Leibniz pi calculation; Learn You a Haskell Exercices

gistfile1.txt

stepReverseSign :: (Fractional a, Ord a) => a -> a -> a
stepReverseSign num step = (if num > 0 then -1 else 1) * (abs(num) + step)

piCalc :: (Fractional a, Integral b, Ord a) => a -> (a, b)
piCalc tolerance = piCalc' 1 0.0 tolerance 0

piCalc' :: (Ord a, Fractional a, Integral b) => a -> a -> a -> b -> (a, b)
piCalc' denominator pi tolerance iterations =
  | abs( pi' - pi) < tolerance = ( pi', iterations )
  | otherwise = piCalc' (stepReverseSign denominator) pi' tolerance (iterations + 1)
  where pi' = pi + (4/denominator)
  
{-
Prelude> :l exercises.hs
[1 of 1] Compiling Main             ( exercises.hs, interpreted )

exercises.hs:70:55: error:
    • Couldn't match expected type ‘a -> a’ with actual type ‘a’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          piCalc' :: forall a b.
                     (Ord a, Fractional a, Integral b) =>
                     a -> a -> a -> b -> (a, b)
        at exercises.hs:67:12
    • In the second argument of ‘piCalc'’, namely ‘pi'’
      In the expression:
        piCalc'
          (stepReverseSign denominator) pi' tolerance (iterations + 1)
      In an equation for ‘piCalc'’:
          piCalc' denominator pi tolerance iterations
            | abs (pi' - pi) < tolerance = (pi', iterations)
            | otherwise
            = piCalc'
                (stepReverseSign denominator) pi' tolerance (iterations + 1)
            where
                pi' = pi + (4 / denominator)
    • Relevant bindings include
        pi' :: a (bound at exercises.hs:71:9)
        tolerance :: a (bound at exercises.hs:68:24)
        pi :: a (bound at exercises.hs:68:21)
        denominator :: a (bound at exercises.hs:68:9)
        piCalc' :: a -> a -> a -> b -> (a, b) (bound at exercises.hs:68:1)
Failed, modules loaded: none.
-}