Class: Porolog::Variable

Inherits:
Object show all
Defined in:
lib/porolog/variable.rb

Overview

A Porolog::Variable is used to hold instantiations during the process of satisfying a goal. It implements a variable of a goal. It allows instantiations to be made and removed as the goal is attempted to be satisfied.

Defined Under Namespace

Classes: Error, GoalError, MultipleValuesError, SelfReferentialError, UnexpectedError

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(name, goal) ⇒ Variable

Initializes the Variable and attaches it to the Goal.

Raises:



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# File 'lib/porolog/variable.rb', line 44

def initialize(name, goal)
  raise GoalError, "Not a Goal: #{goal.inspect}" unless goal.is_a?(Goal)
  @goal = goal
  name  = name.to_sym  if name.is_a?(String)
  
  case name
    when Symbol
      @name   = name
      @values = []
    when Variable
      @name   = name.name
      @values = []
    when Value
      @name   = name.value
      @values = [name]
    else
      @name   = name.to_s
      @values = [Value.new(name, goal)]
  end
  
  @instantiations = []
  @goal.variable(self)
end

Instance Attribute Details

#goal ⇒ Porolog::Goal 



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# File 'lib/porolog/variable.rb', line 26

class Variable
  
  # Error class for rescuing or detecting any Variable error.
  class Error                 < PorologError  ; end
  # Error class indicating a Variable has been instantiated to multiple different values at the same time.
  class MultipleValuesError   < Error         ; end
  # Error class indicating a Variable has been created without a Goal.
  class GoalError             < Error         ; end
  # Error class indicating a Variable has been instantiated to a value that contains itself.
  class SelfReferentialError  < Error         ; end
  # Error class indicating an unexpected scenario has occurred.
  class UnexpectedError       < Error         ; end
  
  attr_accessor :name, :goal, :instantiations, :values
  
  # Initializes the Variable and attaches it to the Goal.
  # @param name [Object] the name used to refer to the Variable.
  # @param goal [Porolog::Goal] the Goal the Variable is to be attached to.
  def initialize(name, goal)
    raise GoalError, "Not a Goal: #{goal.inspect}" unless goal.is_a?(Goal)
    @goal = goal
    name  = name.to_sym  if name.is_a?(String)
    
    case name
      when Symbol
        @name   = name
        @values = []
      when Variable
        @name   = name.name
        @values = []
      when Value
        @name   = name.value
        @values = [name]
      else
        @name   = name.to_s
        @values = [Value.new(name, goal)]
    end
    
    @instantiations = []
    @goal.variable(self)
  end
  
  # Converts a Variable back to a Symbol.
  # @return [Symbol, nil] the name of the Variable.
  def to_sym
    @name&.to_sym
  end
  
  # @return [Symbol] the type of the Variable, which should be :variable.
  def type
    :variable
  end
  
  # @return [String] pretty representation.
  def inspect
    "#{@goal.myid}.#{@name.inspect}"
  end
  
  # @param visited [Array] used to prevent infinite recursion.
  # @param depth [Integer] the current level of indentation.
  # @param index [Object, nil] the index into this Variable that is instantiated.
  # @param self_index [Object, nil] the index where this Variable is instantiated.
  # @return [String] the inspect of the Variable showing instantiations using indentation.
  def inspect_with_instantiations(visited = [], depth = 0, index = nil, self_index = nil)
    return if visited.include?(self)
    
    index_str = index      && "[#{index.inspect}]"      || ''
    prefix    = self_index && "[#{self_index.inspect}]" || ''
    name      = "#{'  ' * depth}#{prefix}#{@goal.myid}.#{@name.inspect}#{index_str}"
    
    name = "#{name} = #{@values.map(&:inspect).join(',')}#{index_str}" if @values && !@values.empty? && @instantiations.empty?
    
    others = @instantiations.map{|instantiation|
      [
        instantiation.variable1.inspect_with_instantiations(visited + [self], depth + 1, instantiation.index1, instantiation.index2),
        instantiation.variable2.inspect_with_instantiations(visited + [self], depth + 1, instantiation.index2, instantiation.index1),
      ].compact
    }.reject(&:empty?)
    
    [
      name,
      *others,
    ].join("\n")
  end
  
  # @return [Object,self] returns the current value of the Variable based on its current instantiations.
  #   If there are no concrete instantiations, it returns itself, indicating no value.
  # @param visited [Array] prevents infinite recursion.
  def value(visited = [])
    return nil if visited.include?(self)
    visited = visited + [self]
    
    # -- Collect values --
    values = [*@values]
    
    @instantiations.each do |instantiation|
      values += instantiation.values_for(self, visited)
    end
   
    values.uniq!
   
    # -- Filter trivial values --
    values = [[nil]] if values == [[UNKNOWN_TAIL], [nil]] || values == [[nil], [UNKNOWN_TAIL]]
   
    values = values.reject{|value| value == UNKNOWN_TAIL }
   
    values_values = values.map{|value| value.value(visited) }.reject{|value| value == UNKNOWN_ARRAY }
    
    # -- Condense Values --
    result = if values_values.size > 1
      # -- Potentially Multiple Values Found --
      unifications = []
      if values_values.all?{|value| value.is_a?(Array) }
        # -- All Values Are Arrays --
        values = values.reject{|value| value == UNKNOWN_ARRAY } if values.size > 2 && values.include?(UNKNOWN_ARRAY)
        values_goals = values.map{|value|
          value.respond_to?(:goal) && value.goal || value.variables.map(&:goal).first || values.variables.map(&:goal).first
        }

        if values.size > 2
          merged, unifications = Porolog::unify_many_arrays(values, values_goals, visited)
        elsif values.size == 2
          no_variables = values.map(&:variables).flatten.empty?
          if no_variables
            left_value  = values[0].value.value
            right_value = values[1].value.value
            if left_value.last == UNKNOWN_TAIL && right_value.first == UNKNOWN_TAIL
              return [*left_value[0...-1], *right_value[1..-1]]
            elsif right_value.last == UNKNOWN_TAIL && left_value.first == UNKNOWN_TAIL
              return [*right_value[0...-1], *left_value[1..-1]]
            elsif left_value != right_value
              return nil
            end
          end
          merged, unifications = Porolog::unify_arrays(*values, *values_goals, visited)
        else
          # :nocov: NOT REACHED
          merged, unifications = values.first, []
          # :nocov:
        end
        
