Raising Exceptions in Ruby

Direct Answer

Ruby's raise stops normal execution and sends an exception up the call stack. raise "message" creates a RuntimeError; raise SomeError, "message" creates the named error; and raise error_object sends an existing instance. Inside a rescue clause, bare raise re-raises the current exception with its original backtrace, which is the right choice when the handler cannot fully recover.

Deep Dive & Architecture

The important effect of raise is control transfer. Ruby does not return a special error value and it does not run the next line. It starts unwinding frames until a compatible rescue is found, while ensure clauses encountered along the way still run.

The chosen class communicates category; the message supplies instance detail. ArgumentError says a caller broke an argument contract. A domain class can say a business rule failed. Using RuntimeError for every condition forces handlers to inspect prose and makes unrelated failures look identical.

Re-raising is different from wrapping. Bare raise in a handler preserves the same object and backtrace. Raising a new higher-level exception may be appropriate at an abstraction boundary, but the original error should remain available through logging or an explicit cause so diagnosis does not lose its starting point.

An exception is costly mainly in clarity when used as routine branching. Methods should document whether absence, validation failure, and external failure arrive as values or exceptions. Consistent contracts let callers handle each outcome without broad rescue blocks.

Code Example

class Order
  attr_reader :items

  def initialize
    @items = []
  end

  def add(product, quantity)
    @items << [product, quantity]
  end
end

def add_item(order, product, quantity)
  raise ArgumentError, 'quantity must be positive' unless quantity.positive?

  order.add(product, quantity)
rescue ArgumentError
  raise                          # same object and backtrace
end

# A caller can rescue the known contract failure:
order = Order.new
begin
  add_item(order, 'book', 0)
rescue ArgumentError => error
  puts error.message             # => quantity must be positive
end

Common Mistakes & Pitfalls

Raising for normal control flow (use a return value), or raise with no class (RuntimeError) where a specific class fits.