IsNull

A typeclass to determine if a given container is null.

Strongly inspired by mono-traversable, but with a simpler goal: supporting IsNull and nested IsNull operations.

Supported functions:

isNull :: a -> Bool
notNull :: a -> Bool
-- | Nested isNull
isNullN :: (Foldable f) => f a -> Bool
-- | Monadic isNull
isNullM :: Monad m => m a -> m Bool
-- | Monadic Nested isNull
isNullNM :: (Monad m, Foldable f) => m (f a) -> m Bool
-- | Alternative <|> operator does not always operate as choice,
-- at least not in an intuitive way (for example with lists).
-- This one does:
<\> :: a -> a -> a

The N stands for (non-recursive) nested, such that:

isNullN (Just "abc") == False
isNullN (Just ""   ) == True
isNullN (Nothing   ) == True

While the isNull function is equivalent to (==) mempty for most of the instances, not all Foldables are monoids, and not all monoids mempty means null:

• Either is an example of a Foldable which is not a Monoid, but where it makes sense to consider a Left as an 'Null' value. While this is not strictly true, the Left option does carries a value, we take the more liberal approach: Empty ~ Null ~ Invalid Value. If you need proper type reasoning, you should not be using this package, just regular pattern matching instead.

• Product Monoid instance is 1. Hardly qualifies as an Empty or Null value. For this reason no default implementation is provided for the Monoid class. It's up to you to use (==) mempty instead.

Bugs, suggestions and comments are most welcomed!