        merged.value(visited).to_a
      else
        # -- Not All Values Are Arrays --
        values.each_cons(2){|left,right|
          unification = Porolog::unify(left, right, @goal, @goal, visited)
          if unification && unifications
            unifications += unification
          else
            unifications = nil
          end
        }
        if unifications
          values.min_by{|value|
            case value
              when Porolog::Variable, Symbol                      then  2
              when Porolog::UNKNOWN_TAIL, Porolog::UNKNOWN_ARRAY  then  9
              else                                                      0
            end
          } || self
        else
          raise MultipleValuesError, "Multiple values detected for #{inspect}: #{values.inspect}"
        end
      end
    else
      # -- One (or None) Value Found --
      values.min_by{|value|
        case value
          when Variable, Symbol             then  2
          when UNKNOWN_TAIL, UNKNOWN_ARRAY  then  9
          else                              0
        end
      } || self
    end
    
    # -- Splat Tail --
    if result.is_a?(Array) && result.size == 1 && result.first.is_a?(Tail)
      result = result.first.value
    end
    
    result
  end
  
  # Instantiates Variable to another experssion.
  # @param other [Object] the other value (or object) being instantiated.
  # @param index_into_other [] the index into the other value.
  # @param index_into_self [] the index into this Variable.
  # @return [Porolog::Instantiation,nil] the instantiation made.
  # @example
  #   # To instantiate the third element of x to the second element of y,
  #   # x = [a,b,c,d,e]
  #   #          |
  #   # y =   [p,q,r,s]
  #     x = goal.variable(:x)
  #     y = goal.variable(:y)
  #     x.instantiate(y, 1, 2)
  # @example
  #   # To instantiate the tail of x to y,
  #     x.instantiate(y, nil, :tail)
  def instantiate(other, index_into_other = nil, index_into_self = nil)
    raise SelfReferentialError, "Cannot instantiate self-referential definition for #{(self.variables & other.variables).inspect}"  unless (self.variables & other.variables).empty?
    
    # -- Check Instantiation is Unifiable --
    unless self.value.is_a?(Variable) || other.value.is_a?(Variable)
      # -- Determine Other Goal --
      other_goal = nil
      other_goal = other.goal if other.respond_to?(:goal)
      other_goal ||= self.goal
      
      # -- Check Unification --
      unless Porolog::unify(self, other, self.goal, other_goal)
        self.goal.log << "Cannot unify: #{self.inspect} and #{other.inspect}"
        return nil
      end
    end
    
    # -- Create Instantiation --
    instantiation = Instantiation.new(self, index_into_self, other, index_into_other)
    
    # -- Create Reverse Assymetric Instantiations --
    if other.value.is_a?(Array) && other.value.last.is_a?(Tail)
      array = other.value
      if array.length == 2
        if array.first.is_a?(Variable)
          Instantiation.new(array.first, nil, self, :head)
        end
        if array.last.is_a?(Tail) && array.last.value.is_a?(Variable)
          Instantiation.new(array.last.value, nil, self, :tail)
        end
      end
    end
    
    # -- Return --
    instantiation
  end
  
  # Removes this Variable by removing all of its insantiations.
  # @return [Boolean] success.
  def remove
    @instantiations.dup.each(&:remove)
    @instantiations[0..-1] = []
    true
  end
  
  # Removes instantiations from another goal.
  # @param other_goal [Porolog::Goal] the Goal of which instantiations are to be removed.
  # @return [Boolean] success.
  def uninstantiate(other_goal)
    @instantiations.delete_if do |instantiation|
      instantiation.remove  if instantiation.other_goal_to(self) == other_goal
    end
    
    @values.delete_if{|value| value.goal == other_goal }
    true
  end
  
  # Indexes the value of the Variable.
  # @param index [Object] the index into the value.
  # @return [Object, nil] the value at the index in the value of the Variable.
  def [](index)
    value = self.value
    value = value.value if value.is_a?(Value)
    
    case value
      when Array
        case index
          when Integer
            value[index]
          when Symbol
            case index
              when :head
                value[0]
              when :tail
                value[1..-1]
              else
                nil
            end
          else
            nil
        end
      else
        nil
    end
  end
  
  # @return [Array<Porolog::Variable>] the Variables in itself, which is just itself.
  def variables
    [self]
  end
  
  # Compares Variables.
  # @param other [Porolog::Variable] the other Variable.
  # @return [Boolean] whether the two Variables have the same name in the same Goal.
  def ==(other)
    other.is_a?(Variable) && @name == other.name && @goal == other.goal
  end
  
end

#instantiations ⇒ Array<Porolog::Instantiation> 



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# File 'lib/porolog/variable.rb', line 26

class Variable
  
  # Error class for rescuing or detecting any Variable error.
  class Error                 < PorologError  ; end
  # Error class indicating a Variable has been instantiated to multiple different values at the same time.
  class MultipleValuesError   < Error         ; end
  # Error class indicating a Variable has been created without a Goal.
  class GoalError             < Error         ; end
  # Error class indicating a Variable has been instantiated to a value that contains itself.
  class SelfReferentialError  < Error         ; end
  # Error class indicating an unexpected scenario has occurred.
  class UnexpectedError       < Error         ; end
  
  attr_accessor :name, :goal, :instantiations, :values
  
  # Initializes the Variable and attaches it to the Goal.
  # @param name [Object] the name used to refer to the Variable.
  # @param goal [Porolog::Goal] the Goal the Variable is to be attached to.
  def initialize(name, goal)
    raise GoalError, "Not a Goal: #{goal.inspect}" unless goal.is_a?(Goal)
    @goal = goal
    name  = name.to_sym  if name.is_a?(String)
    
    case name
      when Symbol
        @name   = name
        @values = []
      when Variable
        @name   = name.name
        @values = []
      when Value
        @name   = name.value
        @values = [name]
      else
        @name   = name.to_s
        @values = [Value.new(name, goal)]
    end
    
