Question

The following function generates a ‘stack level too deep (SystemStackError)’ for n = 5,000

def factorial(n)
  n == 0 ? 1 : factorial(n -1) * n
end

Is there a way to avoid this error using continuations/callcc?

Note:

I know this can be implemented without recursion. e.g.

def factorial2(n)
  (1..n).inject(1) {|result, n| result * n } 
end

Answer 1

Sure. Continuations can do anything! However, you’ll end up reinventing one of two things: a loop or a function call. On my machine, which does tail-call optimization by default, tail recursion with call/cc does not get optimized. The program quickly bogs down as each callcc apparently captures the entire call stack. That code being broken, here is a call/cc loop:

def fact( n )
    (n, f, k) = callcc { |k| [ n, 1, k ] }
    if ( n == 0 ) then return f
    else k.call n-1, n*f, k
    end
end

Note: previously I’d forgotten that call/cc isn’t just about initiating a continuation-passing chain and got confused about the need for recursion, hence the comments below.