YAML is a human-friendly data serialization format. The specification for this language and many examples are available from the Official YAML web site. You may also want to check the YAML Wikipedia article.

yamerl is a pure Erlang application which is able to parse YAML 1.1 and YAML 1.2 documents, as well as JSON documents. It only depends on standard Erlang/OTP applications; no external dependency is required. It doesn't use native code either (neither port drivers nor NIFs).

yamerl can be used inside Elixir projects, like any other Erlang library. You can find an example later in this README.

yamerl is distributed under the terms of the 2-clause BSD license; see LICENSE.

yamerl uses Rebar 3 as its build system so it can be integrated to many common build systems.

yamerl is available as a Hex.pm package. Thus you can simply list it as a package dependency in your rebar.config:

{deps, [yamerl]}.

Erlang.mk knows about yamerl. You just need to add yamerl as a dependency in your Makefile:

DEPS = yamerl

You can use yamerl in your Elixir project. yamerl is available as a Hex.pm package. Thus you can simply list its name in your mix.exs:

def project do
  [
    deps: [{:yamerl, "~> 0.8.0"}]
  ]
end

Before using yamerl, the application must be started:

application:start(yamerl).

Now, one can use the yamerl_constr module to parse and construct a list of documents from:

• an in-memory document (string or binary);
• a regular file;
• a stream.

Because a YAML input stream may contain multiple documents, yamerl_constr always returns a list of documents, even if the input stream only contains one.

yamerl_constr:string("Hello World!").

% List of documents; here, only one document.
[
 % Document #1; contains a single scalar.
 "Hello World!"
]

Here, the returned value is a list of documents containing one document. This document has a scalar as its sole node.

Considering the following YAML 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

yamerl_constr:file("applications.yaml").

% 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"}
  ]
 ]
]

The developer is responsible for reading the stream and provide the chunks to yamerl.

% Initialize a new construction state. It takes a term describing the
% source; it may be any Erlang term.
Parser0 = yamerl_constr:new({file, "<stdin>"}),

% Read chunks and feed the parser. A new parser state is returned.
{continue, Parser1} = yamerl_constr:next_chunk(Parser0, Chunk1),
% ...
{continue, Parser2} = yamerl_constr:next_chunk(Parser1, Chunk2),

% When the stream ends, tell the parser it's the last chunk.
Documents = yamerl_constr:last_chunk(Parser2, Chunk3).

yamerl_constr comes with two built-in modes:

• It can output simple documents, eg. documents based on basic Erlang structures (strings, numbers, lists, proplists). This is the default mode.
• It can output detailed documents using records. These records carry more information such as line/column, tag URI, YAML node type, module used to construct it, etc.

If we use the following YAML document:

# system.yaml
- os: FreeBSD
  version: 9.0-RELEASE-p3

• Simple documents:

yamerl_constr:file("system.yaml").

% List of documents.
[
    % List of mappings.
    [
     % Mapping with two entries.
     [
      {"os", "FreeBSD"},
      {"version","9.0-RELEASE-p3"}
     ]
    ]
]

• Full documents:

yamerl_constr:file("system.yaml", [{detailed_constr, true}]).

% List of documents.
[
    % Document with a list as its root node.
    {yamerl_doc,
     {yamerl_seq, yamerl_node_seq, "tag:yaml.org,2002:seq", [{line, 2}, {column, 1}], [
      % Mapping #1.
      {yamerl_map, yamerl_node_map, "tag:yaml.org,2002:map", [{line, 2}, {column, 3}], [
       {
        % Mapping entry #1.
        {yamerl_str, yamerl_node_str, "tag:yaml.org,2002:str", [{line, 2}, {column, 3}], "os"},
        {yamerl_str, yamerl_node_str, "tag:yaml.org,2002:str", [{line, 2}, {column, 7}], "FreeBSD"}
       }, {
        % Mapping entry #2.
        {yamerl_str, yamerl_node_str, "tag:yaml.org,2002:str", [{line, 3}, {column, 3}], "version"},
        {yamerl_str, yamerl_node_str, "tag:yaml.org,2002:str", [{line, 3}, {column, 12}], "9.0-RELEASE-p3"}
       }
      ]}
     ],
     1}
    }
]

Here is a complete example:

1. You first need to add yamerl to the dependencies list in mix.exs:

# mix.exs, created with `mix new myapp` and updated to have `yamerl` as
# a dependency.
defmodule Myapp.Mixfile do
 use Mix.Project

 def project do
   [app: :myapp,
    version: "0.1.0",
    elixir: "~> 1.3",
    build_embedded: Mix.env == :prod,
    start_permanent: Mix.env == :prod,
    deps: deps()]
 end

 # Configuration for the OTP application
 #
 # Type "mix help compile.app" for more information
 def application do
   [applications: [:logger]]
 end

 # Dependencies can be Hex packages:
 #
 #   {:mydep, "~> 0.3.0"}
 #
 # Or git/path repositories:
 #
 #   {:mydep, git: "https://github.com/elixir-lang/mydep.git", tag: "0.1.0"}
 #
 # Type "mix help deps" for more examples and options
 defp deps do
   [
     {:yamerl, "~> 0.4.0"}
   ]
 end
end

2. Start the yamerl application and use the constructor, either in simple or detailed mode:

# lib/myapp.ex
defmodule Myapp do
 def simple(filename) do
   # The yamerl application must be started before any use of it.
   Application.start(:yamerl)

   :yamerl_constr.file(filename)
 end

 def detailed(filename) do
   # The yamerl application must be started before any use of it.
   Application.start(:yamerl)

   :yamerl_constr.file(filename, [:detailed_constr])
 end
end

Now let's use the Myapp module to parse the same YAML example file as above:

# system.yaml
- os: FreeBSD
  version: 9.0-RELEASE-p3

• Parsing in simple mode:

Myapp.simple("system.yaml")

# List of documents.
[
    # List of mappings.
    [
     # Mapping with two entries.
     [
      {'os', 'FreeBSD'},
      {'version', '9.0-RELEASE-p3'}
     ]
    ]
]

• Parsing in detailed mode:

Myapp.detailed("system.yaml")

# List of documents.
[
    # Document with a list as its root node.
    yamerl_doc:
    {:yamerl_seq, :yamerl_node_seq, 'tag:yaml.org,2002:seq', [line: 2, column: 1],
     [
      # Mapping #1.
      {:yamerl_map, :yamerl_node_map, 'tag:yaml.org,2002:map', [line: 2, column: 3],
       [
        # Mapping entry #1.
        {
         {:yamerl_str, :yamerl_node_str, 'tag:yaml.org,2002:str', [line: 2, column: 3], 'os'},
         {:yamerl_str, :yamerl_node_str, 'tag:yaml.org,2002:str', [line: 2, column: 7], 'FreeBSD'}
        },
        # Mapping entry #2.
        {
         {:yamerl_str, :yamerl_node_str, 'tag:yaml.org,2002:str', [line: 3, column: 3], 'version'},
         {:yamerl_str, :yamerl_node_str, 'tag:yaml.org,2002:str', [line: 3, column: 12], '9.0-RELEASE-p3'}
        }
       ]
      }
     ],
     1
    }
]

See https://hexdocs.pm/yamerl/ for a complete user guide and reference manual.