Haskell declaring an instance of Show class

Question

I am trying to wrap my mind around classes and data structures in Haskell, specifically declaring a type instance of it. I can get it to work with some classes and data types but not all of them so I must be missing something. Specifically I have the following data declaration:

data LinkedList a = End | Link a (LinkedList a)

I want to declare an instance of Show for that type so that the output looks something close to "el1, el2, el3, el4, ..."

instance Show LinkedList where
    show (End) = "."
    show (Link a b) = show a ++ "," ++ show b

As expected this doesn't work... Any idea why? I think I understand what "data" and "type" mean but I am not sure whether I am comfortable with classes and instances. Thank you

What error do you get? Please edit your question to include this important detail. — Code-Apprentice
– Code-Apprentice, Commented Sep 8, 2018 at 16:54

chi · Accepted Answer · 2018-09-08 16:53:25Z

3

instance Show LinkedList where

LinkedList is not a type, LinkedList a is a type. Correcting that, we get

instance Show (LinkedList a) where

Then , we get another error because we call show on values of type a. We need to require that a belongs to class Show, too.

instance Show a => Show (LinkedList a) where

This should now work.

answered Sep 8, 2018 at 16:53

chi

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4 Comments

omykhron Over a year ago

Thank you! So why isn't LinkedList a type?

chi Over a year ago

@OmarKahol Beacuse it lacks an argument. It is a constructor of kind * -> *, i.e. (roughly) a function from types to types, and we need to pass it an argument to get a type.

amalloy Over a year ago

@OmarKahol LinkedList is a "type-level function" which produces types. It's not a type in the same way that (* 5) isn't a number, but if you pass it a number you get a number back.

Jon Purdy Over a year ago

@OmarKahol: You can learn more about kinds with the :kind command in GHCi. :kind Show gives * -> Constraint because Show is a typeclass that takes a type of kind * (a “base type”, one that contains values) and returns a constraint; :kind LinkedList gives * -> * because LinkedList is a type constructor that takes a type of kind * and produces a type, also of kind *. So you can’t write Show LinkedList because * and * -> * don’t match—it’s a kind error, which is like a type error at the type level. * can also be written as Type (imported from GHC.Types).

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