package.pathとpackage.cpath

package.path の内容

print(package.path)
-- ./?.lua;./?.luac;/usr/local/share/lua/5.4/?.lua;...

-- Dots in module names become directory separators
-- require("foo.bar") -> searches for foo/bar.lua
print(("foo.bar"):gsub("%.", "/"))  -- foo/bar

検索パスの追加

-- Add multiple directories
package.path = table.concat({
  "./?.lua",
  "./lib/?.lua",
  "./lib/?/init.lua",
  package.path,  -- keep existing paths
}, ";")

-- Now require("json") searches:
-- ./json.lua -> ./lib/json.lua -> ./lib/json/init.lua -> ...

C 拡張用の package.cpath

print(package.cpath)
-- ./?.so;/usr/local/lib/lua/5.4/?.so;...

-- Add a local C library directory
package.cpath = "./clib/?.so;" .. package.cpath

-- require("myextension") will now search:
-- ./clib/myextension.so

LUA_PATH 環境変数

-- Set before launching Lua:
-- export LUA_PATH="./?.lua;./lib/?.lua;;"
-- The ";;" expands to the built-in default path

-- In Lua, check what was set:
print(package.path)

-- You can also use LUA_CPATH for C paths:
-- export LUA_CPATH="./clib/?.so;;"

package.searchpath

local file, err = package.searchpath("json", package.path)
if file then
  print("Found:", file)
else
  print("Not found. Tried:\n" .. err)
end

-- Useful for checking if a module exists:
local function moduleExists(name)
  return package.searchpath(name, package.path) ~= nil
end
print(moduleExists("json"))

require の検索アルゴリズム

-- Simulate what require does:
local function myRequire(name)
  -- 1. Check cache
  if package.loaded[name] ~= nil then
    return package.loaded[name]
  end
  -- 2. Check preload
  if package.preload[name] then
    return package.preload[name]()
  end
  -- 3. Search path
  local file = package.searchpath(name, package.path)
  if file then
    return dofile(file)  -- simplified
  end
  error("module not found: " .. name)
end

ローダー関数

-- Add a custom searcher that loads from a table
local builtins = {
  myconfig = function()
    return {host="localhost", port=8080}
  end
}

table.insert(package.searchers, 1, function(name)
  local loader = builtins[name]
  if loader then return loader end
end)

local cfg = require("myconfig")
print(cfg.host, cfg.port)   -- localhost  8080

モジュール名の規則

-- Good: hierarchical, lowercase
-- require("myapp.db.connection")
-- require("myapp.utils.string")

-- Bad: dashes cause syntax issues in dot notation
-- local my-module = require("my-module")  -- SYNTAX ERROR
local mymodule = require("my_module")  -- ok

パスのデバッグ

-- Debug require failures:
local ok, err = pcall(require, "missing_module")
if not ok then
  -- The error message shows all tried paths
  print("require failed:")
  print(err)
  print("\nCurrent package.path:")
  for path in package.path:gmatch("[^;]+") do
    print("  " .. path)
  end
end

デフォルトパスへのリセット

-- Minimal path for embedded environment
package.path  = "./?.lua;./lib/?.lua"
package.cpath = "./?.so"

-- Or get just the current directory patterns:
local function justLocal()
  return "./?.lua;./?/init.lua"
end

print("Minimal path:", justLocal())

クイックチェック

復習：package.path