Find if a string has unique characters in Lua with Set implementation

is_unique.lua

#!/usr/bin/env lua

local lib = require "lib"
local totable = lib.totable
local Set = lib.Set
local tablelength = lib.tablelength
local contains = lib.contains

-- isuniqe function implementation
-- print(isuniqe("aabc")) -- would return false
function isuniqe(str)
  strtab = totable(str)
  return tablelength(Set(strtab)) == string.len(str) 
end


if arg[0] == lib.GetCurrentLuaFile() then
-- if arg[0] == 'is_unique.lua' then
  print(isuniqe("aabc"))
  print(isuniqe("abc"))
end

lib.lua

local lib = {}

-- get current filename
function lib.GetCurrentLuaFile()
    local source = debug.getinfo(2, "S").source
    if source:sub(1,1) == "@" then
        return source:sub(2)
    end
end

-- Basic Set Implementation
-- shanda = Set{"1", "2", "3", "a", "a", "b", "c",}
-- print(shanda["1"]) -- would be true
function lib.Set(t)
  local set = {}
  for _, v in ipairs(t) do set[v] = true end
  return set
end

-- Tablefy: convert string to table: returns table address
-- print(tablelength(totable("rakib"))) -- would become {"r","a","k","i","b"}
function lib.totable(str)
  local t = {}
  for i = 1, #str do t[i] = str:sub(i, i) end
  return t
end

-- Get length of Set, table etc
-- print(tablelength(shanda))
function lib.tablelength(T)
  local count = 0
  for _ in pairs(T) do count = count + 1 end
  return count
end

-- Check if element exists in Set
-- print(contains(shanda,"1"))
function lib.contains(set, e)
  return set[e]
end

return lib