calculator.rb

require 'dentaku'

class ExtendedCalculator < Dentaku::Calculator
  def initialize(*args)
    super

    add_exponent
  end

  private

  def add_exponent
    self.add_function(
      name: :exp,
      type: :numeric,
      signature: [:numeric, :numeric],
      body: ->(mantissa, exponent) { mantissa ** exponent }
    )
  end
end

class Guard
  def initialize(proc = nil, &block)
    @proc = if block_given?
              block
            else
              proc
            end
  end

  def call(args = {})
    return true unless @proc

    @proc.call(*args.values)
  end
end

class Formula
  InvalidArgs = Class.new(StandardError)

  def initialize(formula, guard = Guard.new)
    @formula = formula
    @guard   = guard
  end

  def calculate(args = {})
    raise InvalidArgs unless @guard.call(args)

    calculator.evaluate(@formula, args)
  end

  protected

  def calculator
    @calculator ||= ExtendedCalculator.new
  end
end

f1 = Formula.new("x/(exp(x,2) - 4)", Guard.new {|x| x > 0 } )

begin
  puts "F2(2): #{f1.calculate(x: 2)}"
rescue => e
  puts e
end

begin
  puts "F1(-1): #{f1.calculate(x: -1)}"
rescue => e
  puts "Invalid args"
  puts e
end

f2 = Formula.new("x + y")

begin
  puts "F2(-1, 2): #{f2.calculate(x: 1, y: 2)}"
rescue => e
  puts e
end