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 automatically cross the ancestors list (classes in the hierarchy and included modules) to find a matching visit method (like a statically typed language does thanks to method overloading)
• it works without polluting visitable objects interface with an accept method (not possible with a statically typed language like C++ or Java); consequently it does not require to explicitly set visited objects visitable
• it allows class instance visitors (when Eavi::Visitor is included) and singleton visitors (when Eavi::Visitor is extended), all-in-one
• it comes with its own little internal Domain-Specific Language (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 define like this:

class Jsonifier
  include Eavi::Visitor

  def_visit Array do |array|
    # some code…
  end

  def_visit Hash do |hash|
    # some code…
  end

  def_visit String, Fixnum do |string|
    # some code…
  end

  # […]
end

jsonifier = Jsonifier.new
jsonifier.visit(an_object)

You can build a singleton visitor 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 2.3, i5-4670K 3.40GHz CPU):

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

Benchmarking…

• Average at depth 0: 2.6x slower
• Average at depth 3: 5.45x slower
• Average when extended: 3.94x slower
• Average when included: 4.11x slower
• Total average: 4.02x slower

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

This benchmark shows that a visit method call is on average only 4x slower than a standard method call.

License

Eavi is licensed under the MIT License.