Matter AI | Code Reviewer Documentation

Lua Anti-Patterns Overview

Global Variables by Default

-- Anti-pattern: Implicit global variables
function process_data()
  result = 42  -- Implicitly global!
  return result
end

-- Better approach: Use local variables
function process_data()
  local result = 42  -- Explicitly local
  return result
end

Always use the local keyword when declaring variables. In Lua, variables are global by default, which can lead to unexpected behavior and name collisions.

Not Using Modules

-- Anti-pattern: Global functions and variables
function calculate_area(radius)
  return math.pi * radius * radius
end

function calculate_circumference(radius)
  return 2 * math.pi * radius
end

-- Better approach: Use modules
local Circle = {}

function Circle.area(radius)
  return math.pi * radius * radius
end

function Circle.circumference(radius)
  return 2 * math.pi * radius
end

return Circle

Organize your code into modules instead of using global functions and variables. This improves maintainability and reduces the risk of name collisions.

Using Tables Inefficiently

-- Anti-pattern: Inefficient table usage
local list = {}
for i = 1, 1000 do
  table.insert(list, i)  -- Inefficient for large lists
end

-- Better approach: Pre-allocate indices
local list = {}
for i = 1, 1000 do
  list[i] = i  -- More efficient direct assignment
end

Use direct index assignment instead of table.insert() when you know the index in advance. This is more efficient, especially for large tables.

String Concatenation in Loops

-- Anti-pattern: String concatenation in loops
local result = ""
for i = 1, 1000 do
  result = result .. "item " .. i .. "\n"  -- Creates many intermediate strings
end

-- Better approach: Use table.concat
local parts = {}
for i = 1, 1000 do
  parts[i] = "item " .. i
end
local result = table.concat(parts, "\n")

Avoid concatenating strings in loops with the .. operator. Instead, collect strings in a table and use table.concat() at the end, which is much more efficient.

Not Using Proper Error Handling

-- Anti-pattern: No error handling
function read_file(filename)
  local file = io.open(filename, "r")  -- Might fail
  local content = file:read("*all")
  file:close()
  return content
end

-- Better approach: Proper error handling
function read_file(filename)
  local file, err = io.open(filename, "r")
  if not file then
    return nil, "Failed to open file: " .. (err or "unknown error")
  end
  
  local content, err = file:read("*all")
  file:close()
  
  if not content then
    return nil, "Failed to read file: " .. (err or "unknown error")
  end
  
  return content
end

-- Usage
local content, err = read_file("data.txt")
if not content then
  print("Error: " .. err)
else
  -- Process content
end

Use proper error handling by checking return values and returning error messages. Lua functions often return nil plus an error message on failure.

Using Numeric For-Loops Incorrectly

-- Anti-pattern: Incorrect numeric for-loop
local t = {10, 20, 30, 40, 50}
for i = 0, #t do  -- Starts at 0, but Lua tables are 1-indexed
  print(t[i])  -- t[0] is nil
end

-- Better approach: Correct indexing
local t = {10, 20, 30, 40, 50}
for i = 1, #t do  -- Lua tables are 1-indexed
  print(t[i])
end

Remember that Lua tables are 1-indexed by default. Start your numeric for-loops at 1 when iterating over sequential tables.

Not Using Proper Scope

-- Anti-pattern: Variables with too broad scope
function process_data(items)
  count = 0  -- Global variable
  for i = 1, #items do
    if is_valid(items[i]) then
      count = count + 1
    end
  end
  return count
end

-- Better approach: Proper variable scoping
function process_data(items)
  local count = 0  -- Local variable
  for i = 1, #items do
    if is_valid(items[i]) then
      count = count + 1
    end
  end
  return count
end

Keep variables in the smallest possible scope. Use local variables instead of global ones whenever possible.

Inefficient Table Clearing

-- Anti-pattern: Inefficient table clearing
function clear_table(t)
  t = {}  -- This creates a new table, doesn't clear the original
end

-- Better approach: Actually clear the table
function clear_table(t)
  for k in pairs(t) do
    t[k] = nil
  end
end

Assigning a new empty table to a variable doesn’t clear the original table. Instead, set each key to nil to actually clear a table.

