Specify is a library to create Comfortable, Explicit, Multi-Layered and Well-Documented Specifications for all your configurations, settings and options in Elixir.

Basic features:

• Configuration is converted to a struct, with fields being parsed to their appropriate types.
• Specify a stack of sources to fetch the configuration from.
• Always possible to override local configuration using plain arguments to a function call.
• Fail-fast on missing or malformed values.
• Auto-generated documentation based on your config specification.

Specify can be used both to create normalized configuration structs during runtime and compile-time using both implicit external configuration sources and explicit arguments to a function call.

Installation

You can install Specify by adding specify to your list of dependencies in mix.exs:

def deps do
  [
    {:specify, "~> 0.4.0"}
  ]
end

Documentation can be found at https://hexdocs.pm/specify.

Examples

Basic usage is as follows:

defmodule Cosette.CastleOnACloud do
  require Specify
  Specify.defconfig do
    @doc "there are no floors for me to sweep"
    field :floors_to_sweep, :integer, default: 0

    @doc "there are a hundred boys and girls"
    field :amount_boys_and_girls, :integer, default: 100

    @doc "The lady all in white holds me and sings a lullaby"
    field :lullaby, :string

    @doc "Crying is usually not allowed"
    field :crying_allowed, :boolean, default: false
  end
end

iex> Cosette.CastleOnACloud.load(explicit_values: [lullaby: "I love you very much", crying_allowed: true])
%Cosette.CastleOnACloud{
  crying_allowed: true,
  floors_to_sweep: 0,
  lullaby: "I love you very much",
  amount_boys_and_girls: 100
}

Mandatory Fields

Notice that since the :lullaby-field is mandatory, if it is not defined in any of the configuration sources, an error will be thrown:

Cosette.CastleOnACloud.load
** (Specify.MissingRequiredFieldsError) Missing required fields for `Elixir.Cosette.CastleOnACloud`: `:lullaby`.
    (specify) lib/specify.ex:179: Specify.prevent_missing_required_fields!/3
    (specify) lib/specify.ex:147: Specify.load/2

Loading from Sources

Loading from another source is easy:

iex> Application.put_env(Cosette.CastleOnACloud, :lullaby, "sleep little darling")
# or: in a Mix config.ex file
config Cosette.CastleOnACloud, lullaby: "sleep little darling"

iex> Cosette.CastleOnACloud.load(sources: [Specify.Provider.MixEnv])
%Cosette.CastleOnACloud{
  crying_allowed: false,
  floors_to_sweep: 0,
  lullaby: "sleep little darling",
  no_boys_and_girls: 100
}

Rather than passing in the sources when loading the configuration, it often makes more sense to specify them when defining the configuration:

defmodule Cosette.CastleOnACloud do
  require Specify
  Specify.defconfig sources: [Specify.Provider.MixEnv] do
    # ...
  end
end

Providers

Providers can be specified by passing them to the sources: option (while loading the configuration structure or while defining it). They can also be set globally by altering the sources: key of the Specify application environment, or per-process using the :sources subkey of the Specify key in the current process’ dictionary (Process.put_env).

Be aware that for bootstrapping reasons, it is impossible to override the :sources field globally in an external source (because Specify would not know where to find it).

Most providers have sensible default values on how they work:

• Specify.Provider.Process will look at the configured key, but will default to the configuration specification module name.
• Specify.Providers.MixEnv will look at the configured application_name and key, but will default to the whole environment of an application (Application.get_all_env) if no key was set, with application_name defaulting to the configuration specification module name.

Writing Providers

Providers implement the Specify.Provider protocol, which consists of only one function: load/2. Its first argument is the implementation’s own struct, the second argument being the configuration specification’s module name. If extra information is required about the configuration specification to write a good implementation, the Reflection function module_name.__specify__ can be used to look these up.

Roadmap

• [x] Compound parsers for collections using {collection_parser, element_parser}-syntax, with provided :list parser.
• [x] Main functionality documentation.
• [x] Parsers documentation.
• [x] Writing basic Tests
  • [x] Specify.Parsers
  • [x] Main Specify module and functionality.
• [x] Thinking on how to handle environment variable names (capitalization, prefixes).
• [x] Environment Variables (System.get_env) provider
• [x] Specify Provider Tests.
• [ ] Better/more examples
• [ ] Stable release

Possibilities for the future

• (D)ETS provider
• CLI arguments provider, which could be helpful for defining e.g. Mix tasks.
• .env files provider.
• JSON and YML files provider.
• Nested configs?
• Possibility to load without raising on parsing falure (instead returning a success/failure tuple?)
• Watching for updates and call a configurable handler function when configuration has changed.

Changelog

• 0.4.2 - Finishes provider tests; bugfix for the MixEnv provider.
• 0.4.1 - Improves documentation.
• 0.4.0 - Name change: from ‘Confy’ to ‘Specify’. This name has been chosen to be more clear about the intent of the library.
• 0.3.0 - Changed overrides: to explicit_values: and added Specify.load_explicit/3 function. (Also added tests and fixed parser bugs).
• 0.2.0 - Initially released version

Attribution

I want to thank Chris Keathley for his interesting library Vapor which helped inspire Specify.