Question

I would like to have a function that checks a property of a class instance:

class ListWithAtLeastOneElement a where
   list :: [a]
   check :: Bool
   check = (length list) >= 1

but I get this error when compiling in GHC:
“The class method ‘check’ mentions none of the type variables of the class ListWithAtLeastOneElement a When checking the class method: check :: Bool In the class declaration for ‘ListWithAtLeastOneElement'”

Is there a better way of doing what I want, or a way to get this to compile in GHC?

Answer 1

You seem to think a class in Haskell is like a class in an OO-language. That is not true. You should use a data type or newtype or type synonym.

newtype NonemptyList a = List1 [a]

fromList :: [a] -> Maybe (NonemptyList a)
fromList []       = Nothing
fromList xs@(_:_) = Just $ List1 xs

check :: NonemptyList a -> Bool
check (List1 xs) = length xs >= 1