Protocols in Elixir & Clojure

generic_size.clj

(ns generic-size)

(defprotocol Size
  (size' [data] "Calculates the size (and not the length!) of a data structure"))

(extend-protocol Size
  String
  (size' [s] (count s))
  clojure.lang.IPersistentMap
  (size' [m] (count m))
  clojure.lang.IPersistentVector
  (size' [v] (count v))
  clojure.lang.IPersistentSet
  (size' [v] (count v))
  ;; fallback implementation
  Object
  (size' [_] 0)
  nil
  (size' [_] 0))

(defrecord User [name age]
  Size
  (size' [_] 2))

generic_size.ex

defprotocol Size do
  @fallback_to_any true
  @doc "Calculates the size (and not the length!) of a data structure"
  def size(data)
end

defimpl Size, for: BitString do
  def size(string), do: byte_size(string)
end

defimpl Size, for: Map do
  def size(map), do: map_size(map)
end

defimpl Size, for: Tuple do
  def size(tuple), do: tuple_size(tuple)
end

defimpl Size, for: MapSet do
  def size(set), do: MapSet.size(set)
end

defmodule User do
  defstruct [:name, :age]
end

defimpl Size, for: User do
  def size(_), do: 2
end

defimpl Size, for: Any do
  def size(_), do: 0
end

cf. Protocols - Elixir