© 2012 Relevance Inc

edn-ruby is a Ruby library to read and write edn (extensible data notation), a subset of Clojure used for transferring data between applications, much like JSON, YAML, or XML.


Add this line to your application's Gemfile:

gem 'edn'

And then execute:

$ bundle

Or install it yourself as:

$ gem install edn


To read a string of edn:

EDN.read('[1 2 {:foo "bar"}]')

To convert a data structure to an edn string:


By default, this will work for strings, symbols, numbers, arrays, hashes, sets, nil, Time, and boolean values.

Value Translations

edn has some different terminology, and some types that do not map cleanly to Ruby.

NOTE: Comments requested on the following.

In edn, you have keywords, which look like Ruby symbols and have the same meaning and purpose. These are converted to Ruby symbols.

You have edn symbols, which generally reflect variable names, but have several purposes. We parse these and return EDN::Type::Symbol values for them, as they are not directly portable into Ruby. To create an edn symbol in Ruby, call EDN::Type::Symbol.new or EDN.symbol with a string argument, or use the convenience unary operator ~ like so: ~"elf/rings".

You have vectors, which map to Ruby arrays, and lists, which are linked lists in Clojure. We map these to EDN::Type::List values, which are type-compatible with arrays. To create an edn list in Ruby, call EDN::Type::List.new or EDN.list with all arguments to go in the list. If you have an array, you will use the splat operator, like so: EDN.list(*[1, 2, 3]). You can also use the ~ unary operator like so: ~[1, 2, 3].

edn has character types, but Ruby does not. These are converted into one-character strings.

Tagged Values

The interesting part of edn is the extensible part. Data can be be tagged to coerce interpretation of it to a particular data type. An example of a tagged data element:

#wolf/pack {:alpha "Greybeard" :betas ["Frostpaw" "Blackwind" "Bloodjaw"]}

The tag (#wolf/pack) will tell any consumers of this data to use a data type registered to handle wolf/pack to represent this data.

The rules for tags from the edn README should be followed. In short, custom tags should have a prefix (the part before the /) designating the user that created them or context they are used in. Non-prefixed tags are reserved for built-in tags.

There are two tags built in by default: #uuid, used for UUIDs, and #inst, used for an instant in time. In edn-ruby, #inst is converted to a Time, and Time values are tagged as #inst. There is not a UUID data type built into Ruby, so #uuid is converted to an instance of EDN::Type::UUID.

Tags that are not registered generate a struct of the type EDN::Type::Unknown with the methods tag and value.

Registering a New Tag For Reading

To register a tag for reading, call the method EDN.register with a tag and one of the following:

  • A block that accepts data and returns a value.
  • A lambda that accepts data and returns a value.
  • A class that has an initialize method that accepts data.


EDN.register("clinton/uri") do |uri|

EDN.register("clinton/date", lambda { |date_array| Date.new(*date_array) }

class Dog
  def initialize(name)
    @name = name

EDN.register("clinton/dog", Dog)

Writing Tags

Writing tags should be done as part of the class's .to_edn method, like so:

class Dog
  def to_edn
    ["#clinton/dog", @name.to_edn].join(" ")

EDN provides a helper method, EDN.tagout:

class Dog
  def to_edn
    EDN.tagout("clinton/dog", @name)

This method calls .to_edn on the second argument and joins the arguments appropriately.


  • Clinton N. Dreisbach


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Added some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request