    @instantiations = []
    @goal.variable(self)
  end
  
  # Converts a Variable back to a Symbol.
  # @return [Symbol, nil] the name of the Variable.
  def to_sym
    @name&.to_sym
  end
  
  # @return [Symbol] the type of the Variable, which should be :variable.
  def type
    :variable
  end
  
  # @return [String] pretty representation.
  def inspect
    "#{@goal.myid}.#{@name.inspect}"
  end
  
  # @param visited [Array] used to prevent infinite recursion.
  # @param depth [Integer] the current level of indentation.
  # @param index [Object, nil] the index into this Variable that is instantiated.
  # @param self_index [Object, nil] the index where this Variable is instantiated.
  # @return [String] the inspect of the Variable showing instantiations using indentation.
  def inspect_with_instantiations(visited = [], depth = 0, index = nil, self_index = nil)
    return if visited.include?(self)
    
    index_str = index      && "[#{index.inspect}]"      || ''
    prefix    = self_index && "[#{self_index.inspect}]" || ''
    name      = "#{'  ' * depth}#{prefix}#{@goal.myid}.#{@name.inspect}#{index_str}"
    
    name = "#{name} = #{@values.map(&:inspect).join(',')}#{index_str}" if @values && !@values.empty? && @instantiations.empty?
    
    others = @instantiations.map{|instantiation|
      [
        instantiation.variable1.inspect_with_instantiations(visited + [self], depth + 1, instantiation.index1, instantiation.index2),
        instantiation.variable2.inspect_with_instantiations(visited + [self], depth + 1, instantiation.index2, instantiation.index1),
      ].compact
    }.reject(&:empty?)
    
    [
      name,
      *others,
    ].join("\n")
  end
  
  # @return [Object,self] returns the current value of the Variable based on its current instantiations.
  #   If there are no concrete instantiations, it returns itself, indicating no value.
  # @param visited [Array] prevents infinite recursion.
  def value(visited = [])
    return nil if visited.include?(self)
    visited = visited + [self]
    
    # -- Collect values --
    values = [*@values]
    
    @instantiations.each do |instantiation|
      values += instantiation.values_for(self, visited)
    end
   
    values.uniq!
   
    # -- Filter trivial values --
    values = [[nil]] if values == [[UNKNOWN_TAIL], [nil]] || values == [[nil], [UNKNOWN_TAIL]]
   
    values = values.reject{|value| value == UNKNOWN_TAIL }
   
    values_values = values.map{|value| value.value(visited) }.reject{|value| value == UNKNOWN_ARRAY }
    
    # -- Condense Values --
    result = if values_values.size > 1
      # -- Potentially Multiple Values Found --
      unifications = []
      if values_values.all?{|value| value.is_a?(Array) }
        # -- All Values Are Arrays --
        values = values.reject{|value| value == UNKNOWN_ARRAY } if values.size > 2 && values.include?(UNKNOWN_ARRAY)
        values_goals = values.map{|value|
          value.respond_to?(:goal) && value.goal || value.variables.map(&:goal).first || values.variables.map(&:goal).first
        }

        if values.size > 2
          merged, unifications = Porolog::unify_many_arrays(values, values_goals, visited)
        elsif values.size == 2
          no_variables = values.map(&:variables).flatten.empty?
          if no_variables
            left_value  = values[0].value.value
            right_value = values[1].value.value
            if left_value.last == UNKNOWN_TAIL && right_value.first == UNKNOWN_TAIL
              return [*left_value[0...-1], *right_value[1..-1]]
            elsif right_value.last == UNKNOWN_TAIL && left_value.first == UNKNOWN_TAIL
              return [*right_value[0...-1], *left_value[1..-1]]
            elsif left_value != right_value
              return nil
            end
          end
          merged, unifications = Porolog::unify_arrays(*values, *values_goals, visited)
        else
          # :nocov: NOT REACHED
          merged, unifications = values.first, []
          # :nocov:
        end
        
        merged.value(visited).to_a
      else
        # -- Not All Values Are Arrays --
        values.each_cons(2){|left,right|
          unification = Porolog::unify(left, right, @goal, @goal, visited)
          if unification && unifications
            unifications += unification
          else
            unifications = nil
          end
        }
        if unifications
          values.min_by{|value|
            case value
              when Porolog::Variable, Symbol                      then  2
              when Porolog::UNKNOWN_TAIL, Porolog::UNKNOWN_ARRAY  then  9
              else                                                      0
            end
          } || self
        else
          raise MultipleValuesError, "Multiple values detected for #{inspect}: #{values.inspect}"
        end
      end
    else
      # -- One (or None) Value Found --
      values.min_by{|value|
        case value
          when Variable, Symbol             then  2
          when UNKNOWN_TAIL, UNKNOWN_ARRAY  then  9
          else                              0
        end
      } || self
    end
    
    # -- Splat Tail --
    if result.is_a?(Array) && result.size == 1 && result.first.is_a?(Tail)
      result = result.first.value
    end
    
    result
  end
  
  # Instantiates Variable to another experssion.
  # @param other [Object] the other value (or object) being instantiated.
  # @param index_into_other [] the index into the other value.
  # @param index_into_self [] the index into this Variable.
  # @return [Porolog::Instantiation,nil] the instantiation made.
  # @example
  #   # To instantiate the third element of x to the second element of y,
  #   # x = [a,b,c,d,e]
  #   #          |
  #   # y =   [p,q,r,s]
  #     x = goal.variable(:x)
  #     y = goal.variable(:y)
  #     x.instantiate(y, 1, 2)
  # @example
  #   # To instantiate the tail of x to y,
  #     x.instantiate(y, nil, :tail)
  def instantiate(other, index_into_other = nil, index_into_self = nil)
    raise SelfReferentialError, "Cannot instantiate self-referential definition for #{(self.variables & other.variables).inspect}"  unless (self.variables & other.variables).empty?
    
