libyaml-based yaml loader for Erlang
C Erlang C++
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yamler, an Erlang YAML Loader

This application loads YAML files into Erlang. This implementation supports:

  • Detailed errors on yaml load failures (line, column, reason)
  • Anchors and aliases
  • Merge tags
  • The tag !atom for explicitely tagging values as atoms.
  • An implicit_atoms mode to interpret values that look atom-ish as atoms.
  • Customizable schemas via callback modules.
  • Mappings returned as native Erlang R17 maps. (passes all unit tests on experimental maps branch, edoc currently broken)

This application embeds the C yaml parser "libyaml" which is compiled as a NIF.


The yaml file...

# demo1.yaml
receipt:     Oz-Ware Purchase Invoice
date:        2007-08-06
    given:   Dorothy
    family:  Gale

    - part_no:   A4786
      descrip:   Water Bucket (Filled)
      price:     1.47
      quantity:  4

    - part_no:   E1628
      descrip:   High Heeled "Ruby" Slippers
      size:      8
      price:     100.27
      quantity:  1

bill_to:  &id001
    street: |
            123 Tornado Alley
            Suite 16
    city:   East Centerville
    state:  KS

ship_to:  *id001

specialDelivery:  >
    Follow the Yellow Brick
    Road to the Emerald City.
    Pay no attention to the
    man behind the curtain.

Load command...

3> yaml:load_file("test/yaml/demo1.yaml", [implicit_atoms]).

{ok,[#{bill_to => #{city => <<"East Centerville">>,
         state => <<"KS">>,
         street => <<"123 Tornado Alley\nSuite 16\n">>},
       customer => #{family => <<"Gale">>,given => <<"Dorothy">>},
       date => <<"2007-08-06">>,
       items => [#{descrip => <<"Water Bucket (Filled)">>,
          part_no => <<"A4786">>,
          price => 1.47,
          quantity => 4},
        #{descrip => <<"High Heeled \"Ruby\" Slippers">>,
          part_no => <<"E1628">>,
          price => 100.27,
          quantity => 1,
          size => 8}],
       receipt => <<"Oz-Ware Purchase Invoice">>,
       ship_to => #{city => <<"East Centerville">>,
         state => <<"KS">>,
         street => <<"123 Tornado Alley\nSuite 16\n">>},
       specialDelivery => <<"Follow the Yellow Brick Road to the Emerald City. Pay no attention to the ma"...>>}]}



Download the code...

$ git clone git://


Build the source and documentation and run tests with...

$ cd yamler
$ rebar compile doc eunit

The NIF module spews a few warnings. This is okay.

Play with it..

$ export ERL_LIBS=$(pwd)
$ erl
Erlang R17A (erts-5.11) [source-16b4dc1] [64-bit] [smp:3:3] [async-threads:10] [hipe] [kernel-poll:false]

Eshell V5.11  (abort with ^G)
1> yaml:load_file("test/yaml/demo1.yaml").
{ok,[#{<<"bill_to">> => #{<<"city">> => <<"East Centerville">>,
         <<"state">> => <<"KS">>, ...


Tag resolution and value construction are implemented in schema callback modules. This application includes 4 different schema callback modules, and others can be implemented if different behavior is required.

The included schemas are:

  • yaml_schema_failsafe
  • yaml_schema_json
  • yaml_schema_core
  • yaml_schema_erlang (default)

The schema is selected with the schema option, for example:

4> yaml:load_file("test/yaml/demo1.yaml", [{schema, yaml_schema_core}]).
{ok,[[{<<"specialDelivery">>, ...

failsafe and json are defined by the yaml spec. They are of limited use, however the schema modules can be instructive if you want to write your own.

The core schema (defined by the yaml spec) has most of what you need to make good use of yaml. The erlang schema extends core with merge and atom support.

To explicitely tag a value as an atom, use the !atom tag, for example:

- !atom an_atom
- !atom yet another atom

To turn on implicit atom detection for untagged values, use the erlang schema and pass in the implicit_atoms options, for example:

5> yaml:load_file("test/yaml/demo1.yaml", [implicit_atoms]). 

The following yaml fragment illustrates which values will be recognized as atoms:

- "not an atom"
- not an atom
- 'an atom'
- an_atom
- anatom
- aNA_t@OM@_
- Not_an_atom
- Notanatom
- "not_an_atom"

Custom Schemas

See the behavior documenation for yaml_schema and the 4 schemas included in this application.


If you are using erlide, I recommend the eclipse yaml editor yedit.

Erlang R17 implements a native map type which is used by yamler to represent yaml mappings. If you need to use an older Erlang see the git branch mapping_as_list.


  • This app does not emit yaml.
  • This app may stumble on enormous yaml files because the entire file, event stream, and constructed documents must fit in memory at once. Also, a scheduler may be stuck in a NIF call for a while when parsing a very large yaml file.