Lua Ropes

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Lua Ropes

Because strings are immutable objects in Lua, it is important to avoid creating them unnecessarily. Very often programs manipulate strings in order to create some textual output. One strategy is to avoid creating intermediate strings, and to output all the pieces of the final text in the right order. The appropriate datatype for this is the rope[1], where

 rope ::= string | list of rope 

do

  local function walk(x,f)

    local typ, ok, bad = type(x),true

    local errmsg = "bad type (%s), %s"

    if typ == "string" then return pcall(f,x)

    elseif typ == "table" then

      local i,n = 1,#x

      while ok and i <= n do

        ok,bad = walk(x[i],f)

        i = i+1

      end -- while

      return ok,bad

    else

      bad = errmsg:format(typ,typ and "" or "something undefined?")

      return nil,bad

    end -- if

  end -- function



  local mt = {

    push = function (self,x)

      local function f(y)

        if y and type(y) ~= "function" then

          self[#self + 1] =y

          return f

        elseif y then

          return function (z)

            return f(y(z))

          end -- function

        end -- if

      end -- function y

      return f(x)

    end; -- function push

    walk = walk;

    flatten = function(self)

      local o = {}

      local f = function (x) o[#o + 1] = x end -- function

      self:walk(f)

      return table.concat(o)

    end; -- function

  }



  function rope()

    return setmetatable({}, {__index = mt})

  end -- function

end -- do



t,x = rope(),rope()

t:push (string.upper) "m" "r" (string.char) (32) "Smith" ;

x:push [[ Good ]] [[morning]];

t:push (x)

t:walk(io.write) --> Mr Smith Good morning

print(t:flatten()) --> Mr Smith Good morning