    # -- Check Instantiation is Unifiable --
    unless self.value.is_a?(Variable) || other.value.is_a?(Variable)
      # -- Determine Other Goal --
      other_goal = nil
      other_goal = other.goal if other.respond_to?(:goal)
      other_goal ||= self.goal
      
      # -- Check Unification --
      unless Porolog::unify(self, other, self.goal, other_goal)
        self.goal.log << "Cannot unify: #{self.inspect} and #{other.inspect}"
        return nil
      end
    end
    
    # -- Create Instantiation --
    instantiation = Instantiation.new(self, index_into_self, other, index_into_other)
    
    # -- Create Reverse Assymetric Instantiations --
    if other.value.is_a?(Array) && other.value.last.is_a?(Tail)
      array = other.value
      if array.length == 2
        if array.first.is_a?(Variable)
          Instantiation.new(array.first, nil, self, :head)
        end
        if array.last.is_a?(Tail) && array.last.value.is_a?(Variable)
          Instantiation.new(array.last.value, nil, self, :tail)
        end
      end
    end
    
    # -- Return --
    instantiation
  end
  
  # Removes this Variable by removing all of its insantiations.
  # @return [Boolean] success.
  def remove
    @instantiations.dup.each(&:remove)
    @instantiations[0..-1] = []
    true
  end
  
  # Removes instantiations from another goal.
  # @param other_goal [Porolog::Goal] the Goal of which instantiations are to be removed.
  # @return [Boolean] success.
  def uninstantiate(other_goal)
    @instantiations.delete_if do |instantiation|
      instantiation.remove  if instantiation.other_goal_to(self) == other_goal
    end
    
    @values.delete_if{|value| value.goal == other_goal }
    true
  end
  
  # Indexes the value of the Variable.
  # @param index [Object] the index into the value.
  # @return [Object, nil] the value at the index in the value of the Variable.
  def [](index)
    value = self.value
    value = value.value if value.is_a?(Value)
    
    case value
      when Array
        case index
          when Integer
            value[index]
          when Symbol
            case index
              when :head
                value[0]
              when :tail
                value[1..-1]
              else
                nil
            end
          else
            nil
        end
      else
        nil
    end
  end
  
  # @return [Array<Porolog::Variable>] the Variables in itself, which is just itself.
  def variables
    [self]
  end
  
  # Compares Variables.
  # @param other [Porolog::Variable] the other Variable.
  # @return [Boolean] whether the two Variables have the same name in the same Goal.
  def ==(other)
    other.is_a?(Variable) && @name == other.name && @goal == other.goal
  end
  
end

#name ⇒ Symbol 



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# File 'lib/porolog/variable.rb', line 26

class Variable
  
  # Error class for rescuing or detecting any Variable error.
  class Error                 < PorologError  ; end
  # Error class indicating a Variable has been instantiated to multiple different values at the same time.
  class MultipleValuesError   < Error         ; end
  # Error class indicating a Variable has been created without a Goal.
  class GoalError             < Error         ; end
  # Error class indicating a Variable has been instantiated to a value that contains itself.
  class SelfReferentialError  < Error         ; end
  # Error class indicating an unexpected scenario has occurred.
  class UnexpectedError       < Error         ; end
  
  attr_accessor :name, :goal, :instantiations, :values
  
  # Initializes the Variable and attaches it to the Goal.
  # @param name [Object] the name used to refer to the Variable.
  # @param goal [Porolog::Goal] the Goal the Variable is to be attached to.
  def initialize(name, goal)
    raise GoalError, "Not a Goal: #{goal.inspect}" unless goal.is_a?(Goal)
    @goal = goal
    name  = name.to_sym  if name.is_a?(String)
    
    case name
      when Symbol
        @name   = name
        @values = []
      when Variable
        @name   = name.name
        @values = []
      when Value
        @name   = name.value
        @values = [name]
      else
        @name   = name.to_s
        @values = [Value.new(name, goal)]
    end
    
    @instantiations = []
    @goal.variable(self)
  end
  
  # Converts a Variable back to a Symbol.
  # @return [Symbol, nil] the name of the Variable.
  def to_sym
    @name&.to_sym
  end
  
  # @return [Symbol] the type of the Variable, which should be :variable.
  def type
    :variable
  end
  
  # @return [String] pretty representation.
  def inspect
    "#{@goal.myid}.#{@name.inspect}"
  end
  
  # @param visited [Array] used to prevent infinite recursion.
  # @param depth [Integer] the current level of indentation.
  # @param index [Object, nil] the index into this Variable that is instantiated.
  # @param self_index [Object, nil] the index where this Variable is instantiated.
  # @return [String] the inspect of the Variable showing instantiations using indentation.
  def inspect_with_instantiations(visited = [], depth = 0, index = nil, self_index = nil)
    return if visited.include?(self)
    
    index_str = index      && "[#{index.inspect}]"      || ''
    prefix    = self_index && "[#{self_index.inspect}]" || ''
    name      = "#{'  ' * depth}#{prefix}#{@goal.myid}.#{@name.inspect}#{index_str}"
    
    name = "#{name} = #{@values.map(&:inspect).join(',')}#{index_str}" if @values && !@values.empty? && @instantiations.empty?
    
    others = @instantiations.map{|instantiation|
      [
        instantiation.variable1.inspect_with_instantiations(visited + [self], depth + 1, instantiation.index1, instantiation.index2),
        instantiation.variable2.inspect_with_instantiations(visited + [self], depth + 1, instantiation.index2, instantiation.index1),
      ].compact
    }.reject(&:empty?)
    
    [
      name,
      *others,
    ].join("\n")
  end
  
  # @return [Object,self] returns the current value of the Variable based on its current instantiations.
  #   If there are no concrete instantiations, it returns itself, indicating no value.
  # @param visited [Array] prevents infinite recursion.
  def value(visited = [])
    return nil if visited.include?(self)
    visited = visited + [self]
    
    # -- Collect values --
    values = [*@values]
    
    @instantiations.each do |instantiation|
      values += instantiation.values_for(self, visited)
    end
   
    values.uniq!
   
