I'm currently studying semigroups and trying to figure out how to determine which type variables need a semigroup instance. Here are a couple of examples from Evan Cameron's github (https://github.com/leshow/haskell-programming-book/blob/master/src/Ch15ex.hs):

(1)

data Validation a b

  = Failure a

    | Success b

    deriving (Eq, Show)

instance Semigroup a => Semigroup (Validation a b) where

  Success a <> Success b = Success a

   Failure a <> Success b = Success b

   Success a <> Failure b = Success a

   Failure a <> Failure b = Failure (a <> b)

* Why doesn't 'b' need an instance of semigroup?

(2)

newtype AccumulateRight a b = AccumulateRight (Validation a b) deriving (Eq, Show)

instance Semigroup b => Semigroup (AccumulateRight a b) where

AccumulateRight (Success a) <>AccumulateRight (Failure b) =AccumulateRight (Success a)

AccumulateRight (Failure a) <>AccumulateRight (Success b) =AccumulateRight (Success b)

AccumulateRight (Failure a) <>AccumulateRight (Failure b) =AccumulateRight (Failure a)