Using stack with Ansible

I recently started to use stack for all of my personal Haskell projects. Stack is a tool to manage the dependencies of your Haskell projects, and is an alternative to cabal-install. One of the killer features of stack is the ability to download a pre-built copy of GHC for your platform. This is awesome for continuous integration setups, and for automated deployments, because it becomes very easy to install both GHC and your dependencies using one simple tool.

I also use Ansible to manage my personal server configurations. Ansible allows you to declaratively specify what you want your server to look like, in terms of installed packages, open ports, users, etc. Installing a recent version of GHC on an Ubuntu server with Ansible used to be a somewhat painful multi-step process. However, with the addition of stack to the Haskell tool chest, that is no longer so.

In this blog post, I present the very simple role I created for installing stack on an Ubuntu server using Ansible.

The Task List

Installing stack is surprisingly simple. You just have to:

1. Add the trusted apt keys for the stack deb repo
2. Add the deb repo to your apt sources
3. Install stack using apt-get

That 3 step process is shown in the following list of Ansible tasks:

---
- name: Add stack repo keys
  apt_key:
    keyserver: keyserver.ubuntu.com
    id: 575159689BEFB442

- name: Add stack deb repo
  apt_repository: repo="deb http://download.fpcomplete.com/ubuntu/{{ ansible_distribution_release }} stable main"

- name: Install stack
  apt: name=stack
       state=present

Note the use of the ansible_distribution_release variable, which gives you the name of the Ubuntu version (for example, 14.04 has a release name of “trusty”).

Building a Project

If you want to install stack on a server, I assume you want to build projects with it! If you already have a cabal file and a stack.yaml file, then building a project with stack is a simply two-step process:

1. stack setup downloads the correct version of GHC if it is not already installed
2. stack build builds your project

The following list of tasks is what I used to use to build my personal website using hakyll (I now just upload the files directly using scp, but it is still a good example):

---
- name: Check out git head
  git: repo={{ git_repo }}
       dest={{ repo_location }}

- name: Build website
  shell: stack setup && stack build && stack exec -- jdreaver-site rebuild
  args:
    chdir: "{{ repo_location }}"
  environment:  # Prevent UTF-8 encoding errors
    LANG: "C"
    LC_CTYPE: "en_US.UTF-8"

- name: Symlink _site/ to deploy location
  file: src={{ repo_location }}/_site
        dest={{ deploy_location }}
        state=link

Conclusion

This is a very short blog post, but the reason it is short is because stack has made it so! I was Googling for ways to install a recent GHC version using Ansible, and I didn’t find a stack-based solution. Therefore, I decided to make this post. I hope it helps at least one person!