isNothing

isNothing is a function in the Haskell programming language that checks whether a value of type Maybe a is Nothing. It is part of the Data.Maybe module and is commonly used to test the absence of a value wrapped in a Maybe.

In Haskell, the Maybe type represents an optional value and has two constructors: Nothing and Just a. The isNothing function has the type Maybe a -> Bool and is defined by pattern matching on these constructors: isNothing Nothing = True; isNothing (Just _) = False. Thus, it returns True when the input is Nothing and False when it is Just something.

isNothing is the complement of isJust, another widely used function for Maybe values. While isNothing detects the absence of a value, isJust detects the presence of a value within Just.

Typical usage examples include evaluating whether a Maybe value indicates the absence of data, or guiding control flow in guards and case expressions. For instance, isNothing Nothing evaluates to True, whereas isNothing (Just 5) evaluates to False.

In practice, isNothing helps make code more explicit about handling missing values, reducing the need for verbose pattern matching in simple conditional checks. It is commonly paired with isJust in documentation and examples to illustrate the dichotomy between Nothing and Just. See also: isJust, Maybe, Data.Maybe.