Haskell: Stack Overflow in self-defined Show typeclass

Question

As a task, I have to get the Show typeclass of my program to be implemented in a way that giveMoney is used appropiately to change the notes to the highest possible change. So far, I know that I can get a list of the change when using giveMoney (value [list of notes]), however, I get a stack overflow exception when I try to use Money [list of notes] in my current implementation:

data EuroNote = Five | Ten | Twenty | Fifty | Hundred | TwoHundred | FiveHundred       deriving Show

data  Money = Money [EuroNote]


value = foldr (+) 0 . map notevalue

notevalue Five = 5 
notevalue Ten = 10
notevalue Twenty = 20
notevalue Fifty = 50
notevalue Hundred = 100
notevalue TwoHundred = 200
notevalue FiveHundred = 500

giveMoney euros
            | euros >= 500 = [FiveHundred] ++ giveMoney(euros - 500)
            | euros >= 200 = [TwoHundred] ++ giveMoney(euros - 200)
            | euros >= 100 = [Hundred] ++ giveMoney(euros - 100)
            | euros >= 50 = [Fifty] ++ giveMoney(euros - 50)
            | euros >= 20 = [Twenty] ++ giveMoney(euros - 20)
            | euros >= 10 = [Ten] ++ giveMoney(euros - 10)
            | euros >= 5 = [Five]
            | euros == 0 = []

instance Show Money where
show (Money notes) = giveMoney (value notes)

Answer 1

There are two problems with the code.

1. Indentation with your code

Once you make that correct, it throws a type error for this code:

instance Show Money where
  show (Money notes) = giveMoney (value notes)

You can rectify that by converting the code to:

instance Show Money where
  show (Money notes) = show $ giveMoney (value notes)

Demo in ghci:

ghci> Money [Ten]
[Ten]