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

One possibility would be

newtype Even n = Even n
getEven (Even n) = 2*n

squared :: Num n => Even n -> Even n
squared (Even n) = Even (2*n*n)