Learning Functional Programming in Go

Maybe

Maybe is a functor that maps every type to the same type with an additional Nothing value. Maybe is like an optional value (note that types are the objects in our category):

data Maybe a = Just a | Nothing

The value of Maybe Int can be either just a number, such as Just 2, or Nothing.

The Maybe type maps types to types. For example, it maps Char to Maybe Char. fmap, defined in the following snippet, shows how every a -> b function has a corresponding version, Maybe a -> Maybe b, which just returns Nothing when given Nothing and behaves normally otherwise:

instance Functor Maybe wherefmap f Nothing = Nothingfmap f (Just x) = Just (f x)