Creating Haskell instance declarations

Question

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!

Answer 1

First of all, by default, type synonyms (that is, stuff defined using type) aren't legal in instance declarations. There's a GHC extensions to allow this, however.

Beyond that, in this specific case, show needs to return a String; your instance is trying to return a... type synonym name, which doesn't even make sense to begin with, and besides refers to a list of Int, which is the wrong return type for show.

Finally, D is a function type--what is that supposed to show, anyway? There's really not much you can meaningfully do with a Show instance on a function type, in most cases.

If you just want it to say "this is type D", you could write an instance like this:

{-# LANGUAGE TypeSynonymInstances #-}
instance Show D where show _ = "Stack -> Stack"

I'm not sure how useful that is in practice, though.