Elixir YAML Parsing

I’ve been writing about using Dialyzer with Elixir lately. But, I’ve started working on a new side project and had a need to parse YAML files. So, this is a post of opportunity in which I’ll share my experineces learning to use yamerl. Expect more such posts in the future as I work on this new project.

Yamerl is an Erlang module for parsing YAML files. I didn’t find any good resources on using yamerl with Elixir. So I’m writing up what I’ve learned in this post. yamerl is available on github and has a README that gives good instructions for Erlang. I’ll try to translate some of that into Elixr.

I wrote this post about a week ago and after writing it I cam across Parsing YAML in Elixir from Alexander Panek. The post goes into good detail on both yamerl and yamler. Definitely worth a read.

Installation:

The instructions say to use rebar but my Elixir project uses mix. Fortunately, mix has support for rebar based projects and it works with yamerl. I was able to simply include yamerl as a github based dependency:

defp deps do [ {:yamerl, github: "yakaz/yamerl"} ] end

and run

mix deps.get mix deps.compile

Setup

The yamerl process needs to be started. In an Elixir / Mix application this is as simple as adding yamerl to the applications section of mix.exs:

def application do [applications: [:logger, :yamerl]] end

Parsing a YAML file in Elixir

The yamerl README has the following test YAML data

# applications.yaml - application: kernel version: 2.15.3 path: /usr/local/lib/erlang/lib/kernel-2.15.3 - application: stdlib version: 1.18.3 path: /usr/local/lib/erlang/lib/stdlib-1.18.3 - application: sasl version: 2.2.1 path: /usr/local/lib/erlang/lib/sasl-2.2.1

I’ve saved this data in a file called applications.yaml so I can test load it. Then I started up iex

$ iex -S mix

In iex I loaded up the YAML file like this:

iex(1)> :yamerl_constr.file("applications.yaml") [[[{'application', 'kernel'}, {'version', '2.15.3'}, {'path', '/usr/local/lib/erlang/lib/kernel-2.15.3'}], [{'application', 'stdlib'}, {'version', '1.18.3'}, {'path', '/usr/local/lib/erlang/lib/stdlib-1.18.3'}], [{'application', 'sasl'}, {'version', '2.2.1'}, {'path', '/usr/local/lib/erlang/lib/sasl-2.2.1'}]]]

Extracting the data

First of all we need to know how the parsed data is formatted. This is a direct quote from the README:

% List of documents; again, only one document here. [ % List of mappings.  [ % Mapping, represented as a proplist: each entry has the form {Key, Value}.  [ {"application", "kernel"}, {"version", "2.15.3"}, {"path", "/usr/local/lib/erlang/lib/kernel-2.15.3"} ], [ {"application", "stdlib"}, {"version", "1.18.3"}, {"path", "/usr/local/lib/erlang/lib/stdlib-1.18.3"} ], [ {"application", "sasl"}, {"version", "2.2.1"}, {"path", "/usr/local/lib/erlang/lib/sasl-2.2.1"} ] ] ]

Ok, this is very general especially in that it provides support for multiple documents. For my needs (just like the example), I have only one document. I can extract the first document from the list like this:

iex(10)> [ document | _ ] = :yamerl_constr.file("applications.yaml") # Entire structure omitted iex(11)> document [[{'application', 'kernel'}, {'version', '2.15.3'}, {'path', '/usr/local/lib/erlang/lib/kernel-2.15.3'}], [{'application', 'stdlib'}, {'version', '1.18.3'}, {'path', '/usr/local/lib/erlang/lib/stdlib-1.18.3'}], [{'application', 'sasl'}, {'version', '2.2.1'}, {'path', '/usr/local/lib/erlang/lib/sasl-2.2.1'}]]

Now, I’ll look at the first mapping which coresponds to the kernel:

iex(12)> [ kernel | _ ] = document # Entire document omitted iex(13)> kernel [{'application', 'kernel'}, {'version', '2.15.3'}, {'path', '/usr/local/lib/erlang/lib/kernel-2.15.3'}]

Before I went back and read the README carefully I thought this was a Keyword list but it turns out that it isn’t. I was very confused why the Keyword module couldn’t look up any of the values in the data structure. After I went back and read the README more diligently I realized that the data structure is an Erlang proplist which can be accessed with the :proplists module like this:

iex(14)> :proplists.get_value('version', kernel) '2.15.3' iex(15)> :proplists.get_value('path', kernel) '/usr/local/lib/erlang/lib/kernel-2.15.3' iex(16)> :proplists.get_value('application', kernel) 'kernel'

It is worth noting that the keys and values in the proplist are stored as Erlang style strings which are given the type char_list in Elixir. To convert the values to Elixir String we can use List.to_string/1 like this:

iex(17)> :proplists.get_value('version', kernel) |> List.to_string "2.15.3" iex(18)> :proplists.get_value('path', kernel) |> List.to_string "/usr/local/lib/erlang/lib/kernel-2.15.3" iex(19)> :proplists.get_value('application', kernel) |> List.to_string "kernel"

In my case I’ll be using literals for the keys so I can just use single quotes to make char_list literals. But, if you have String variables in Elixir and want to use them as keys when getting values from the proplist then you can use String.to_char_list/1 to convert.