Error Handling: pcall, xpcall, and error

Lua doesn't have try/catch, instead errors are handled with the built-in functions pcall, xpcall, error, and assert.

Raising errors

local function withdraw(balance, amount)
  if amount > balance then
    error("insufficient funds")
  end
  return balance - amount
end

Calling error(message) immediately stops execution and unwinds the call stack, exactly like throw in other languages, except the "catch" is a separate function call rather than a block.

Catching errors with pcall

local ok, result = pcall(withdraw, 100, 150)

if ok then
  print("New balance: " .. result)
else
  print("Failed: " .. result) -- result holds the error message here
end

pcall ("protected call") runs a function and catches any error it raises. It always returns a boolean first (true if no error, false if one occurred), followed by either the function's return value(s), or the error message.

assert: a shortcut for a common check

local function withdraw(balance, amount)
  assert(amount <= balance, "insufficient funds")
  return balance - amount
end

assert(condition, message) is sugar for "if condition is falsy, error(message)", used constantly to validate function arguments up front.

xpcall: catching errors with a custom handler

local function riskyOperation()
  error({ code = 42, reason = "something broke" }) -- errors can be any value, not just strings
end

local ok, err = xpcall(riskyOperation, function(e)
  if type(e) == "table" then
    return "Error " .. e.code .. ": " .. e.reason
  end
  return tostring(e)
end)

print(err) -- Error 42: something broke

xpcall is like pcall but takes a second argument, a message handler that runs while the stack is still unwound (useful for attaching a stack trace, or normalizing non-string error values like the table above).