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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
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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