A Ruby interpreter for propositional logic

logical.rb

# simple propositional logic

class Token
  attr_reader :type, :value

  def initialize(type, value)
    @type = type
    @value = value
  end
end

class Lexer
  def initialize(input)
    @input = input
  end

  def tokens
    @input.split('').map do |char|
      # for each `char`, there are only 6 possible things to do
      case char
      when ' '
        next
      when '~'
        Token.new(:NOT, '~')
      when '&'
        Token.new(:AND, '&')
      when 'v'
        Token.new(:OR, 'v')
      when '>'
        Token.new(:IFSO, '>')
      when 'T'
        Token.new(:TRUE, 'T')
      when 'F'
        Token.new(:FALSE, 'F')
      end
    end.compact
  end
end

module AST
  class Atom
    attr_reader :value

    def initialize(value)
      @value = value
    end
  end

  class UnaryOperation
    attr_reader :operand

    def initialize(operand)
      @operand = operand
    end
  end

  class BinaryOperation
    attr_reader :left, :right

    def initialize(left, right)
      @left = left
      @right = right
    end
  end

  class Negation < UnaryOperation
  end

  class Conjunction < BinaryOperation
  end

  class Disjunction < BinaryOperation
  end

  class Implication < BinaryOperation
  end
end

class Parser
  def initialize(tokens)
    @tokens = tokens
    @stream = @tokens.to_enum
    @current_token = @stream.next
  end

  def parse
    expression
  end

  # expression : formula ((AND | OR | IFSO) formula)*
  def expression
    result = formula
    token = @current_token

    if token.type == :AND
      eat(:AND)
      result = AST::Conjunction.new(result, formula)
    elsif token.type == :OR
      eat(:OR)
      result = AST::Disjunction.new(result, formula)
    elsif token.type == :IFSO
      eat(:IFSO)
      result = AST::Implication.new(result, formula)
    end

    result
  end

  # formula :: (NOT)? term
  def formula
    token = @current_token

    if token.type == :NOT
      eat(:NOT)
      return AST::Negation.new(term)
    else
      return term
    end
  end

  # term :: TRUE | FALSE
  def term
    token = @current_token

    if token.type == :TRUE
      eat(:TRUE)
      return AST::Atom.new(true)
    elsif token.type == :FALSE
      eat(:FALSE)
      return AST::Atom.new(false)
    else
      raise "#{token.value} is an invalid term"
    end
  end

  def eat(token_type)
    raise 'ERROR' unless @current_token.type == token_type

    begin
      @current_token = @stream.next
    rescue StopIteration
    end
  end
end

class Interpreter
  def initialize(ast)
    @ast = ast
  end

  def interpret
    visit(@ast)
  end

  def visit(node)
    if node.is_a? AST::Atom
      visit_atom(node)
    elsif node.is_a? AST::Negation
      visit_negation(node)
    elsif node.is_a? AST::Conjunction
      visit_conjunction(node)
    elsif node.is_a? AST::Disjunction
      visit_disjunction(node)
    elsif node.is_a? AST::Implication
      visit_implication(node)
    end
  end

  def visit_atom(node)
    node.value
  end

  def visit_negation(node)
    case visit(node.operand)
    when true
      false
    when false
      true
    end
  end

  def visit_conjunction(node)
    case [visit(node.left), visit(node.right)]
    when [false, false]
      false
    when [false, true]
      false
    when [true, false]
      false
    when [true, true]
      true
    end
  end

  def visit_disjunction(node)
    case [visit(node.left), visit(node.right)]
    when [false, false]
      false
    when [false, true]
      true
    when [true, false]
      true
    when [true, true]
      true
    end
  end

  def visit_implication(node)
    case [visit(node.left), visit(node.right)]
    when [false, false]
      true
    when [false, true]
      true
    when [true, false]
      false
    when [true, true]
      true
    end
  end
end

def interpret(string)
  tokens = Lexer.new(string).tokens
  ast = Parser.new(tokens).parse
  Interpreter.new(ast).interpret
end

# the `interpret` method is what we will eventually build
def assert_interpret_equals(expression, result)
  error_msg = "Expected '#{expression}' to evaluate to #{result}"
  raise error_msg unless interpret(expression) == result
end

def run_tests
  assert_interpret_equals('T', true)
  assert_interpret_equals('F', false)

  assert_interpret_equals('~T', false)
  assert_interpret_equals('~F', true)

  assert_interpret_equals('T & T', true)
  assert_interpret_equals('T & F', false)
  assert_interpret_equals('F & T', false)
  assert_interpret_equals('F & F', false)

  assert_interpret_equals('T v T', true)
  assert_interpret_equals('T v F', true)
  assert_interpret_equals('F v T', true)
  assert_interpret_equals('F v F', false)

  assert_interpret_equals('T > T', true)
  assert_interpret_equals('T > F', false)
  assert_interpret_equals('F > T', true)
  assert_interpret_equals('F > F', true)

  assert_interpret_equals('~F & F', false)
  assert_interpret_equals('F v ~T', false)

  'SUCCESS!'
end