Example of how you could use Ruby's WeakRef to perform weak memoization

weak_memoization_through_rubys_weak_ref.rb

require 'weakref'

# This module allows methods to be memoized, but the memoized result can still be GC'd.
# So the object can live for a long time. Eg maybe it's created on server initialization
# and lasts for the lifetime of a server. If the calculated result is large,
# then memoizing it doesn't permanently allocate that memory.
module WeakMemo
  def memoize(name)
    method = instance_method name
    ivar   = :"@_memoized_#{name}"
    define_method name do
      memoized = instance_variable_get ivar
      return memoized.__getobj__ if memoized&.weakref_alive?
      instance_variable_set ivar, WeakRef.new(method.bind_call(self))
      redo
    end
    name
  end
end

# This class just has the pure logic for the calculation
Calculate = Struct.new :arg1, :arg2 do
  extend WeakMemo

  memoize def call
    arg1 + arg2 # represents some arbitrary calculation
  end
end

calc = Calculate.new 'a', 'b'
calc.call # => "ab"

# Returns the memoized object (same object id)
calc.call.object_id # => 60
calc.call.object_id # => 60

# But if the result has been GC'd, it recalculates it (new object id)
GC.start
calc.call.object_id # => 80