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I used the existing anchors generated by Docbook, so the anchor part should be a no-op. This could be useful depending on the infrastructure we choose to use, and it is better to be explicit than rely on Docbook's id generating algorithms. I got rid of the metadata segments of the Markdown files, because they are outdated, inaccurate, and could make people less willing to change them without speaking with the author.
107 lines
5.0 KiB
Markdown
107 lines
5.0 KiB
Markdown
# Agda {#agda}
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## How to use Agda
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Agda can be installed from `agda`:
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```
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$ nix-env -iA agda
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```
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To use Agda with libraries, the `agda.withPackages` function can be used. This function either takes:
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* A list of packages,
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* or a function which returns a list of packages when given the `agdaPackages` attribute set,
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* or an attribute set containing a list of packages and a GHC derivation for compilation (see below).
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For example, suppose we wanted a version of Agda which has access to the standard library. This can be obtained with the expressions:
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```
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agda.withPackages [ agdaPackages.standard-library ]
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```
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or
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```
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agda.withPackages (p: [ p.standard-library ])
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```
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or can be called as in the [Compiling Agda](#compiling-agda) section.
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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.
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Agda will not by default use these libraries. To tell Agda to use the library we have some options:
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* Call `agda` with the library flag:
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```
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$ agda -l standard-library -i . MyFile.agda
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```
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* Write a `my-library.agda-lib` file for the project you are working on which may look like:
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```
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name: my-library
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include: .
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depend: standard-library
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```
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* Create the file `~/.agda/defaults` and add any libraries you want to use by default.
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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).
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## Compiling Agda
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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.
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This can be overridden by a different version of `ghc` as follows:
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```
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agda.withPackages {
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pkgs = [ ... ];
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ghc = haskell.compiler.ghcHEAD;
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}
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```
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## Writing Agda packages
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To write a nix derivation for an Agda library, first check that the library has a `*.agda-lib` file.
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A derivation can then be written using `agdaPackages.mkDerivation`. This has similar arguments to `stdenv.mkDerivation` with the following additions:
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* `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).
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* `libraryName` should be the name that appears in the `*.agda-lib` file, defaulting to `pname`.
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* `libraryFile` should be the file name of the `*.agda-lib` file, defaulting to `${libraryName}.agda-lib`.
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### Building Agda packages
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The default build phase for `agdaPackages.mkDerivation` simply runs `agda` on the `Everything.agda` file.
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If something else is needed to build the package (e.g. `make`) then the `buildPhase` should be overridden.
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Additionally, a `preBuild` or `configurePhase` can be used if there are steps that need to be done prior to checking the `Everything.agda` file.
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`agda` and the Agda libraries contained in `buildInputs` are made available during the build phase.
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### Installing Agda packages
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The default install phase copies Agda source files, Agda interface files (`*.agdai`) and `*.agda-lib` files to the output directory.
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This can be overridden.
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By default, Agda sources are files ending on `.agda`, or literate Agda files ending on `.lagda`, `.lagda.tex`, `.lagda.org`, `.lagda.md`, `.lagda.rst`. The list of recognised Agda source extensions can be extended by setting the `extraExtensions` config variable.
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## Adding Agda packages to Nixpkgs
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To add an Agda package to `nixpkgs`, the derivation should be written to `pkgs/development/libraries/agda/${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:
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```
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{ mkDerivation, standard-library, fetchFromGitHub }:
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```
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and `mkDerivation` should be called instead of `agdaPackages.mkDerivation`. Here is an example skeleton derivation for iowa-stdlib:
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```
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mkDerivation {
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version = "1.5.0";
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pname = "iowa-stdlib";
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src = ...
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libraryFile = "";
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libraryName = "IAL-1.3";
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buildPhase = ''
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patchShebangs find-deps.sh
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make
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'';
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}
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```
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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`.
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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).
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