Lua Writing and using modules Writing the module

Example

--- trim: a string-trimming module for Lua
-- Author, date, perhaps a nice license too
--
-- The code here is taken or adapted from  material in
-- Programming in Lua, 3rd ed., Roberto Ierusalimschy

-- trim_all(string) => return string with white space trimmed on both sides 
local trim_all = function (s)
  return (string.gsub(s, "^%s*(.-)%s*$", "%1"))
end

-- trim_left(string) => return string with white space trimmed on left side only
local trim_left = function (s)
  return (string.gsub(s, "^%s*(.*)$", "%1"))
end

-- trim_right(string) => return string with white space trimmed on right side only
local trim_right = function (s)
  return (string.gsub(s, "^(.-)%s*$", "%1"))
end

-- Return a table containing the functions created by this module
return {
  trim_all = trim_all,
  trim_left = trim_left,
  trim_right = trim_right
}

An alternative approach to the one above is to create a top-level table and then store the functions directly in it. In that idiom, our module above would look like this:

-- A conventional name for the table that will hold our functions
local M = {}

-- M.trim_all(string) => return string with white space trimmed on both sides
function M.trim_all(s)
  return (string.gsub(s, "^%s*(.-)%s*$", "%1"))
end

-- M.trim_left(string) => return string with white space trimmed on left side only
function M.trim_left(s)
  return (string.gsub(s, "^%s*(.*)$", "%1"))
end

-- trim_right(string) => return string with white space trimmed on right side only
function M.trim_right(s)
  return (string.gsub(s, "^(.-)%s*$", "%1"))
end


return M

From the point of view of the caller, there is little difference between the two styles. (One difference worth mentioning is that the first style makes it more difficult for users to monkeypatch the module. This is either a pro or a con, depending on your point of view. For more detail about this, see this blog post by Enrique García Cota.)