Example to Article on Lua Declarative Programming

declarative.lua

local function build_message_box_imperative(gui_builder)
  local my_dialog = gui_builder:dialog()
  my_dialog:set_title("Message Box")
  
  local my_label = gui_builder:label()
  my_label:set_font_size(20)
  my_label:set_text("Hello, world!")
  my_dialog:add(my_label)
  
  local my_button = gui_builder:button()
  my_button:set_title("OK")
  my_dialog:add(my_button)
  
  return my_dialog
end

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local function declarative_method(method)
  return function(self, name)
    return function(data)
      return method(self, name, data)
    end
  end
end

--------------------------------------------------------------------------------

local gui = { }

gui.dialog = declarative_method(function(self, title, element_list)
  return function(gui_builder) 
    local my_dialog = gui_builder:dialog()
    my_dialog:set_title(title)
    
    for i = 1, #element_list do
      my_dialog:add(
          element_list[i](gui_builder)
        )
    end
    
    return my_dialog      
  end
end)

gui.label = declarative_method(function(self, text, parameters)
  return function(gui_builder) 
    local my_label = gui_builder:label()

    my_label:set_text(text)
    if parameters.font_size then
      my_label:set_font_size(parameters.font_size)
    end
    
    return my_label
  end
end)

gui.button = declarative_method(function(self, title, parameters)
  return function(gui_builder) 
    local my_button = gui_builder:button()

    my_button:set_title(title)
    -- Our button doesn't have any parameters somehow. :-)

    return my_button
  end
end)

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local build_message_box_declarative = gui:dialog "Message Box"
{
  gui:label "Hello, world!" { font_size = 20 };
  gui:button "OK" { };
}

--------------------------------------------------------------------------------

local gui_builder = { }

function gui_builder:dialog()
  local dialog = { }
  function dialog:set_title(title)
    self.title_ = title
  end
  function dialog:add(gui_element)
    self[#self + 1] = gui_element
  end
  function dialog:dump()
    local dump = 
         "dialog { "
      .. "title = " .. ("%q"):format(self.title_ or "(no_title)") .. "; "

    for i = 1, #self do
      dump = dump .. self[i]:dump()
    end
      
    dump = dump
      .. "}; "
    return dump
  end
  return dialog
end

function gui_builder:label()
  local label = { }
  function label:set_text(text)
    self.text_ = text
  end
  function label:set_font_size(font_size)
    self.font_size_ = font_size
  end
  function label:dump()
    return "label { "
      .. "font_size = " .. (self.font_size_ or 10) .. "; "
      .. "text = " .. ("%q"):format(self.text_ or "(no_text)") .. "; "
      .. "}; "
  end
  return label
end

function gui_builder:button()
  local button = { }
  function button:set_title(title)
    self.title_ = title
  end
  function button:dump()
    return "button { "
      .. "title = " .. ("%q"):format(self.title_ or "(no_title)") .. "; "
      .. "}; "
  end
  return button
end

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local EXPECTED = 
     'dialog { '
  .. 'title = "Message Box"; '
  .. 'label { font_size = 20; text = "Hello, world!"; }; '
  .. 'button { title = "OK"; }; '
  .. '}; '

print("EXPECTED")
print(EXPECTED)
print("IMPERATIVE")
print(build_message_box_imperative(gui_builder):dump())
print("DECLARATIVE")
print(build_message_box_declarative(gui_builder):dump())

assert(build_message_box_imperative(gui_builder):dump() == EXPECTED)
assert(build_message_box_declarative(gui_builder):dump() == EXPECTED)

print("OK")