Hello, complete noob to Haskell here with probably an even noobier question. I'm trying to get ghci output working and am stuck on instance declarations. How could I declare an instance for "(Show (Stack -> Stack))" given:

data Cmd = LD Int
         | ADD
         | MULT
         | DUP
         deriving Show

type Prog = [Cmd]

type Stack = [Int]

type D = Stack -> Stack

I've been trying to create a declaration like:

instance Show D where show = Stack

but all my attempts have resulted in illegal instance declarations. Any help and/or references much appreciated!