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If you haven't figured this out since you asked -- it's a matter of confusing (IMO bad) variable names. Check the data definition:

data Validation a b
= Failure a
| Success b
deriving (Eq, Show)

Failures are always type a, and successes are always type b. The type variables used in the first line correspond to these. But in the definitions of (<>), they are just local values. The instance could be rewritten like so:

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

Success x <> Success y = Success x

Failure x <> Success y = Success y

Success x <> Failure y = Success x

Failure x <> Failure y = Failure (x <> y)

On Thu, Jan 26, 2017 at 1:55 PM, Atrudyjane <atrudyjane@protonmail.com> wrote:

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)

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

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

Thank you,
Andrea

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