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{-# LANGUAGE Trustworthy #-}{-# LANGUAGE NoImplicitPrelude #-}------------------------------------------------------------------------------- |-- Module      :  Data.Maybe-- Copyright   :  (c) The University of Glasgow 2001-- License     :  BSD-style (see the file libraries/base/LICENSE)---- Maintainer  :  libraries@haskell.org-- Stability   :  stable-- Portability :  portable---- The Maybe type, and associated operations.-------------------------------------------------------------------------------moduleData.Maybe(Maybe(Nothing,Just),maybe,isJust,isNothing,fromJust,fromMaybe,listToMaybe,maybeToList,catMaybes,mapMaybe)whereimportGHC.Base-- $setup-- Allow the use of some Prelude functions in doctests.-- >>> import Prelude ( (*), odd, show, sum )-- ----------------------------------------------------------------------------- Functions over Maybe-- | The 'maybe' function takes a default value, a function, and a 'Maybe'-- value.  If the 'Maybe' value is 'Nothing', the function returns the-- default value.  Otherwise, it applies the function to the value inside-- the 'Just' and returns the result.---- ==== __Examples__---- Basic usage:---- >>> maybe False odd (Just 3)-- True---- >>> maybe False odd Nothing-- False---- Read an integer from a string using 'readMaybe'. If we succeed,-- return twice the integer; that is, apply @(*2)@ to it. If instead-- we fail to parse an integer, return @0@ by default:---- >>> import Text.Read ( readMaybe )-- >>> maybe 0 (*2) (readMaybe "5")-- 10-- >>> maybe 0 (*2) (readMaybe "")-- 0---- Apply 'show' to a @Maybe Int@. If we have @Just n@, we want to show-- the underlying 'Int' @n@. But if we have 'Nothing', we return the-- empty string instead of (for example) \"Nothing\":---- >>> maybe "" show (Just 5)-- "5"-- >>> maybe "" show Nothing-- ""--maybe::b->(a->b)->Maybea->bmayben_Nothing=nmaybe_f(Justx)=fx-- | The 'isJust' function returns 'True' iff its argument is of the-- form @Just _@.---- ==== __Examples__---- Basic usage:---- >>> isJust (Just 3)-- True---- >>> isJust (Just ())-- True---- >>> isJust Nothing-- False---- Only the outer constructor is taken into consideration:---- >>> isJust (Just Nothing)-- True--isJust::Maybea->BoolisJustNothing=FalseisJust_=True-- | The 'isNothing' function returns 'True' iff its argument is 'Nothing'.---- ==== __Examples__---- Basic usage:---- >>> isNothing (Just 3)-- False---- >>> isNothing (Just ())-- False---- >>> isNothing Nothing-- True---- Only the outer constructor is taken into consideration:---- >>> isNothing (Just Nothing)-- False--isNothing::Maybea->BoolisNothingNothing=TrueisNothing_=False-- | The 'fromJust' function extracts the element out of a 'Just' and-- throws an error if its argument is 'Nothing'.---- ==== __Examples__---- Basic usage:---- >>> fromJust (Just 1)-- 1---- >>> 2 * (fromJust (Just 10))-- 20---- >>> 2 * (fromJust Nothing)-- *** Exception: Maybe.fromJust: Nothing--fromJust::Maybea->afromJustNothing=errorWithoutStackTrace"Maybe.fromJust: Nothing"-- yuckfromJust(Justx)=x-- | The 'fromMaybe' function takes a default value and and 'Maybe'-- value.  If the 'Maybe' is 'Nothing', it returns the default values;-- otherwise, it returns the value contained in the 'Maybe'.---- ==== __Examples__---- Basic usage:---- >>> fromMaybe "" (Just "Hello, World!")