EnvParser

If your code uses environment variables, you know that ENV will always surface these as strings. Interpreting these strings as the value you actually want to see/use takes some work, however: for numbers you need to cast with to_i or to_f ... for booleans you need to check for a specific value (ENV['SOME_VAR'] == 'true') ... maybe you want to set non-trivial defaults (something other than 0 or '')? ... maybe you only want to allow values from a limited set? ...

Things can get out of control pretty fast, especially as the number of environment variables in play grows. Tools like dotenv help to make sure you're loading the correct set of variables, but EnvParser makes the values themselves usable with a minimum of effort.

Full documentation is available here, but do read below for a crash course on availble featues!

• If your project uses Bundler:

  • Add one of the following to your application's Gemfile:

    # For on-demand usage ...
    #
    gem 'env_parser'
    
    # To automatically register ENV
    # constants per ".env_parser.yml" ...
    #
    gem 'env_parser', require: 'env_parser/autoregister'

  • And then run a:

    $ bundle install

• Or, you can keep things simple with a manual install:

  $ gem install env_parser

# Returns an ENV value parsed "as" a specific type:
#
EnvParser.parse env_key_as_a_symbol
                as: …                          # ➜ required
                if_unset: …                    # ➜ optional; default value
                from_set: …                    # ➜ optional; an Array or Range
                validated_by: ->(value) { … }  # ➜ optional; may also be given as a block

# Parse an ENV value and register it as a constant:
#
EnvParser.register env_key_as_a_symbol
                   as: …                          # ➜ required
                   within: …                      # ➜ optional; Class or Module
                   if_unset: …                    # ➜ optional; default value
                   from_set: …                    # ➜ optional; an Array or Range
                   validated_by: ->(value) { … }  # ➜ optional; may also be given as a block

# Registers all ENV variables as spec'ed in ".env_parser.yml":
#
EnvParser.autoregister  # Note this is automatically called if your
                        # Gemfile included the "env_parser" gem with
                        # the "require: 'env_parser/autoregister'" option.

# Lets you call "parse" and "register" on ENV itself:
#
EnvParser.add_env_bindings  # ENV.parse will now be a proxy for EnvParser.parse
                            # and ENV.register will now be a proxy for EnvParser.register

• Parsing ENV Values

  At its core, EnvParser is a straight-forward parser for string values (since that's all ENV ever gives you), allowing you to read a given string as a variety of types.

  # Returns ENV['TIMEOUT_MS'] as an Integer,
  # or a sensible default (0) if ENV['TIMEOUT_MS'] is unset.
  #
  timeout_ms = EnvParser.parse ENV['TIMEOUT_MS'], as: :integer

  You can check the full documentation for a list of all as types available right out of the box.

• How About Less Typing?

  EnvParser is all about simplification less typing laziness. If you pass in a symbol instead of a string, EnvParser will look to ENV and use the value from the corresponding (string) key.

  # YAY, LESS TYPING!  😃
  # These two are the same:
  #
  more_typing = EnvParser.parse ENV['TIMEOUT_MS'], as: :integer
  less_typing = EnvParser.parse :TIMEOUT_MS, as: :integer

• Registering Constants From ENV Values

  The EnvParser.register method lets you "promote" ENV variables into their own constants, already parsed into the correct type.

  ENV['API_KEY']  # => 'unbreakable p4$$w0rd'
  
  EnvParser.register :API_KEY, as: :string
  API_KEY  # => 'unbreakable p4$$w0rd'

  By default, EnvParser.register will create the requested constant within the Kernel module (making it available everywhere), but you can specify any class or module you like.

  ENV['BEST_VIDEO']  # => 'https://youtu.be/L_jWHffIx5E'
  
  EnvParser.register :BEST_VIDEO, as: :string, within: URI
  URI::BEST_VIDEO  # => 'https://youtu.be/L_jWHffIx5E'
  BEST_VIDEO  # => raises NameError

  You can also register multiple constants with a single call, which is a bit cleaner.

  EnvParser.register :USERNAME, as: :string
  EnvParser.register :PASSWORD, as: :string
  EnvParser.register :MOCK_API, as: :boolean, within: MyClassOrModule }
  
  # ... is equivalent to ... #
  
  EnvParser.register USERNAME: { as: :string                           },
                     PASSWORD: { as: :string                           },
                     MOCK_API: { as: :boolean, within: MyClassOrModule }

• Okay, But... How About Even Less Typing?

  Calling EnvParser.add_env_bindings binds proxy parse and register methods onto ENV. With these bindings in place, you can call parse or register on ENV itself, which is more legible and feels more straight-forward.

  ENV['SHORT_PI']  # => '3.1415926'
  ENV['BETTER_PI']  # => '["flaky crust", "strawberry filling"]'
  
  # Bind the proxy methods.
  #
  EnvParser.add_env_bindings
  
  ENV.parse :SHORT_PI, as: :float  # => 3.1415926
  ENV.register :BETTER_PI, as: :array  # Your constant is set!

  Note that the proxy ENV.parse method will (naturally) always interpret the value given as an ENV key (converting it to a string, if necessary), which is slightly different from the original EnvParser.parse method.

  ENV['SHORT_PI']  # => '3.1415926'
  
  EnvParser.parse 'SHORT_PI', as: :float  # => 'SHORT_PI' as a float: 0.0
  EnvParser.parse :SHORT_PI , as: :float  # => ENV['SHORT_PI'] as a float: 3.1415926
  
  # Bind the proxy methods.
  #
  EnvParser.add_env_bindings
  
  ENV.parse 'SHORT_PI', as: :float  # => ENV['SHORT_PI'] as a float: 3.1415926
  ENV.parse :SHORT_PI , as: :float  # => ENV['SHORT_PI'] as a float: 3.1415926

  Note also that the ENV.parse and ENV.register binding is done safely and without polluting the method space for other objects.