Not Using Metatables Appropriately

-- Anti-pattern: Reinventing common patterns
local Point = {}

function Point.new(x, y)
  local self = {}
  self.x = x or 0
  self.y = y or 0
  
  function self.add(other)
    return Point.new(self.x + other.x, self.y + other.y)
  end
  
  return self
end

-- Better approach: Use metatables
local Point = {}
Point.__index = Point

function Point.new(x, y)
  local self = setmetatable({}, Point)
  self.x = x or 0
  self.y = y or 0
  return self
end

function Point:add(other)
  return Point.new(self.x + other.x, self.y + other.y)
end

-- Even better: Operator overloading with metatables
function Point.__add(a, b)
  return Point.new(a.x + b.x, a.y + b.y)
end

-- Usage
local p1 = Point.new(1, 2)
local p2 = Point.new(3, 4)
local p3 = p1 + p2  -- Uses __add metamethod

Use metatables to implement object-oriented patterns, operator overloading, and other advanced features in a clean and efficient way.

Using Inefficient Patterns for OOP

-- Anti-pattern: Inefficient OOP pattern
function create_person(name, age)
  local person = {}
  
  person.name = name
  person.age = age
  
  person.greet = function(self)
    return "Hello, I'm " .. self.name
  end
  
  person.have_birthday = function(self)
    self.age = self.age + 1
  end
  
  return person
end

-- Better approach: Prototype-based OOP
local Person = {}
Person.__index = Person

function Person.new(name, age)
  local self = setmetatable({}, Person)
  self.name = name
  self.age = age
  return self
end

function Person:greet()
  return "Hello, I'm " .. self.name
end

function Person:have_birthday()
  self.age = self.age + 1
end

Use prototype-based OOP with metatables instead of creating new function objects for each instance. This is more memory-efficient and performs better.

Not Using Proper Closures

-- Anti-pattern: Not using closures effectively
function create_counter()
  local counter = {count = 0}
  
  function counter.increment()
    counter.count = counter.count + 1
    return counter.count
  end
  
  function counter.get()
    return counter.count
  end
  
  return counter
end

-- Better approach: Use closures
function create_counter()
  local count = 0
  
  return {
    increment = function()
      count = count + 1
      return count
    end,
    
    get = function()
      return count
    end
  }
end

Use closures to encapsulate state without exposing it directly. This provides better encapsulation and can be more memory-efficient.

Not Using Proper Module Structure

-- Anti-pattern: Poor module structure
-- mymodule.lua
function hello()
  print("Hello, World!")
end

function goodbye()
  print("Goodbye, World!")
end

-- These are now global functions

-- Better approach: Proper module structure
-- mymodule.lua
local M = {}

function M.hello()
  print("Hello, World!")
end

function M.goodbye()
  print("Goodbye, World!")
end

return M

-- Usage
-- local mymodule = require("mymodule")
-- mymodule.hello()

Structure your modules properly by returning a table of functions and values. This prevents polluting the global namespace.

Using Unnecessary Upvalues

-- Anti-pattern: Unnecessary upvalues
function create_functions()
  local functions = {}
  
  for i = 1, 10 do
    functions[i] = function() return i end  -- i is an upvalue
  end
  
  return functions
end

-- Better approach: Avoid unnecessary upvalues
function create_functions()
  local functions = {}
  
  for i = 1, 10 do
    local value = i  -- Create a new local variable
    functions[i] = function() return value end
  end
  
  return functions
end

Be careful with upvalues in loops. Create a new local variable inside the loop to capture the current value, not the final value of the loop variable.

Not Using Proper Error Propagation

-- Anti-pattern: Swallowing errors
function process_file(filename)
  local file = io.open(filename, "r")
  if not file then
    print("Error opening file")
    return
  end
  
  -- Process file...
  file:close()
end

-- Better approach: Propagate errors
function process_file(filename)
  local file, err = io.open(filename, "r")
  if not file then
    return nil, "Error opening file: " .. err
  end
  
  -- Process file...
  local result = file:read("*all")
  file:close()
  
  return result
end

-- Usage
local result, err = process_file("data.txt")
if not result then
  -- Handle error
  print(err)
else
  -- Use result
end

Propagate errors up the call stack instead of handling them locally or swallowing them. This allows the caller to decide how to handle errors.

