Safe Haskell | Safe |
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The Maybe
type and operations
The Maybe
type encapsulates an optional value. A value of type
either contains a value of type Maybe
aa
(represented as
),
or it is empty (represented as Just
aNothing
). 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.
listToMaybe :: [a] -> Maybe aSource
The listToMaybe
function returns Nothing
on an empty list
or
where Just
aa
is the first element of the list.
maybeToList :: Maybe a -> [a]Source
The maybeToList
function returns an empty list when given
Nothing
or a singleton list when not given Nothing
.
Specification
module Data.Maybe( Maybe(Nothing, Just), isJust, isNothing, fromJust, fromMaybe, listToMaybe, maybeToList, catMaybes, mapMaybe, maybe ) where maybe :: b -> (a -> b) -> Maybe a -> b maybe n _ Nothing = n maybe _ f (Just x) = f x 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