Option type

Article summary

In programming languages (especially functional programming languages) and type theory, an option type or maybe type is a polymorphic type that represents encapsulation of an optional value; e.g., it is used as the return type of functions which may or may not return a meaningful value when they are applied. It consists of a constructor which either is empty (often named None or Nothing ), or which encapsulates the original data type A (often written Just A or Some A ). A distinct, but related concept outside of functional programming, which is popular in object-oriented programming, is called nullable types (often expressed as A? ). The core difference between option types and nullable types is that option types support nesting (e.g. Maybe (Maybe String) ≠ Maybe String ), while nullable types do not (e.g. String?? = String? ).

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