This is the main behaviour describing RulEx and all the available callbacks and functions needed to fully implement rules evaluation.

Optionally, using RulEx.Behaviour to generate your custom RulEx module can also implement the RulEx.Encoding behaviour.

Usage

A custom RulEx module can be defined by simply using RulEx.Behaviour

defmodule MyApp.Rules do
  use RulEx.Behaviour
end

When using the default set of rules defined by RulEx, this is not needed as a default implementing already exists in RulEx.

Extending RulEx operands

RulEx behaviour provides the ability to extend it's supported operands arbitrarily via the operand/3 callback, by default this will yield an error for all non reserved operands as defined by the type RulEx.op.

defmodule MyApp.CustomRules do
  use RulEx.Behaviour

  # Here we're matching against our custom operand name
  # afterwards we assert that the single expression
  # given to us will yield `nil`.
  #
  # A catchall operand for all non-implemented operands
  # exists and will yield back an error.
  #
  def operand("nil?", [arg], db) do
    with value <- value(arg, db),
         do: {:ok, is_nil(value)}
  end
end

Defining a default encoding mechanism

RulEx behaviour optionally allows you to provide a custom encoding mechanism to translate RulEx expression into whatever encoding you want/need.

This is achieve by either setting the application environment to define a config for under RulEx.Behaviour key with a keyword list with :default key set to the encoding module (implementing the behaviour RulEx.Encoding).

# In your `config/config.exs`
config :rul_ex, RulEx.Behaviour, default: MyApp.RulEx.Encoder

Further more, when defining the module you can pass encoder option when using this behaviour as such.

defmodule MyApp.CustomRules do
  use RulEx.Behaviour, encoder: MyApp.RulEx.Encoder
end

Do note that passing the :encoder option overrides any application configurations set in the environment.

You can disable the encoder behaviour all together by passing the option :without_encoder when using this behaviour, as such.

defmodule MyApp.CustomRules do
  use RulEx.Behaviour, :without_encoder
end

If no explicit encoding mechanism is provided, an the option :without_encoder isn't passed, then using this behaviour to build your custom RulEx module will use RulEx.Encoding.Json as an encoder to implement the RulEx.Encoding behaviour.