 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. |