Is it possible to define a function on a subset of an existing type?

Question

I\'m new to Haskell and would like to know whether it\'s possible to define a function that is only defined on a subset of an already existing type, without actually having

Answer 1

Wrap the subset in a newtype

newtype EvenInteger = EvenInteger {
    unEvenInteger :: Integer
} deriving (Show, Eq, Ord, Num)

mkEvenInteger :: Integer -> Maybe EvenInteger
mkEvenInteger n = case n % 2 of
    0 -> Just $ EvenInteger n
    _ -> Nothing

squared :: EvenInteger -> EvenInteger
squared n = n * n