Instance of Monad for Multiple Data Constructors

Question

I've started Haskell not too long ago, and am now teaching myself about monads, so I've come up with a dumb example for myself here to assist me in understanding an aspect of monads.

Given a type Certainty defined as

data Certainty a = Definitely a | Perhaps a | Nope

and an instance of Monad for said type

instance Monad Certainty where
  return = ...

how would the method return be defined for Certainty and each of its data constructors?

Answer 1

you have to pick one - based on the naming I would suggest

return value = Definitely value

remember that return a >>= f must be f a

bind should probably be

(Definitely a) >>= f = f a
(Perhaps a)    >>= f = case f a of
                          Definitely b -> Perhaps b
                          Perhaps b    -> Perhaps b
                          Nope         -> Nope
Nope           >>= _ = Nope