IsNull: A typeclass to determine if a given value is null.

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A typeclass to determine if a given foldable type (or other) is empty ~ null ~ invalid. The definition is intentionally vague, to cover types from Either to Text and Sets.

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Change log None available
Dependencies base (>=4.5 && <4.8), base-compat (>=0.5 && <0.6), bytestring (>=0.10.0 && <1.0), containers (>=0.5 && <0.6), text (>=0.11.3 && <1.3) [details]
License BSD-3-Clause
Author João Cristóvão
Category Data
Home page
Source repo head: git clone
Uploaded by jcristovao at 2014-05-08T09:40:55Z




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Readme for IsNull-

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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:

Bugs, suggestions and comments are most welcomed!