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module Maybe( isJust, isNothing, fromJust, fromMaybe, listToMaybe, maybeToList, catMaybes, mapMaybe, -- ...and what the Prelude exports Maybe(Nothing, Just), maybe ) where isJust, isNothing :: Maybe a -> Bool fromJust :: Maybe a -> a fromMaybe :: a -> Maybe a -> a listToMaybe :: [a] -> Maybe a maybeToList :: Maybe a -> [a] catMaybes :: [Maybe a] -> [a] mapMaybe :: (a -> Maybe b) -> [a] -> [b] |
The type constructor Maybe is defined in Prelude as
data Maybe a = Nothing | Just a
The purpose of the Maybe type is to provide a method of dealing with
illegal or optional values without terminating the program, as would
happen if error were used, and without using IOError from the IO
monad, which would cause the expression to become monadic. A correct
result is encapsulated by wrapping it in Just; an incorrect result
is returned as Nothing.
Other operations on Maybe are provided as part of the monadic classes in the Prelude.