Using Inefficient Table Traversal

-- Anti-pattern: Inefficient table traversal
local t = {a = 1, b = 2, c = 3}

for i = 1, #t do  -- This only works for array-like tables
  print(t[i])
end

-- Better approach: Use pairs for hash tables
local t = {a = 1, b = 2, c = 3}

for k, v in pairs(t) do
  print(k, v)
end

-- Use ipairs for array-like tables
local arr = {10, 20, 30}

for i, v in ipairs(arr) do
  print(i, v)
end

Use pairs() for traversing hash tables and ipairs() for traversing array-like tables. Don’t use the length operator (#) for non-sequential tables.

Not Using Proper Documentation

-- Anti-pattern: Poor or no documentation
function process_data(data, options)
  -- Implementation...
end

-- Better approach: Proper documentation
---Process data according to specified options
---@param data table The data to process
---@param options table|nil Optional settings with the following fields:
---                         - verbose (boolean): Whether to output verbose logs
---                         - max_items (number): Maximum number of items to process
---@return table The processed data
---@return string|nil Error message if an error occurred
function process_data(data, options)
  -- Implementation...
end

Document your functions and modules properly. Include parameter descriptions, return values, and usage examples. Consider using a documentation format like LuaDoc or EmmyLua annotations.

Not Using Proper Testing

-- Anti-pattern: Manual testing or no testing
function add(a, b)
  return a + b
end

-- Manual test
print(add(2, 3) == 5 and "OK" or "FAIL")

-- Better approach: Use a testing framework
local luaunit = require("luaunit")

function add(a, b)
  return a + b
end

function test_add()
  luaunit.assertEquals(add(2, 3), 5)
  luaunit.assertEquals(add(-1, 1), 0)
  luaunit.assertEquals(add(0, 0), 0)
end

luaunit.run("test_add")

Write proper tests for your code using a testing framework like LuaUnit, Busted, or luatest. This makes it easier to verify that your code works as expected and to catch regressions.

Using Inefficient Algorithms

-- Anti-pattern: Inefficient algorithms
function find_element(t, value)
  for i = 1, #t do
    if t[i] == value then
      return i
    end
  end
  return nil
end

-- Better approach: Use more efficient data structures
function create_lookup_table(t)
  local lookup = {}
  for i, v in ipairs(t) do
    lookup[v] = i
  end
  return lookup
end

-- Usage
local data = {10, 20, 30, 40, 50}
local lookup = create_lookup_table(data)

-- O(1) lookup instead of O(n)
local index = lookup[30]  -- Returns 3

Choose appropriate algorithms and data structures for your task. Consider time and space complexity, especially for operations that are performed frequently or with large data sets.

Not Using Proper Resource Management

-- Anti-pattern: Poor resource management
function process_file(filename)
  local file = io.open(filename, "r")
  if not file then return nil end
  
  if some_condition() then
    return early_result  -- File is never closed!
  end
  
  -- Process file...
  file:close()
  return result
end

-- Better approach: Ensure resources are released
function process_file(filename)
  local file, err = io.open(filename, "r")
  if not file then return nil, err end
  
  -- Use pcall to ensure file is closed even if an error occurs
  local success, result = pcall(function()
    if some_condition() then
      return early_result
    end
    
    -- Process file...
    return final_result
  end)
  
  file:close()  -- Always close the file
  
  if not success then
    return nil, result  -- result contains the error message
  end
  return result
end

Ensure that resources like files, network connections, and database connections are properly closed, even in error cases or early returns.

Not Using Proper Configuration Management

-- Anti-pattern: Hardcoded configuration
function connect_to_database()
  return db.connect("localhost", 5432, "mydb", "user", "password")
end

-- Better approach: Externalize configuration
local config = require("config")

function connect_to_database()
  return db.connect(
    config.db.host,
    config.db.port,
    config.db.name,
    config.db.user,
    config.db.password
  )
end

-- config.lua
return {
  db = {
    host = os.getenv("DB_HOST") or "localhost",
    port = tonumber(os.getenv("DB_PORT")) or 5432,
    name = os.getenv("DB_NAME") or "mydb",
    user = os.getenv("DB_USER") or "user",
    password = os.getenv("DB_PASSWORD") or "password"
  }
}

Externalize configuration instead of hardcoding values. This makes your code more flexible and easier to deploy in different environments.

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