  All additional examples below will assume that ENV bindings are already in place, for brevity's sake.

• Sensible Defaults

  If the ENV variable you want is unset (nil) or blank (''), the return value is a sensible default for the given as type: 0 or 0.0 for numbers, an empty string/array/hash, etc. Sometimes you want a non-trivial default, however. The if_unset option lets you specify a default that better meets your needs.

  ENV.parse :MISSING_VAR, as: :integer  # => 0
  ENV.parse :MISSING_VAR, as: :integer, if_unset: 250  # => 250

  Note these default values are used as-is with no type conversion, so exercise caution.

  ENV.parse :MISSING_VAR, as: :integer, if_unset: 'Careful!'  # => 'Careful!' (NOT AN INTEGER)

• Selecting From A Set

  Sometimes setting the as type is a bit too open-ended. The from_set option lets you restrict the domain of allowed values.

  ENV.parse :API_TO_USE, as: :symbol, from_set: %i[internal external]
  ENV.parse :NETWORK_PORT, as: :integer, from_set: (1..65535), if_unset: 80
  
  # And if the value is not in the allowed set ...
  #
  ENV.parse :TWELVE, as: :integer, from_set: (1..5)  # => raises EnvParser::ValueNotAllowedError

• Custom Validation Of Parsed Values

  You can write your own, more complex validations by passing in a validated_by lambda or an equivalent block. The lambda/block should take one value and return true if the given value passes the custom validation.

  # Via a "validated_by" lambda ...
  #
  ENV.parse :MUST_BE_LOWERCASE, as: :string, validated_by: ->(value) { value == value.downcase }
  
  # ... or with a block!
  #
  ENV.parse(:MUST_BE_LOWERCASE, as: :string) { |value| value == value.downcase }
  ENV.parse(:CONNECTION_RETRIES, as: :integer, &:positive?)

• Defining Your Own EnvParser "as" Types

  If you use a particular validation many times or are often manipulating values in the same way after EnvParser has done its thing, you may want to register a new type altogether. Defining a new type makes your code both more maintainable (all the logic for your special type is only defined once) and more readable (your parse calls aren't littered with type-checking cruft).

  Something as repetitive as:

  a = ENV.parse :A, as: :int, if_unset: 6
  raise unless passes_all_my_checks?(a)
  
  b = ENV.parse :B, as: :int, if_unset: 6
  raise unless passes_all_my_checks?(b)

  ... is perhaps best handled by defining a new type:

  EnvParser.define_type(:my_special_type_of_number, if_unset: 6) do |value|
    value = value.to_i
    unless passes_all_my_checks?(value)
      raise(EnvParser::ValueNotConvertibleError, 'cannot parse as a "special type number"')
    end
  
    value
  end
  
  a = ENV.parse :A, as: :my_special_type_of_number
  b = ENV.parse :B, as: :my_special_type_of_number

• The autoregister Call

  Consolidating all of your EnvParser.register calls into a single place only makes sense. A single EnvParser.autoregister call take a filename to read and process as a series of constant registration requests. If no filename is given, the default ".env_parser.yml" is assumed.

  You'll normally want to call EnvParser.autoregister as early in your application as possible. For Rails applications (and other frameworks that call require 'bundler/setup'), requiring the EnvParser gem via ...

  gem 'env_parser', require: 'env_parser/autoregister'

  ... will automatically make the autoregistration call for you as soon as the gem is loaded (which should be early enough for most uses). If this is still not early enough for your needs, you can always require 'env_parser/autoregister' yourself even before bundler/setup is invoked.

• The ".env_parser.yml" File

  If you recall, multiple constants can be registered via a single EnvParser.register call:

  EnvParser.register :USERNAME, as: :string
  EnvParser.register :PASSWORD, as: :string
  EnvParser.register :MOCK_API, as: :boolean, within: MyClassOrModule }
  
  # ... is equivalent to ... #
  
  EnvParser.register USERNAME: { as: :string                           },
                     PASSWORD: { as: :string                           },
                     MOCK_API: { as: :boolean, within: MyClassOrModule }

  The autoregistraton file is intended to read as a YAML version of what you'd pass to the single-call version of EnvParser.register: a single hash with keys for each of the constants you'd like to register, with each value being the set of options to parse that constant.

  The equivalent autoregistration file for the above would be:

  USERNAME:
    as: :string
  
  PASSWORD:
    as: :string
  
  MOCK_API:
    as: :boolean
    within: MyClassOrModule

  Because no Ruby statements can be safely represented via YAML, the set of EnvParser.register options available via autoregistration is limited to as, within, if_unset, and from_set. As an additional restriction, from_set (if given) must be an array, as ranges cannot be represented in YAML.

Additional features coming in the future:

• Continue to round out the as type selection as ideas come to mind, suggestions are made, and pull requests are submitted.

Bug reports and pull requests are welcome at: https://github.com/nestor-custodio/env_parser

After checking out the repo, run bin/setup to install dependencies. Then, run bundle exec rspec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

Linting is courtesy of Rubocop (bundle exec rubocop) and documentation is built using Yard (bundle exec yard). Please ensure you have a clean bill of health from Rubocop and that any new features and/or changes to behaviour are reflected in the documentation before submitting a pull request.

EnvParser is available as open source under the terms of the MIT License.