What is a Functor instance in Haskell?

What is a Functor instance in Haskell?

A Functor instance in Haskell is a type class that allows you to apply a function over wrapped values, using the fmap function.

Can you give me an example of a Functor instance?

Sure! For example, the Maybe type is a Functor. You can define its instance like this:

instance Functor Maybe where
    fmap _ Nothing  = Nothing
    fmap f (Just x) = Just (f x)

What does fmap do in this context?

fmap applies a function to the value inside the Maybe type, if it exists. If it's Nothing, it simply returns Nothing.

Are there other types that can be Functors?

Yes, many types can be Functors, such as lists, tuples, and even custom data types!