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