RE: Menus and submenus (WAS: Forward Declaration)

[RLake@...]

OK, here is a fully-worked example.

I figured that people might be using the crippled Windows NT console (at 
least, that's what I have here)
so I didn't put in any VT100 codes; this will therefore do something on 
pretty well any console.

The initial do... end block (about 150 lines including comments) 
implements a Menu "class" -- actually it creates a single menu object 
called "Menu" -- which you can then play with as you will. The example at 
the end (starting with the comment "OK, let's give it a whirl") shows most 
of the possibilities.

Adding "Back" automatically when the menu is created is really redundant: 
it could just as well be done by drawmenu. Certain modifications will be 
needed to enable this, but it would allow the possibility of multiple 
"Back" paths.

I tried to put in comments, but the code really depends on understanding 
first-order functions. And tail calls (which is how it moves from one menu 
to the next, or to the previous, for that matter.)

On the other hand, it may be a useful learning experience.

R.

------ Cut here and put it in "menu.lua" then execute "lua menu.lua" 
------

do

  local curriedMethod, method, meta = {}, {}, {}

  -- __index either executes a method from method or curries a method from
  -- curriedMethod with its self argument. This allows all calls to be 
with
  -- "." rather than ":" and also allows you to write obj.foo instead of
  -- obj.foo() for methods which don't require arguments. It might
  -- not be great design, but it is interesting. :)
 
  function meta:__index(key)
    local func = method[key]
    if func then
      return func(self)
     else
      func = curriedMethod[key]
      if func then
        local rv = function(a, b) return func(self, a, b) end
        self[key] = rv
        return rv
      end
    end
  end

  -- quick and dirty display routine.
  local DASHES = string.rep('-', 80)
  local DOUBLES = string.rep('=', 80)

  local function drawmenu(self)
    local maxsize = string.len(self.name) + 2
    local item = 0
    for i = 1, self.n do
      local sz = 6 + string.len(self[1][i])
      if maxsize < sz then maxsize = sz end
    end
    if maxsize > 75 then maxsize = 75 end
    local sepformat = "  +%-"..maxsize.."."..maxsize.."s+\n"
    local nameformat = "  | %-"..(maxsize - 2).."."..(maxsize-2).."s |\n"
    local itemformat = "  | %2i. %-"..(maxsize - 6).."."..(maxsize-6).."s 
|\n"
    local sepline = string.format(sepformat, DASHES)
    io.write("\n", string.format(sepformat, DOUBLES))
    io.write(string.format(nameformat, self.name))
    io.write(string.format(sepformat, DOUBLES))
    for i = 1, self.n do
      if self[2][i] then
        item = item + 1
        io.write(string.format(itemformat, item, self[1][i]))
      else
        io.write(sepline)
      end
    end
    io.write(sepline)
  end

  -- Equally quick and dirty menu execution. Tail calls the function
  -- associated with the selected menu item.
  local function domenu(self)
    drawmenu(self)
    io.write("\n\nSelect a menu item: ")
    while true do
      local choice = io.read("*l")
      if choice == nil then return false end
      local _, _, item = string.find(choice, "^%s*(%d+)%s*$")
      if item then
        item = item + 0 -- force numeric conversion
        for i = 1, self.n do
          if self[2][i] then
            if item == 1 then return self[2][i]() end
            item = item - 1
          end
        end
      end
      io.write("\nSelection not valid. Try again: ")
    end
  end

  -- Create a new menu with given name and back reference.
  local function newmenu(name, back)
    return setmetatable({
      {}, {},  -- [1] is the menu label, [2] is the associated function
      name = name,
      back = back,
      n = 0
    },
    meta)
  end

  -- insert a label and a function at the end of a menu
  local function put(self, name, action)
    local n = self.n + 1
    self.n = n
    self[1][n] = name
    self[2][n] = action
    return self
  end

  -- Now the actual menu methods.
  -- add(label, id)
  function curriedMethod:add(name, id)
    return put(self, name, function() return id end)
  end
 
  -- I personally would use functions instead of ids
  curriedMethod.addf = put

  -- create and open a submenu with the given name
  function curriedMethod:sub(name)
    local submenu = newmenu(self.name .. " / " .. name, self)
    put(self, name.." -->", function() return domenu(submenu) end)
    return submenu
  end

  -- create a new, unrelated menu. You cannot use super afterwards
  function curriedMethod:new(name)
    return newmenu(name)
  end
 
  -- go back to the previous level, after introducing the automatic Back 
label
  -- unless this is a top-level menu
  function method:super()
    local mom = self.back
    if mom then
      put(self, "-")
      put(self, "<-- Back", function() return domenu(self.back) end)
      return self.back
     else return self
    end
  end

  -- insert a separator line
  function method:sep()
    return put(self, "-")
  end

  -- and a function to actually execute the thing
  curriedMethod.run = domenu

  -- Finally, we define the Menu "object"
  -- This is a bit of a kludge, because all menus respond to "new"
  -- in the same way. So you could actually just use Menu as your
  -- top-level menu.
  Menu = newmenu("")
end

-- OK, let's give it a whirl

local function about_dialog()
  io.write [[
 
  Menu system written by Rici Lake in order to demonstrate
  some interesting syntactic possibilities in Lua

  This program is released into the public domain. But if you find
  it useful, you could certainly buy me a coffee sometime

]]
  return "About menu"
end

local ID_CAMPAIGN,   ID_RANDOMMAP,   ID_LOADGAME,   ID_EXIT =
      "ID_CAMPAIGN", "ID_RANDOMMAP", "ID_LOADGAME", false
 
mainMenu = Menu.new "Main"
    .sub "New"
        .add("New Campaign", ID_CAMPAIGN)
        .add("New Random Map", ID_RANDOMMAP)
        .super
    .add("Load Game", ID_LOADGAME)
    .sep
    .addf("About", about_dialog)
    .sep
    .add("Exit", ID_EXIT)

while true do 
  local selection = mainMenu.run()
  if not selection then break end
  print("Selected: ", selection)
end