Problem Statement

While implementing a recursive function, it often happens that we must resort to a separate   helper function   to handle the actual recursion. This is usually the case when directly calling the current function would waste too many resources (stack space, execution time), causing unwanted side-effects,   and/or the function doesn't have the right arguments and/or return values. So we end up inventing some silly name like   foo2   or   foo_helper.   I have always found it painful to come up with a proper name, and see some disadvantages: Some languages allow you to embed recursion directly in-place.   This might work via a label, a local gosub instruction, or some special keyword. Anonymous recursion can also be accomplished using the   Y combinator.

If possible, demonstrate this by writing the recursive version of the fibonacci function   (see Fibonacci sequence)   which checks for a negative argument before doing the actual recursion.

Let's start with the solution:

$ include "seed7_05.s7i";

const func integer: fib (in integer: x) is func
  result
    var integer: fib is 0;
  local
    const func integer: fib1 (in integer: n) is func
      result
        var integer: fib1 is 0;
      begin
        if n < 2 then
          fib1 := n;
        else
          fib1 := fib1(n-2) + fib1(n-1);
        end if;
      end func;
  begin
    if x < 0 then
      raise RANGE_ERROR;
    else
      fib := fib1(x);
    end if;
  end func;

const proc: main is func
  local
    var integer: i is 0;
  begin
    for i range 0 to 4 do
      writeln(fib(i));
    end for;
  end func;