Eavi

Eavi (for Easy Visitor) is a Ruby visitor pattern helper.

You can find here the well documented Wikipedia article about the visitor pattern.

Benefits

• it cross the ancestors list (classes and modules in the hierarchy) to find an associated visit method (like overloading in some statically typed language)
• it works without polluting visitable objects interface with an accept method; consequently all objects are visitable
• it allows visitors as class instances (when Eavi::Visitor is included) and visitors as modules (when Eavi::Visitor is extended), all-in-one gem
• it comes with an explicit API, let's call it a DSL (see code examples below)
• it raises a custom error (Eavi::NoVisitMethodError, a subtype of TypeError) if a visitor cannot handle an object

How to use

A visitor can be defined like this:

# An object to JSON serializer example
class Jsonifier
  include Eavi::Visitor

  def_visit String do |object|
    %!"#{object}"!
  end

  def_visit Integer, Float do |object|
    object.to_s
  end

  def_visit Array do |array|
    "[" + array.map { |e| visit(e) }.join(",") + "]"
  end

  def_visit Hash do |hash|
    "{" + hash.map do |key, value|
      visit(key, as: String) + ":" + visit(value)
    end.join(",") + "}"
  end
end

jsonifier = Jsonifier.new
jsonifier.visit("foo")                         #=> '"foo"'
jsonifier.visit(5)                             #=> '5'
jsonifier.visit(7.5)                           #=> '7.5'
jsonifier.visit([1, 2.5, "bar"])               #=> '[1,2.5,"bar"]'
jsonifier.visit({ a: 3, b: 4.5, c: "baz" })    #=> '{"a":3,"b":4.5,"c":"baz"}'
jsonifier.visit({ value: { a: 1, b: "boo" } }) #=> '{"value":{"a":1,"b":"boo"}}'

jsonifier.visit(/this is a cool gem/)
#=> raises "no visit method in #<Jsonifier:...> for Regexp instances (Eavi::NoVisitMethodError)"

You can build a visitor module/class too, using extend instead of include:

module Jsonifier
  extend Eavi::Visitor

  # […]
end

Jsonifier.visit(an_object)

And feel free to alias the visit method:

module Jsonifier
  extend Eavi::Visitor

  alias_visit_method :serialize

  # […]
end

Jsonifier.serialize(an_object)

Benchmark

This is a benchmark output (MRI 3.0.3p157, i7-7500U CPU @ 2.70GHz):

This benchmark compare the speed of a visit call and a standard method call.

Benchmarking…

• Average at depth 0: 22.38x slower
• Average at depth 3: 56.66x slower
• Average when extended: 38.21x slower
• Average when included: 40.83x slower
• Global average: 39.52x slower

The depth is the number of digs before matching an existing visit method using the visited object's ancestors' list.

This benchmark shows that a visit method call is on average 40x slower than a standard method call (the call only, not the body of the method).

The benchmark is located in the benchmark folder and can be run with rake run:benchmark.

License

Eavi is licensed under the MIT License.