error()関数

エラーを発生させる

local function divide(a, b)
  if b == 0 then
    error("division by zero")
  end
  return a / b
end

print(divide(10, 2))    -- 5.0
-- divide(10, 0)        -- ERROR: division by zero

エラーレベル

local function assertPositive(n, name)
  if n <= 0 then
    error((name or "value") .. " must be positive, got " .. n, 2)
    -- level 2: blame the caller, not this function
  end
  return n
end

local function compute(x)
  assertPositive(x, "x")   -- error points here if x <= 0
  return math.sqrt(x)
end

compute(-5)  -- error: "x must be positive, got -5" at compute() call

テーブルオブジェクトを使ったエラー

local function openDB(host, port)
  if port < 1 or port > 65535 then
    error({code="INVALID_PORT", port=port,
           msg="port out of range: " .. port})
  end
  -- ... connect
  return {host=host, port=port}
end

local ok, err = pcall(openDB, "localhost", -1)
if not ok and type(err) == "table" then
  print("Code:", err.code)    -- INVALID_PORT
  print("Port:", err.port)    -- -1
  print("Msg:", err.msg)
end

糖衣構文としてのassert()

local function sqrt(n)
  assert(type(n) == "number", "expected number, got " .. type(n))
  assert(n >= 0, "sqrt of negative: " .. n)
  return math.sqrt(n)
end

print(sqrt(16))    -- 4.0
print(sqrt(2))     -- 1.4142...
-- sqrt("hi")      -- ERROR: expected number, got string

error()とreturn nil,errの違い

-- Exception style (programming error)
local function mustExist(t, key)
  local v = t[key]
  if v == nil then error("required key missing: " .. key, 2) end
  return v
end

-- Functional style (expected failure)
local function findUser(id)
  -- ... database query
  return nil, "user not found"  -- expected: user may not exist
end

メタメソッド内のエラー

local SafeDiv = {}
SafeDiv.__index = SafeDiv

SafeDiv.__div = function(a, b)
  if b.value == 0 then
    error("SafeDiv: division by zero", 2)
  end
  return SafeDiv.new(a.value / b.value)
end

function SafeDiv.new(v)
  return setmetatable({value=v}, SafeDiv)
end

local a = SafeDiv.new(10)
local b = SafeDiv.new(0)
local ok, err = pcall(function() return a / b end)
print(ok, err)

カスタムエラー型

local function newError(kind, msg, extra)
  return setmetatable(
    {kind=kind, message=msg, extra=extra},
    {__tostring = function(e)
      return "[" .. e.kind .. "] " .. e.message
    end}
  )
end

local E = {
  notFound = function(name) return newError("NOT_FOUND","not found: "..name) end,
  badInput = function(msg)  return newError("BAD_INPUT", msg) end,
}

local ok, err = pcall(error, E.notFound("config.json"))
if not ok then print(err.kind, err.message) end

エラーの伝播

local function step3() error("step3 failed") end
local function step2() step3() end
local function step1() step2() end

local ok, err = pcall(step1)
if not ok then
  -- err includes the source location
  print("Caught at top level:", err)
end
-- Error: input:1: step3 failed

debug.traceback によるスタックトレース

local function buggy()
  local t = nil
  return t.field   -- nil indexing: error
end

local ok, err = xpcall(buggy, function(e)
  return debug.traceback(e, 2)  -- full stack trace
end)

if not ok then
  print(err)  -- full traceback
end

エラーの再スロー

local function loadAndParse(path)
  local ok, err = pcall(function()
    local f = io.open(path, "r")
    if not f then error("cannot open: " .. path) end
    local content = f:read("a")
    f:close()
    return content
  end)
  if not ok then
    error("loadAndParse failed: " .. err, 0)  -- re-raise with context
  end
end

local ok2, msg = pcall(loadAndParse, "missing.txt")
print(ok2, msg)

理解度チェック

まとめ: error()