-- "Hello, World!"---- >>> fromMaybe "" Nothing-- ""---- Read an integer from a string using 'readMaybe'. If we fail to-- parse an integer, we want to return @0@ by default:---- >>> import Text.Read ( readMaybe )-- >>> fromMaybe 0 (readMaybe "5")-- 5-- >>> fromMaybe 0 (readMaybe "")-- 0--fromMaybe::a->Maybea->afromMaybedx=casexof{Nothing->d;Justv->v}-- | The 'maybeToList' function returns an empty list when given-- 'Nothing' or a singleton list when not given 'Nothing'.---- ==== __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 Text.Read ( readMaybe )-- >>> sum $ maybeToList (readMaybe "3")-- 3-- >>> sum $ maybeToList (readMaybe "")-- 0--maybeToList::Maybea->[a]maybeToListNothing=[]maybeToList(Justx)=[x]-- | The 'listToMaybe' function returns 'Nothing' on an empty list-- or @'Just' a@ where @a@ is the first element of the list.---- ==== __Examples__---- Basic usage:---- >>> listToMaybe []-- Nothing---- >>> listToMaybe [9]-- Just 9---- >>> listToMaybe [1,2,3]-- Just 1---- Composing 'maybeToList' with 'listToMaybe' should be the identity-- on singleton/empty lists:---- >>> maybeToList $ listToMaybe [5]-- [5]-- >>> maybeToList $ listToMaybe []-- []---- But not on lists with more than one element:---- >>> maybeToList $ listToMaybe [1,2,3]-- [1]--listToMaybe::[a]->MaybealistToMaybe=foldr(const.Just)Nothing{-# INLINElistToMaybe#-}-- We define listToMaybe using foldr so that it can fuse via the foldr/build-- rule. See #14387-- | The 'catMaybes' function takes a list of 'Maybe's and returns-- a list of all the 'Just' values.---- ==== __Examples__---- Basic usage:---- >>> catMaybes [Just 1, Nothing, Just 3]-- [1,3]---- When constructing a list of 'Maybe' values, 'catMaybes' can be used-- to return all of the \"success\" results (if the list is the result-- of a 'map', then 'mapMaybe' would be more appropriate):---- >>> import Text.Read ( readMaybe )-- >>> [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]-- [Just 1,Nothing,Just 3]-- >>> catMaybes $ [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]-- [1,3]--catMaybes::[Maybea]->[a]catMaybesls=[x|Justx<-ls]-- | The 'mapMaybe' function is a version of 'map' which can throw-- out elements.  In particular, the functional argument returns-- something of type @'Maybe' b@.  If this is 'Nothing', no element-- is added on to the result list.  If it is @'Just' b@, then @b@ is-- included in the result list.---- ==== __Examples__---- Using @'mapMaybe' f x@ is a shortcut for @'catMaybes' $ 'map' f x@-- in most cases:---- >>> import Text.Read ( readMaybe )-- >>> let readMaybeInt = readMaybe :: String -> Maybe Int-- >>> mapMaybe readMaybeInt ["1", "Foo", "3"]-- [1,3]-- >>> catMaybes $ map readMaybeInt ["1", "Foo", "3"]-- [1,3]---- If we map the 'Just' constructor, the entire list should be returned:---- >>> mapMaybe Just [1,2,3]-- [1,2,3]--mapMaybe::(a->Maybeb)->[a]->[b]mapMaybe_[]=[]mapMaybef(x:xs)=letrs=mapMaybefxsincasefxofNothing->rsJustr->r:rs{-# NOINLINE[1]mapMaybe#-}{-# RULES"mapMaybe"[~1]forallfxs.mapMaybefxs=build(\cn->foldr(mapMaybeFBcf)nxs)"mapMaybeList"[1]forallf.foldr(mapMaybeFB(:)f)[]=mapMaybef#-}{-# INLINE[0]mapMaybeFB#-}-- See Note [Inline FB functions] in GHC.ListmapMaybeFB::(b->r->r)->(a->Maybeb)->a->r->rmapMaybeFBconsfxnext=casefxofNothing->nextJustr->consrnext