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 

In other words, a rope is a tree whose leaves are strings. The important operation on trees is the tree-walk, applying a procedure to each leaf.

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

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Last edited June 4, 2007 1:43 pm GMT (diff)