[Haskell-beginners] Traversable Instance of the Constant type

Atrudyjane atrudyjane at protonmail.com
Thu Apr 20 05:53:05 UTC 2017

Pretty sure I'm getting bogged down on something simple, but how does the traverse function actually embed the Constant type in the f context passed to it? Or is it the pure defined in the f context that's being called?

Thank you,

newtype Constant a b = Constant { getConstant :: a } deriving (Eq, Show)

instance Functor (Constant a) where
fmap f (Constant a) = Constant a

instance Monoid a => Applicative (Constant a) where
pure a = Constant { getConstant = mempty }
Constant a <*> Constant b = Constant { getConstant = a `mappend` b }

instance Foldable (Constant a) where
foldMap f (Constant a) = mempty

instance Traversable (Constant a) where
traverse f (Constant a) = pure $ Constant a

λ> traverse (\x -> Just x) (Constant 1)
Just (Constant {getConstant = 1})

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