111 lines
5 KiB
Markdown
111 lines
5 KiB
Markdown
---
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title: Agda
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author: Alex Rice (alexarice)
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date: 2020-01-06
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---
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# 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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