    # -- Filter trivial values --
    values = [[nil]] if values == [[UNKNOWN_TAIL], [nil]] || values == [[nil], [UNKNOWN_TAIL]]
   
    values = values.reject{|value| value == UNKNOWN_TAIL }
   
    values_values = values.map{|value| value.value(visited) }.reject{|value| value == UNKNOWN_ARRAY }
    
    # -- Condense Values --
    result = if values_values.size > 1
      # -- Potentially Multiple Values Found --
      unifications = []
      if values_values.all?{|value| value.is_a?(Array) }
        # -- All Values Are Arrays --
        values = values.reject{|value| value == UNKNOWN_ARRAY } if values.size > 2 && values.include?(UNKNOWN_ARRAY)
        values_goals = values.map{|value|
          value.respond_to?(:goal) && value.goal || value.variables.map(&:goal).first || values.variables.map(&:goal).first
        }

        if values.size > 2
          merged, unifications = Porolog::unify_many_arrays(values, values_goals, visited)
        elsif values.size == 2
          no_variables = values.map(&:variables).flatten.empty?
          if no_variables
            left_value  = values[0].value.value
            right_value = values[1].value.value
            if left_value.last == UNKNOWN_TAIL && right_value.first == UNKNOWN_TAIL
              return [*left_value[0...-1], *right_value[1..-1]]
            elsif right_value.last == UNKNOWN_TAIL && left_value.first == UNKNOWN_TAIL
              return [*right_value[0...-1], *left_value[1..-1]]
            elsif left_value != right_value
              return nil
            end
          end
          merged, unifications = Porolog::unify_arrays(*values, *values_goals, visited)
        else
          # :nocov: NOT REACHED
          merged, unifications = values.first, []
          # :nocov:
        end
        
        merged.value(visited).to_a
      else
        # -- Not All Values Are Arrays --
        values.each_cons(2){|left,right|
          unification = Porolog::unify(left, right, @goal, @goal, visited)
          if unification && unifications
            unifications += unification
          else
            unifications = nil
          end
        }
        if unifications
          values.min_by{|value|
            case value
              when Porolog::Variable, Symbol                      then  2
              when Porolog::UNKNOWN_TAIL, Porolog::UNKNOWN_ARRAY  then  9
              else                                                      0
            end
          } || self
        else
          raise MultipleValuesError, "Multiple values detected for #{inspect}: #{values.inspect}"
        end
      end
    else
      # -- One (or None) Value Found --
      values.min_by{|value|
        case value
          when Variable, Symbol             then  2
          when UNKNOWN_TAIL, UNKNOWN_ARRAY  then  9
          else                              0
        end
      } || self
    end
    
    # -- Splat Tail --
    if result.is_a?(Array) && result.size == 1 && result.first.is_a?(Tail)
      result = result.first.value
    end
    
    result
  end
  
  # Instantiates Variable to another experssion.
  # @param other [Object] the other value (or object) being instantiated.
  # @param index_into_other [] the index into the other value.
  # @param index_into_self [] the index into this Variable.
  # @return [Porolog::Instantiation,nil] the instantiation made.
  # @example
  #   # To instantiate the third element of x to the second element of y,
  #   # x = [a,b,c,d,e]
  #   #          |
  #   # y =   [p,q,r,s]
  #     x = goal.variable(:x)
  #     y = goal.variable(:y)
  #     x.instantiate(y, 1, 2)
  # @example
  #   # To instantiate the tail of x to y,
  #     x.instantiate(y, nil, :tail)
  def instantiate(other, index_into_other = nil, index_into_self = nil)
    raise SelfReferentialError, "Cannot instantiate self-referential definition for #{(self.variables & other.variables).inspect}"  unless (self.variables & other.variables).empty?
    
    # -- Check Instantiation is Unifiable --
    unless self.value.is_a?(Variable) || other.value.is_a?(Variable)
      # -- Determine Other Goal --
      other_goal = nil
      other_goal = other.goal if other.respond_to?(:goal)
      other_goal ||= self.goal
      
      # -- Check Unification --
      unless Porolog::unify(self, other, self.goal, other_goal)
        self.goal.log << "Cannot unify: #{self.inspect} and #{other.inspect}"
        return nil
      end
    end
    
    # -- Create Instantiation --
    instantiation = Instantiation.new(self, index_into_self, other, index_into_other)
    
    # -- Create Reverse Assymetric Instantiations --
    if other.value.is_a?(Array) && other.value.last.is_a?(Tail)
      array = other.value
      if array.length == 2
        if array.first.is_a?(Variable)
          Instantiation.new(array.first, nil, self, :head)
        end
        if array.last.is_a?(Tail) && array.last.value.is_a?(Variable)
          Instantiation.new(array.last.value, nil, self, :tail)
        end
      end
    end
    
    # -- Return --
    instantiation
  end
  
  # Removes this Variable by removing all of its insantiations.
  # @return [Boolean] success.
  def remove
    @instantiations.dup.each(&:remove)
    @instantiations[0..-1] = []
    true
  end
  
  # Removes instantiations from another goal.
  # @param other_goal [Porolog::Goal] the Goal of which instantiations are to be removed.
  # @return [Boolean] success.
  def uninstantiate(other_goal)
    @instantiations.delete_if do |instantiation|
      instantiation.remove  if instantiation.other_goal_to(self) == other_goal
    end
    
    @values.delete_if{|value| value.goal == other_goal }
    true
  end
  
  # Indexes the value of the Variable.
  # @param index [Object] the index into the value.
  # @return [Object, nil] the value at the index in the value of the Variable.
  def [](index)
    value = self.value
    value = value.value if value.is_a?(Value)
    
    case value
      when Array
        case index
          when Integer
            value[index]
          when Symbol
            case index
              when :head
                value[0]
              when :tail
                value[1..-1]
              else
                nil
            end
          else
            nil
        end
      else
        nil
    end
  end
  
