Documentation

The class of type of * -> * which may be empty. There is only one law for HasEmpty and its enforced by the type. The law is that there exists a value in type f for which a is fully generic. This alone is often enough to uniquely determine the value for a given, data type.

Additional laws for HasEmpty work backwards, empty has a relationships with other classes. So if f is a member of one of these classes, the following should hold.

Functor identity

g <$> empty = empty

Applicative annihilation left

x <*> empty = empty

Applicative annihilation right

empty <*> x = empty

Monad identity

empty >>= f = empty

Alternative empty

empty = Alternative.empty

MonadPlus mzero

empty = mzero

Foldable identity

foldr f x empty = empty