Michie

Michie (pronounced /ˈmɪki/, like “Mickey”) memoizes methods either passed by method name or defined in a block. Unlike other meomization libraries, Michie encapsulates its memoization in a single module which it prepends over the original method.

Installation

Add Michie to your gemfile:

gem "michie", "~> 0.2.0"

Usage

Simply extend a class with Michie and define methods in a block passed to the memoize method:

class BillingApi
  extend Michie

  memoize do
    def fetch_aggregate_data
      # time-consuming request
    end
  end
end

Alternatively, you can pass the method name(s) to memoize after they have been defined:

class BillingApi
  extend Michie

  def fetch_aggregate_data
    # ...
  end
  memoize :fetch_aggregate_data
end

The method(s) are now memoized:

api = BillingApi.new
api.fetch_aggregate_data
#=> (calls original #fetch_aggregate_data method)

api.fetch_aggregate_data
#=> returns value memoized by Michie

Unlike other memoization libraries that use alias_method to reference the original method, leaving artifacts in the including class, memoization in Michie is encapsulated in a module.

This module is dynamically created and prepended by Michie#memoize:

BillingApi.ancestors
#=> [<#Michie::Memoizer (methods: fetch_aggregate_data)>, BillingApi, Object, Kernel, BasicObject]

memoizer = BillingApi.ancestors[0]
memoizer.instance_methods(false)
#=> [:fetch_aggregate_data]

The memoizer method uses an instance variable to memoize the value returned from the original method, and calls the original method with super:

def fetch_aggregate_data
  return @__michie_m_fetch_aggregate_data if defined?(@__michie_m_fetch_aggregate_data)

  result = super
  @__michie_m_fetch_aggregate_data = result
  result
end

By default, Michie generates an instance variable name prefix combining a "base" string __michie_ with either an m_ (normal methods), b_ (bang methods ending in !) or q_ (query methods ending in ?). This prefix is combined with the name of the method to be memoized to generate the instance variable name. The base prefix can be changed by passing a prefix option to memoize (see specs for details).

Since Michie uses the presence of an instance variable to signal memoization, false and nil values can be memoized (unlike techniques which use ||=). Michie also respects method visibility, so you can use it to memoize your private and protected methods.

Passing eager: true to memoize will eagerly call all methods defined in the memoize block as soon as an instance is created:

class BillingApi
  extend Michie

  memoize(eager: true) do
    def fetch_aggregate_data
      # ...
    end
  end
end

api = BillingApi.new
# `fetch_agregate_data` has already been memoized

Beware that Michie does not memoize methods that take arguments; this is a design decision to make the code as simple and readable as possible.

Reference

The gem is named after Donald Michie (1923-2007), the British AI researcher who invented memoization. Not to be confused with a Rust memoization library with the same name.

License

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