  # @return [Array<Porolog::Variable>] the Variables in itself, which is just itself.
  def variables
    [self]
  end
  
  # Compares Variables.
  # @param other [Porolog::Variable] the other Variable.
  # @return [Boolean] whether the two Variables have the same name in the same Goal.
  def ==(other)
    other.is_a?(Variable) && @name == other.name && @goal == other.goal
  end
  
end

#values ⇒ Array<Porolog::Value> 



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# File 'lib/porolog/variable.rb', line 26

class Variable
  
  # Error class for rescuing or detecting any Variable error.
  class Error                 < PorologError  ; end
  # Error class indicating a Variable has been instantiated to multiple different values at the same time.
  class MultipleValuesError   < Error         ; end
  # Error class indicating a Variable has been created without a Goal.
  class GoalError             < Error         ; end
  # Error class indicating a Variable has been instantiated to a value that contains itself.
  class SelfReferentialError  < Error         ; end
  # Error class indicating an unexpected scenario has occurred.
  class UnexpectedError       < Error         ; end
  
  attr_accessor :name, :goal, :instantiations, :values
  
  # Initializes the Variable and attaches it to the Goal.
  # @param name [Object] the name used to refer to the Variable.
  # @param goal [Porolog::Goal] the Goal the Variable is to be attached to.
  def initialize(name, goal)
    raise GoalError, "Not a Goal: #{goal.inspect}" unless goal.is_a?(Goal)
    @goal = goal
    name  = name.to_sym  if name.is_a?(String)
    
    case name
      when Symbol
        @name   = name
        @values = []
      when Variable
        @name   = name.name
        @values = []
      when Value
        @name   = name.value
        @values = [name]
      else
        @name   = name.to_s
        @values = [Value.new(name, goal)]
    end
    
    @instantiations = []
    @goal.variable(self)
  end
  
  # Converts a Variable back to a Symbol.
  # @return [Symbol, nil] the name of the Variable.
  def to_sym
    @name&.to_sym
  end
  
  # @return [Symbol] the type of the Variable, which should be :variable.
  def type
    :variable
  end
  
  # @return [String] pretty representation.
  def inspect
    "#{@goal.myid}.#{@name.inspect}"
  end
  
  # @param visited [Array] used to prevent infinite recursion.
  # @param depth [Integer] the current level of indentation.
  # @param index [Object, nil] the index into this Variable that is instantiated.
  # @param self_index [Object, nil] the index where this Variable is instantiated.
  # @return [String] the inspect of the Variable showing instantiations using indentation.
  def inspect_with_instantiations(visited = [], depth = 0, index = nil, self_index = nil)
    return if visited.include?(self)
    
    index_str = index      && "[#{index.inspect}]"      || ''
    prefix    = self_index && "[#{self_index.inspect}]" || ''
    name      = "#{'  ' * depth}#{prefix}#{@goal.myid}.#{@name.inspect}#{index_str}"
    
    name = "#{name} = #{@values.map(&:inspect).join(',')}#{index_str}" if @values && !@values.empty? && @instantiations.empty?
    
    others = @instantiations.map{|instantiation|
      [
        instantiation.variable1.inspect_with_instantiations(visited + [self], depth + 1, instantiation.index1, instantiation.index2),
        instantiation.variable2.inspect_with_instantiations(visited + [self], depth + 1, instantiation.index2, instantiation.index1),
      ].compact
    }.reject(&:empty?)
    
    [
      name,
      *others,
    ].join("\n")
  end
  
  # @return [Object,self] returns the current value of the Variable based on its current instantiations.
  #   If there are no concrete instantiations, it returns itself, indicating no value.
  # @param visited [Array] prevents infinite recursion.
  def value(visited = [])
    return nil if visited.include?(self)
    visited = visited + [self]
    
    # -- Collect values --
    values = [*@values]
    
    @instantiations.each do |instantiation|
      values += instantiation.values_for(self, visited)
    end
   
    values.uniq!
   
    # -- Filter trivial values --
    values = [[nil]] if values == [[UNKNOWN_TAIL], [nil]] || values == [[nil], [UNKNOWN_TAIL]]
   
    values = values.reject{|value| value == UNKNOWN_TAIL }
   
    values_values = values.map{|value| value.value(visited) }.reject{|value| value == UNKNOWN_ARRAY }
    
    # -- Condense Values --
    result = if values_values.size > 1
      # -- Potentially Multiple Values Found --
      unifications = []
      if values_values.all?{|value| value.is_a?(Array) }
        # -- All Values Are Arrays --
        values = values.reject{|value| value == UNKNOWN_ARRAY } if values.size > 2 && values.include?(UNKNOWN_ARRAY)
        values_goals = values.map{|value|
          value.respond_to?(:goal) && value.goal || value.variables.map(&:goal).first || values.variables.map(&:goal).first
        }

        if values.size > 2
          merged, unifications = Porolog::unify_many_arrays(values, values_goals, visited)
        elsif values.size == 2
          no_variables = values.map(&:variables).flatten.empty?
          if no_variables
            left_value  = values[0].value.value
            right_value = values[1].value.value
            if left_value.last == UNKNOWN_TAIL && right_value.first == UNKNOWN_TAIL
              return [*left_value[0...-1], *right_value[1..-1]]
            elsif right_value.last == UNKNOWN_TAIL && left_value.first == UNKNOWN_TAIL
              return [*right_value[0...-1], *left_value[1..-1]]
            elsif left_value != right_value
              return nil
            end
          end
          merged, unifications = Porolog::unify_arrays(*values, *values_goals, visited)
        else
          # :nocov: NOT REACHED
          merged, unifications = values.first, []
          # :nocov:
        end
        
        merged.value(visited).to_a
      else
        # -- Not All Values Are Arrays --
        values.each_cons(2){|left,right|
          unification = Porolog::unify(left, right, @goal, @goal, visited)
          if unification && unifications
            unifications += unification
          else
            unifications = nil
          end
        }
        if unifications
          values.min_by{|value|
            case value
              when Porolog::Variable, Symbol                      then  2
              when Porolog::UNKNOWN_TAIL, Porolog::UNKNOWN_ARRAY  then  9
              else                                                      0
            end
          } || self
        else
          raise MultipleValuesError, "Multiple values detected for #{inspect}: #{values.inspect}"
        end
      end
    else
      # -- One (or None) Value Found --
      values.min_by{|value|
        case value
          when Variable, Symbol             then  2
          when UNKNOWN_TAIL, UNKNOWN_ARRAY  then  9
          else                              0
        end
      } || self
    end
    
