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content/blog/crystal_nix.md
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---
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title: Building a Basic Crystal Project with Nix
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date: 2020-02-16T14:31:42-08:00
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tags: ["Crystal", "Nix"]
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---
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I really like the idea of Nix: you can have reproducible builds, written more or less
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declaratively. I also really like the programming language [Crystal](https://crystal-lang.org/),
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which is a compiled Ruby derivative. Recently, I decided to try learn NixOS as a package author,
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and decided to make a Crystal project of mine, [pegasus](https://github.com/DanilaFe/pegasus),
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my guinea pig. In this post, I will document my experience setting up Nix with Crystal.
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### Getting Started
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Pegasus is a rather simple package in terms of the build process - it has no dependencies, and
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can be built with nothing but a Crystal compiler. Thus, I didn't have to worry about
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dependencies. However, the `nixpkgs` repository does have a way to specify build dependencies
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for a Nix project: [`crystal2nix`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/compilers/crystal/crystal2nix.nix).
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`crystal2nix` is another Nix package, which consists of a single Crystal binary program of
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the same name. It translates a `shards.lock` file, generated by Crystal's `shards` package
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manager, into a `shards.nix` file, which allows Nix to properly build the dependencies
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of a Crystal package. If you have a project with a `shards.lock` file, you can use `shards2nix`
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inside a `nix-shell` as follows:
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```Bash
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nix-shell -p crystal2nix --run crystal2nix
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```
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The above command says, create an environment with the `crystal2nix` package, and run the
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program. Note that you should run this
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[inside the project's root](https://github.com/NixOS/nixpkgs/blob/21bfc57dd9eb5c7c58b6ab0bfa707cbc7cf04e98/pkgs/development/compilers/crystal/build-package.nix#L2). Also note that if you
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don't depend on other Crystal packages, you will not have a `shards.lock`, and running
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`crystal2nix` is unnecessary.
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The Crystal folder in the `nixpkgs` repository contains one more handy utility:
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`buildCrystalPackage`. This is a function exported by the `crystal` Nix package, which
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significantly simplifies the process of building a Crystal binary package. We can
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look to `crystal2nix.nix` (linked above) for a concrete example. We can observe the following
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attributes:
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* `pname` - the name of the package.
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* `version` - the
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{{< sidenote "right" "version-note" "version" >}}
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In my example code, I set the Nix package version to the commit hash. Doing this alone
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is probably not the best idea, since it will prevent version numbers from being ordered.
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However, version <code>0.1.0</code> didn't make sense either, since the project technically
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doesn't have a release yet. You should set this to an actual package version if you have
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one.
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{{< /sidenote >}} of the package, as usual.
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* `crystalBinaries.<xxx>.src` - the source Crystal file for binary `xxx`.
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Using these attributes, I concocted the following expression for pegasus and all
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of its included programs:
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```nix
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{ stdenv, crystal, fetchFromGitHub }:
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let
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version = "0489d47b191ecf8501787355b948801506e7c70f";
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src = fetchFromGitHub {
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owner = "DanilaFe";
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repo = "pegasus";
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rev = version;
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sha256 = "097m7l16byis07xlg97wn5hdsz9k6c3h1ybzd2i7xhkj24kx230s";
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};
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in
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crystal.buildCrystalPackage {
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pname = "pegasus";
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inherit version;
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inherit src;
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crystalBinaries.pegasus.src = "src/pegasus.cr";
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crystalBinaries.pegasus-dot.src = "src/tools/dot/pegasus_dot.cr";
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crystalBinaries.pegasus-sim.src = "src/tools/sim/pegasus_sim.cr";
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crystalBinaries.pegasus-c.src = "src/generators/c/pegasus_c.cr";
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crystalBinaries.pegasus-csem.src = "src/generators/csem/pegasus_csem.cr";
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crystalBinaries.pegasus-crystal.src = "src/generators/crystal/pegasus_crystal.cr";
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crystalBinaries.pegasus-crystalsem.src = "src/generators/crystalsem/pegasus_crystalsem.cr";
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}
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```
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Here, I used Nix's `fetchFromGitHub` helper function. It clones a Git repository
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from `https://github.com/<owner>/<repo>`, checks out the `rev` commit or branch,
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and makes sure that it matches the `sha256` hash. The hash check is required so
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that Nix can maintain the reproducibility of the build: if the commit is changed,
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the code to compile may not be the same, and thus, the package would be different. The
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hash helps detect such changes. To generate the hash, I used `nix-prefetch-git`,
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which tries to clone the repository and compute its hash.
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In the case that your project has a `shards.nix` file generated as above, you will also
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need to add the following line inside your `buildCrystalPackage` call:
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```
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shardsFile = ./shards.nix;
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```
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The `shards.nix` file will contain all the dependency Git repositories, and the
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`shardsFile` attribute will forward this list to `buildCrystalPackage`, which
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will handle their inclusion in the package build.
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That's pretty much it! The `buildCrystalPackage` Nix function does most of the heavy
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lifting for Crystal binary packages. Please also check out
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[this web page](https://edef.eu/~qyliss/nixlib/file/nixpkgs/doc/languages-frameworks/crystal.section.md.html):
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I found out from it that `pname` had to be used instead of `name`, and it also has some information
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regarding additional compiler options and build inputs.
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### Appendix: A Small Caveat
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I was running the `crystal2nix` (and doing all of my Nix-related work) in a NixOS virtual
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machine. However, my version of NixOS was somewhat out of date (`19.04`), and I could
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not retrieve `crystal2nix`. I had to switch channels to `nixos-19.09`, which is the current
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stable version of NixOS.
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There was one more difficulty involved in
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[switching channels](https://nixos.wiki/wiki/Nix_channels): I had to do it as root.
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It so happens that if you add a channel as non-root user, your system will still use
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the channel specified by root, and thus, you will experience the update. You can spot
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this issue in the output of `nix-env -u`; it will complain of duplicate packages.
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