异常处理如何工作？

begin
  result = risky_operation
rescue ArgumentError => e          # rescue a SPECIFIC exception type
  puts "Bad argument: #{e.message}"
rescue StandardError => e          # broader catch (StandardError, not Exception)
  puts "Error: #{e.message}"
  retry if attempts < 3            # retry can re-run the begin block
else
  puts "succeeded"                  # runs if NO exception
ensure
  cleanup                           # ALWAYS runs (success or failure)
end

rescue => e              # ✅ bare rescue catches StandardError (the right default)
rescue StandardError     # ✅ explicit, same thing
rescue Exception         # ❌ AVOID — catches EVERYTHING including system signals
                         #    (Interrupt/Ctrl-C, SystemExit) — can break the program

raise ArgumentError, "Amount must be positive"    # raise a built-in
raise "Something failed"                            # raises RuntimeError

# custom exception class
class InsufficientFundsError < StandardError       # inherit from StandardError
  def initialize(msg = "Not enough funds")
    super
  end
end
raise InsufficientFundsError

def process
  do_work
rescue => e            # rescue WITHOUT begin — applies to the whole method body
  handle(e)
ensure
  cleanup
end