require 'babl'
require 'date'

Author = Struct.new(:name, :birthyear)
Article = Struct.new(:author, :title, :body, :date, :comments)
Comment = Struct.new(:author, :date, :body)

# Let's define some data
data = [
    Article.new(
        Author.new("Fred", 1990),
        'Introducing BABL',
        'Blablabla',
        DateTime.now,
        [
            Comment.new(
                Author.new("Luke", 1991),
                DateTime.now,
                'Great gem'
            )
        ]
    )
]

# Define a template
template = Babl.source {

    # A template can be stored in a variable ("inline partial") and re-used later.
    # For instance, this one serializes an Author.
    author = object(
        name: _,
        birthyear: _
    )

    # Produce a JSON object
    object(

        # Visit each article of from collection and produce a JSON object for each elements
        articles: each.object(

            # nav(:iso8601) can also be seen as a method call.
            # The method #iso8601 will be called on the date during rendering.
            date: _.nav(:iso8601),

            # '_' is a shortcut for 'nav(:title)' and 'nav(:body)'
            title: _,
            body: _,

            # You can chain another template using call()
            author: _.call(author),

            # Visit each comment, and produce a JSON object for each of them.
            comments: _.each.object(

                # In Ruby, .() is just a syntax sugar for .call()
                author: _.(author),

                # Type assertions can be (optionally) specified.
                # - They add runtime type checks
                # - They are added to JSON-Schema
                body: _.string,
                date: _.nav(:iso8601).string
            )
        )
    )
}

# All the magic happens here: the template is transformed into a fast serializer.
compiled_template = template.compile

# Serialize some data into JSON
compiled_template.json(data)

# =>
# {
#     "articles":[
#       {
#         "date":"2017-09-07T08:42:42+02:00",
#         "title":"Introducing BABL",
#         "body":"Blablabla",
#         "author":{
#           "name":"Fred",
#           "birthyear":1990
#         },
#         "comments":[
#           {
#             "author":{
#               "name":"Luke",
#               "birthyear":1991
#             },
#             "body":"Great gem",
#             "date":"2017-09-07T08:42:42+02:00"
#           }
#         ]
#       }
#     ]
#   }

# render() is like json(), but produces a Hash instead of a JSON
compiled_template.render(data)

# Output a JSON-Schema description of the template
compiled_template.json_schema