    # -- Splat Tail --
    if result.is_a?(Array) && result.size == 1 && result.first.is_a?(Tail)
      result = result.first.value
    end
    
    result
  end
  
  # Instantiates Variable to another experssion.
  # @param other [Object] the other value (or object) being instantiated.
  # @param index_into_other [] the index into the other value.
  # @param index_into_self [] the index into this Variable.
  # @return [Porolog::Instantiation,nil] the instantiation made.
  # @example
  #   # To instantiate the third element of x to the second element of y,
  #   # x = [a,b,c,d,e]
  #   #          |
  #   # y =   [p,q,r,s]
  #     x = goal.variable(:x)
  #     y = goal.variable(:y)
  #     x.instantiate(y, 1, 2)
  # @example
  #   # To instantiate the tail of x to y,
  #     x.instantiate(y, nil, :tail)
  def instantiate(other, index_into_other = nil, index_into_self = nil)
    raise SelfReferentialError, "Cannot instantiate self-referential definition for #{(self.variables & other.variables).inspect}"  unless (self.variables & other.variables).empty?
    
    # -- Check Instantiation is Unifiable --
    unless self.value.is_a?(Variable) || other.value.is_a?(Variable)
      # -- Determine Other Goal --
      other_goal = nil
      other_goal = other.goal if other.respond_to?(:goal)
      other_goal ||= self.goal
      
      # -- Check Unification --
      unless Porolog::unify(self, other, self.goal, other_goal)
        self.goal.log << "Cannot unify: #{self.inspect} and #{other.inspect}"
        return nil
      end
    end
    
    # -- Create Instantiation --
    instantiation = Instantiation.new(self, index_into_self, other, index_into_other)
    
    # -- Create Reverse Assymetric Instantiations --
    if other.value.is_a?(Array) && other.value.last.is_a?(Tail)
      array = other.value
      if array.length == 2
        if array.first.is_a?(Variable)
          Instantiation.new(array.first, nil, self, :head)
        end
        if array.last.is_a?(Tail) && array.last.value.is_a?(Variable)
          Instantiation.new(array.last.value, nil, self, :tail)
        end
      end
    end
    
    # -- Return --
    instantiation
  end
  
  # Removes this Variable by removing all of its insantiations.
  # @return [Boolean] success.
  def remove
    @instantiations.dup.each(&:remove)
    @instantiations[0..-1] = []
    true
  end
  
  # Removes instantiations from another goal.
  # @param other_goal [Porolog::Goal] the Goal of which instantiations are to be removed.
  # @return [Boolean] success.
  def uninstantiate(other_goal)
    @instantiations.delete_if do |instantiation|
      instantiation.remove  if instantiation.other_goal_to(self) == other_goal
    end
    
    @values.delete_if{|value| value.goal == other_goal }
    true
  end
  
  # Indexes the value of the Variable.
  # @param index [Object] the index into the value.
  # @return [Object, nil] the value at the index in the value of the Variable.
  def [](index)
    value = self.value
    value = value.value if value.is_a?(Value)
    
    case value
      when Array
        case index
          when Integer
            value[index]
          when Symbol
            case index
              when :head
                value[0]
              when :tail
                value[1..-1]
              else
                nil
            end
          else
            nil
        end
      else
        nil
    end
  end
  
  # @return [Array<Porolog::Variable>] the Variables in itself, which is just itself.
  def variables
    [self]
  end
  
  # Compares Variables.
  # @param other [Porolog::Variable] the other Variable.
  # @return [Boolean] whether the two Variables have the same name in the same Goal.
  def ==(other)
    other.is_a?(Variable) && @name == other.name && @goal == other.goal
  end
  
end

Instance Method Details

#==(other) ⇒ Boolean

Compares Variables.



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# File 'lib/porolog/variable.rb', line 319

def ==(other)
  other.is_a?(Variable) && @name == other.name && @goal == other.goal
end

#[](index) ⇒ Object?

Indexes the value of the Variable.



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# File 'lib/porolog/variable.rb', line 285

def [](index)
  value = self.value
  value = value.value if value.is_a?(Value)
  
  case value
    when Array
      case index
        when Integer
          value[index]
        when Symbol
          case index
            when :head
              value[0]
            when :tail
              value[1..-1]
            else
              nil
          end
        else
          nil
      end
    else
      nil
  end
end

#inspect ⇒ String 



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# File 'lib/porolog/variable.rb', line 80

def inspect
  "#{@goal.myid}.#{@name.inspect}"
end

#inspect_with_instantiations(visited = [], depth = 0, index = nil, self_index = nil) ⇒ String 



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# File 'lib/porolog/variable.rb', line 89

def inspect_with_instantiations(visited = [], depth = 0, index = nil, self_index = nil)
  return if visited.include?(self)
  
  index_str = index      && "[#{index.inspect}]"      || ''
  prefix    = self_index && "[#{self_index.inspect}]" || ''
  name      = "#{'  ' * depth}#{prefix}#{@goal.myid}.#{@name.inspect}#{index_str}"
  
  name = "#{name} = #{@values.map(&:inspect).join(',')}#{index_str}" if @values && !@values.empty? && @instantiations.empty?
  
  others = @instantiations.map{|instantiation|
    [
      instantiation.variable1.inspect_with_instantiations(visited + [self], depth + 1, instantiation.index1, instantiation.index2),
      instantiation.variable2.inspect_with_instantiations(visited + [self], depth + 1, instantiation.index2, instantiation.index1),
    ].compact
  }.reject(&:empty?)
  
  [
    name,
    *others,
  ].join("\n")
end

#instantiate(other, index_into_other = nil, index_into_self = nil) ⇒ Porolog::Instantiation?

