lamdba.rb

# If you were curious how to do it with lambdas,
# I think this way makes the method definition look ugly but w/e
def method(arr, fn = ->(e){ "" }) 
  arr[:no_key]
rescue => e
  fn.call(e)
end

method([])
# => ""

method([], ->(e){ raise e })
# TypeError: can't convert Symbol into Integer

you can do it with a block like so:

def method(arr, &block)
  block ||= ->(e){ "" }
  arr[:no_key]
rescue => e
  block.call e
end

method([])
# => ""

method([]) {|e| raise e }
# TypeError: can't convert Symbol into Integer

the definition is slightly longer since you have to assign a lambda to block if one isn't passed, but i like that you can call it with a block