Question

class Test
  def foo
    throw(:label, foo)
    "should never get here"
  end

  def bar
    "bar"
  end
end

test = Test.new

Now I tried the below:

puts("bar -> " + catch(:label) {test.bar})

and got :

bar -> bar
=> nil

Now when I tried:

puts("foo -> " + catch(:label) {test.foo})

I expected that i would get nil but actually got the below:

SystemStackError: stack level too deep
    from /usr/lib/ruby/1.9.1/irb/workspace.rb:80
Maybe IRB bug!

I am unable to explain myself why this. Could anyone help me for the same?

Answer 1

The infinite loop happens outside the throw/catch.

def foo
  throw(:label, foo) # <-
  "should never get here"
end

The value returned has to be generated first, no lazy eval there. So
it calls foo again, and you have your infinite recursion without any
stopping point. If you want nil, use

def foo
  throw(:label, nil) # <-
  "should never get here"
end