No instance for (Show (Double -> Double))

Question

I have the following code which uses Newton's method to approximate the square root of some number. The problems is that when I run it, I get an error..What is wrong and how can I fix it?

newtonRootSequence :: Double -> [Double]
newtonRootSequence d = newtonSequenceGenerator d 1

newtonSequenceGenerator :: Double -> Double -> [Double]
newtonSequenceGenerator d xn = nxplus1 : newtonSequenceGenerator d nxplus1
    where nxplus1 =  (xn + d / xn) / 2

newtonRoot:: Double -> Double -> Double
newtonRoot d epsilon = head ([xs !! index | index <- [1..((length xs) - 1)], (xs !! index) - (xs !! index - 1) <= epsilon]
    where xs = newtonRootSequence d

Error:

 <interactive>:2:1: error:
        * No instance for (Show (Double -> Double))
            arising from a use of `print'
            (maybe you haven't applied a function to enough arguments?)
        * In a stmt of an interactive GHCi command: print it

Running it should be like the following:

$newtonRoot 35

Answer 1

In Haskell all function are curryfied, so, your function

newtonRoot:: Double -> Double -> Double

the are "hidden parenthesis":

newtonRoot:: Double -> (Double -> Double)

if you provide one argument newtonRoot 35 you have

(newtonRoot 35) :: Double -> Double

and a function f :: Double -> Double is not instance of Show type class

You need to finally provide the last argument to your function value:

(newtonRoot 35 2) :: Double

Double can me shown

Answer 2

newtonRoot takes two arguments, a d and an epsilon. You didn't supply an epsilon. Try

> newtonRoot 35 0.1

instead.

There are other errors, too, but this should get you started down the debugging path.