simple_calc.rb

# frozen_string_literal: true
class Calc
  def initialize
    @number   = 0
    @previous = nil
    @op       = nil
  end

  def to_s
    @number.to_s
  end

  (0..9).each do |n|
    define_method "press_#{n}" do
      @number = @number.to_i * 10 + n
    end
  end

  def press_clear
    @number = 0
  end

  {add: '+', sub: '-', times: '*', div: '/'}.each do |meth, op|
    define_method "press_#{meth}" do
      press_equals if @op
      @op = op
      @previous = @number
      @number = nil
    end
  end

  def press_equals
    @number = @previous.send(@op, @number.to_i) unless @previous.nil? || @op.nil? || @number.nil?
    @op = nil
  end
end

number_field = nil
number = Calc.new
Shoes.app height: 250, width: 200, resizable: false do
  background "#EEC".."#996", curve: 5, margin: 2

  stack margin: 2 do
    stack margin: 8 do
      number_field = para strong(number)
    end

    flow width: 218, margin: 4 do
      %w(7 8 9 / 4 5 6 * 1 2 3 - 0 Clr = +).each do |btn|
        button btn, width: 46, height: 46 do
          method = case btn
                   when /[0-9]/  then 'press_' + btn
                   when 'Clr'    then 'press_clear'
                   when '='      then 'press_equals'
                   when '+'      then 'press_add'
                   when '-'      then 'press_sub'
                   when '*'      then 'press_times'
                   when '/'      then 'press_div'
                   end

          number.send(method)
          number_field.replace strong(number)
        end
      end
    end
  end
end