The Haskell 98 Library Report: Maybe Utilities

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## 18`` Maybe Utilities

`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.

### 18.1`` Library `Maybe`

`
module Maybe(
isJust, isNothing,
fromJust, fromMaybe, listToMaybe, maybeToList,
catMaybes, mapMaybe,
-- ...and what the Prelude exports
Maybe(Nothing, Just),
maybe
) where
isJust :: Maybe a -> Bool
isJust (Just a) = True
isJust Nothing = False
isNothing :: Maybe a -> Bool
isNothing = not . isJust
fromJust :: Maybe a -> a
fromJust (Just a) = a
fromJust Nothing = error "Maybe.fromJust: Nothing"
fromMaybe :: a -> Maybe a -> a
fromMaybe d Nothing = d
fromMaybe d (Just a) = a
maybeToList :: Maybe a -> [a]
maybeToList Nothing = []
maybeToList (Just a) = [a]
listToMaybe :: [a] -> Maybe a
listToMaybe [] = Nothing
listToMaybe (a:_) = Just a
catMaybes :: [Maybe a] -> [a]
catMaybes ms = [ m | Just m <- ms ]
mapMaybe :: (a -> Maybe b) -> [a] -> [b]
mapMaybe f = catMaybes . map f

The Haskell 98 Report
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December 2002`