Parse error in simple Haskell Fibonacci implementation

Question

I've tried to make an iterative/tail-recursive version of a function to compute the nth number of the Fibonacci sequence, but I'm getting parse error (possibly incorrect indentation). Why is this happening? The code I'm using:

fib n
    | n < 2 = n
    | otherwise = fibhelper 0 1 2 n
    where fibhelper a b curr num
          | curr == num = a + b
          | curr < num = fibhelper b (a+b) (curr+1) num

To be clear, I'm trying to understand the error - why it's happening, how it should be corrected - and not trying to implement fib efficiently (I understand the popular zipWith implementation here already, for instance).

Thanks!

Answer 1

The guard part has to be indented at least one character relative to the function name. The following thus works:

fib n
    | n < 2 = n
    | otherwise = fibhelper 0 1 2 n
    where fibhelper a b curr num
           | curr == num = a + b  -- moved one character to the left.
           | curr < num = fibhelper b (a+b) (curr+1) num