Type family error

Question

Hello I wrote my first program with type family

{-# LANGUAGE TypeFamilies #-}

data Colist a = Colist (Maybe (a,Colist a))

class Listable l where type Item l :: *
    toList :: l -> [Item l]

instance Listable (Colist a) where 
    type Item (Colist a) = (Colist a)
    toList x = [x]

The program should take Colist a and take it into a list I get the error that 'Item l' couldn't be matched with actual type 'l'. But I wrote that the Item x EQUALS (Colist a). Where is my mistake?

Answer 1

Did you mean...

{-# LANGUAGE TypeFamilies #-}

newtype List a = List (Maybe (a, List a))

class Listable l where
    type Item l :: *
    toList :: l -> [Item l]

instance Listable (List a) where
    type Item (List a) = a

    toList (List Nothing) = []
    toList (List (Just (x, xs)) = x : toList xs

I've changed a few things about your code. Zero, I renamed Colist to List (because there's no such thing as a coinductive list in Haskell). One, I fixed the indentation, making toList a method of the Listable class. Two, I made List a 's instance of Item return a , not List a . Three, I fixed toList to actually return a list of the elements of the List - your version just put the whole List in a singleton list.

I strongly suspect you're misusing type families here though. Here's a much simpler and more Haskell-ish formulation of this class:

class Listable l where
    toList :: l a -> [a]

instance Listable List where
    toList (List Nothing) = []
    toList (List (Just (x, xs)) = x : toList xs

Or you can just derive Foldable and get toList for free:

newtype List a = List (Maybe (a, List a)) deriving Foldable

ghci> import Data.Foldable
ghci> toList $ List (Just ('a', List Nothing))
"a"