A typeclass to determine if a given value is null.

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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!

Used by 1 package:
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