Maybe
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package Data
The Maybe type encapsulates an optional value.
The Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as MJust a), or it is empty (represented as MNothing). Using Maybe is a good way to deal with errors or exceptional cases without resorting to drastic measures such as error.
The Maybe type is also a monad. It is a simple kind of error monad, where all errors are represented by Nothing.
This corresponds to the Data.Maybe in Haskell.
The fromMaybe function takes a default value and and Maybe value.
The fromMaybe function takes a default value and and Maybe value. If the Maybe is MNothing, it returns the default values; otherwise, it returns the value contained in the Maybe.
The isJust function returns True iff its argument is of the form MJust _.
The isNothing function returns True iff its argument is MNothing.
Constructors *
The listToMaybe function returns MNothing on an empty list or MJust a where a is the first element of the list.
The maybe function takes a default value, a function, and a Maybe value.
The maybe function takes a default value, a function, and a Maybe value. If the Maybe value is MNothing, the function returns the default value. Otherwise, it applies the function to the value inside the MJust and returns the result.
The maybeToList function returns an empty list when given Nothing or a singleton list when not given Nothing.
The maybeToOption function returns Empty on MNothing and returns Some on MJust
The optionToMaybe function returns MNothing On Empty and returns MJust on Some