In Haskell, we can check if a value is null/true/false using built-in functions such as null, True, and False.
1. To check if a list is empty (i.e. null), we can use the null function. It returns True if the list is empty and False otherwise. For example:
null [] -- returns True
null [1,2,3] -- returns False
2. To check if a boolean value is True or False, we can simply use the True and False keywords. For example:
True -- returns True, obviously
False -- returns False, obviously
3. To check if a value is Nothing (i.e. null), we can use pattern matching with the Maybe type. For example:
case maybeValue of
Just x -> -- do something with x
Nothing -> -- handle null case
In the above example, maybeValue is a value of type Maybe a, where a is the type of the value we are checking. If maybeValue is Nothing, it will match with the second pattern and we can handle the null case appropriately.