Instantiates Variable to another experssion.

Examples:

# To instantiate the third element of x to the second element of y,
# x = [a,b,c,d,e]
#          |
# y =   [p,q,r,s]
  x = goal.variable(:x)
  y = goal.variable(:y)
  x.instantiate(y, 1, 2)

# To instantiate the tail of x to y,
  x.instantiate(y, nil, :tail)

Raises:



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# File 'lib/porolog/variable.rb', line 225

def instantiate(other, index_into_other = nil, index_into_self = nil)
  raise SelfReferentialError, "Cannot instantiate self-referential definition for #{(self.variables & other.variables).inspect}"  unless (self.variables & other.variables).empty?
  
  # -- Check Instantiation is Unifiable --
  unless self.value.is_a?(Variable) || other.value.is_a?(Variable)
    # -- Determine Other Goal --
    other_goal = nil
    other_goal = other.goal if other.respond_to?(:goal)
    other_goal ||= self.goal
    
    # -- Check Unification --
    unless Porolog::unify(self, other, self.goal, other_goal)
      self.goal.log << "Cannot unify: #{self.inspect} and #{other.inspect}"
      return nil
    end
  end
  
  # -- Create Instantiation --
  instantiation = Instantiation.new(self, index_into_self, other, index_into_other)
  
  # -- Create Reverse Assymetric Instantiations --
  if other.value.is_a?(Array) && other.value.last.is_a?(Tail)
    array = other.value
    if array.length == 2
      if array.first.is_a?(Variable)
        Instantiation.new(array.first, nil, self, :head)
      end
      if array.last.is_a?(Tail) && array.last.value.is_a?(Variable)
        Instantiation.new(array.last.value, nil, self, :tail)
      end
    end
  end
  
  # -- Return --
  instantiation
end

#remove ⇒ Boolean

Removes this Variable by removing all of its insantiations.



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# File 'lib/porolog/variable.rb', line 264

def remove
  @instantiations.dup.each(&:remove)
  @instantiations[0..-1] = []
  true
end

#to_sym ⇒ Symbol?

Converts a Variable back to a Symbol.



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# File 'lib/porolog/variable.rb', line 70

def to_sym
  @name&.to_sym
end

#type ⇒ Symbol 



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# File 'lib/porolog/variable.rb', line 75

def type
  :variable
end

#uninstantiate(other_goal) ⇒ Boolean

Removes instantiations from another goal.



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# File 'lib/porolog/variable.rb', line 273

def uninstantiate(other_goal)
  @instantiations.delete_if do |instantiation|
    instantiation.remove  if instantiation.other_goal_to(self) == other_goal
  end
  
  @values.delete_if{|value| value.goal == other_goal }
  true
end

#value(visited = []) ⇒ Object, self 



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# File 'lib/porolog/variable.rb', line 114

def value(visited = [])
  return nil if visited.include?(self)
  visited = visited + [self]
  
  # -- Collect values --
  values = [*@values]
  
  @instantiations.each do |instantiation|
    values += instantiation.values_for(self, visited)
  end
 
  values.uniq!
 
  # -- Filter trivial values --
  values = [[nil]] if values == [[UNKNOWN_TAIL], [nil]] || values == [[nil], [UNKNOWN_TAIL]]
 
  values = values.reject{|value| value == UNKNOWN_TAIL }
 
  values_values = values.map{|value| value.value(visited) }.reject{|value| value == UNKNOWN_ARRAY }
  
  # -- Condense Values --
  result = if values_values.size > 1
    # -- Potentially Multiple Values Found --
    unifications = []
    if values_values.all?{|value| value.is_a?(Array) }
      # -- All Values Are Arrays --
      values = values.reject{|value| value == UNKNOWN_ARRAY } if values.size > 2 && values.include?(UNKNOWN_ARRAY)
      values_goals = values.map{|value|
        value.respond_to?(:goal) && value.goal || value.variables.map(&:goal).first || values.variables.map(&:goal).first
      }

      if values.size > 2
        merged, unifications = Porolog::unify_many_arrays(values, values_goals, visited)
      elsif values.size == 2
        no_variables = values.map(&:variables).flatten.empty?
        if no_variables
          left_value  = values[0].value.value
          right_value = values[1].value.value
          if left_value.last == UNKNOWN_TAIL && right_value.first == UNKNOWN_TAIL
            return [*left_value[0...-1], *right_value[1..-1]]
          elsif right_value.last == UNKNOWN_TAIL && left_value.first == UNKNOWN_TAIL
            return [*right_value[0...-1], *left_value[1..-1]]
          elsif left_value != right_value
            return nil
          end
        end
        merged, unifications = Porolog::unify_arrays(*values, *values_goals, visited)
      else
        # :nocov: NOT REACHED
        merged, unifications = values.first, []
        # :nocov:
      end
      
      merged.value(visited).to_a
    else
      # -- Not All Values Are Arrays --
      values.each_cons(2){|left,right|
        unification = Porolog::unify(left, right, @goal, @goal, visited)
        if unification && unifications
          unifications += unification
        else
          unifications = nil
        end
      }
      if unifications
        values.min_by{|value|
          case value
            when Porolog::Variable, Symbol                      then  2
            when Porolog::UNKNOWN_TAIL, Porolog::UNKNOWN_ARRAY  then  9
            else                                                      0
          end
        } || self
      else
        raise MultipleValuesError, "Multiple values detected for #{inspect}: #{values.inspect}"
      end
    end
  else
    # -- One (or None) Value Found --
    values.min_by{|value|
      case value
        when Variable, Symbol             then  2
        when UNKNOWN_TAIL, UNKNOWN_ARRAY  then  9
        else                              0
      end
    } || self
  end
  
  # -- Splat Tail --
  if result.is_a?(Array) && result.size == 1 && result.first.is_a?(Tail)
    result = result.first.value
  end
  
  result
end

#variables ⇒ Array<Porolog::Variable> 



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# File 'lib/porolog/variable.rb', line 312

def variables
  [self]
end