A Lua class for accessing individual characters in a string

StringAccessor.lua

--[[ Example:
sa = StringAccessor 'hello'
for c in sa:iter() do
  print(c)
end

print(sa[1],sa[3])
print('end',sa[-1])
--]]

local char,byte = string.char, string.byte
local type,rawget = type,rawget

local function char_at(s,i)
    return char(byte(s,i))
end

StringAccessor = {
    __index = function(t,i) 
            local ti = type(i)
            if ti == 'string' then return rawget(StringAccessor,i) 
            elseif ti == 'number' then
                if i < 0 then i = #t.str + i + 1 end
                if i <= #t.str then return char_at(t.str,i)
                else error('out of range',2)
                end
            else
                error("bad index type",2)
            end
       end;
    __newindex = function(t) error("not modifiable!",2) end;
    iter = function(t)
        local i = 0
        local s = t.str
        return function()
            i = i + 1
            if i > #s then return nil end
            return char_at(s,i)
        end
    end
}

--- Callable constructor
setmetatable(StringAccessor,{
    __call = function(t,str)
        return setmetatable({str=str},StringAccessor)
    end
})