Haskell void function

Question

I'm new to Haskell. In the Monad docs, there is an example use for the void function:

>>> void (Left 8675309)
Left 8675309
>>> void (Right 8675309)
Right ()

I'm having a tough time figuring out why this is the case.

I see that void is defined as: void x = () <$ x and (<$) = fmap . const but I cannot figure out why there is a difference between Left and Right.

Any hints?

Answer 1

Because fmap maps the value defined in the Right, not in the Left. Indeed, Either is defined as:

data Either a b = Left a | Right b

and the Functor of Either a is thus implemented as:

instance Functor (Either a) where
    fmap _ (Left x) = Left x
    fmap f (Right x) = Right (f x)

This makes sense, since the Functor expects a type of kind * -> *, and thus the fmap :: Functor f => (b -> c) -> f b -> f c makes a mapping for Either with fmap :: (b -> c) -> Either a b -> Either a c.

Since void is defined as void = fmap (const ()), this thus means that if we make an analysis per case, we see:

fmap (const ()) (Left x) = Left x
fmap (const ()) (Right x) = Right (const () x) = Right ()