Error Handling

Code

class ValidationError < StandardError
  attr_reader :field
  def initialize(field, msg)
    @field = field
    super(msg)
  end
end

def validate_age(age)
  raise ValidationError.new(:age, "must be >= 0") if age < 0
  raise ValidationError.new(:age, "must be <= 130") if age > 130
  "ok"
end

# begin/rescue/ensure
def process(age)
  begin
    result = validate_age(age)
    puts "result: #{result}"
  rescue ValidationError => e
    puts "Validation failed on #{e.field}: #{e.message}"
  rescue => e
    puts "Unexpected: #{e.class}"
  ensure
    puts "cleanup"
  end
end

process(25)
process(-1)
process(200)

# Retry on transient failure
attempts = 0
begin
  attempts += 1
  raise "boom" if attempts < 3
  puts "succeeded on attempt #{attempts}"
rescue => e
  puts "attempt #{attempts} failed: #{e.message}"
  retry if attempts < 3
end

# else and ensure
def divide(a, b)
  result = a / b
  puts "computed"
  result
rescue ZeroDivisionError
  :undefined
ensure
  puts "always runs"
end

puts divide(10, 2)
puts divide(1, 0)

Explanation

Exceptions inherit from StandardError and are raised with raise, then caught by class with rescue. Custom exception classes carry extra fields for richer error context. ensure runs whether or not an exception was raised, and retry re-executes the begin block, useful for transient failures.

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