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constrained-categories Control.Category.Constrained.Prelude The Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as Just a), or it is empty (represented as Nothing). 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. A richer error monad can be built using the Either type.
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constrained-categories Control.Category.Hask The Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as Just a), or it is empty (represented as Nothing). 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. A richer error monad can be built using the Either type.
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copilot-language Copilot.Language.Prelude The Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as Just a), or it is empty (represented as Nothing). 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. A richer error monad can be built using the Either type.
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quaalude Essentials The Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as Just a), or it is empty (represented as Nothing). 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. A richer error monad can be built using the Either type.
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verset Verset The Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as Just a), or it is empty (represented as Nothing). 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. A richer error monad can be built using the Either type.
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xmonad-contrib XMonad.Config.Prime The Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as Just a), or it is empty (represented as Nothing). 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. A richer error monad can be built using the Either type.
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base Data.Maybe The maybeToList function returns an empty list when given Nothing or a singleton list when given Just.
Examples
Basic usage:>>> maybeToList (Just 7) [7]
>>> maybeToList Nothing []
One can use maybeToList to avoid pattern matching when combined with a function that (safely) works on lists:>>> import GHC.Internal.Text.Read ( readMaybe ) >>> sum $ maybeToList (readMaybe "3") 3 >>> sum $ maybeToList (readMaybe "") 0
maybeNew :: (a -> IO (Ptr b)) -> Maybe a -> IO (Ptr b)base Foreign.Marshal.Utils Allocate storage and marshal a storable value wrapped into a Maybe
maybePeek :: (Ptr a -> IO b) -> Ptr a -> IO (Maybe b)base Foreign.Marshal.Utils Convert a peek combinator into a one returning Nothing if applied to a nullPtr
maybeWith :: (a -> (Ptr b -> IO c) -> IO c) -> Maybe a -> (Ptr b -> IO c) -> IO cbase Foreign.Marshal.Utils Converts a withXXX combinator into one marshalling a value wrapped into a Maybe, using nullPtr to represent Nothing.