Data/Maybe.hs

{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE CPP, 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.
--
-----------------------------------------------------------------------------

module Data.Maybe
 (
 Maybe(Nothing,Just)-- instance of: Eq, Ord, Show, Read,
 -- Functor, Monad, MonadPlus

 , maybe -- :: b -> (a -> b) -> Maybe a -> b

 , isJust -- :: Maybe a -> Bool
 , 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]
 ) where

#ifdef __GLASGOW_HASKELL__
import GHC.Base
#endif

#ifdef __NHC__
import Prelude
import Prelude (Maybe(..), maybe)
import Maybe
 ( isJust
 , isNothing
 , fromJust
 , fromMaybe
 , listToMaybe
 , maybeToList
 , catMaybes
 , mapMaybe
 )
#else

#ifndef __HUGS__
-- ---------------------------------------------------------------------------
-- The Maybe type, and instances

-- | 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 'Data.Either.Either' type.

data Maybe a = Nothing | Just a
 deriving (Eq, Ord)

instance Functor Maybe where
 fmap _ Nothing = Nothing
 fmap f (Just a) = Just (f a)

instance Monad Maybe where
 (Just x) >>= k = k x
 Nothing >>= _ = Nothing

 (Just _) >> k = k
 Nothing >> _ = Nothing

 return = Just
 fail _ = Nothing

-- ---------------------------------------------------------------------------
-- 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.
maybe :: b -> (a -> b) -> Maybe a -> b
maybe n _ Nothing = n
maybe _ f (Just x) = f x
#endif /* __HUGS__ */

-- | The 'isJust' function returns 'True' iff its argument is of the
-- form @Just _@.
isJust :: Maybe a -> Bool
isJust Nothing = False
isJust _ = True

-- | The 'isNothing' function returns 'True' iff its argument is 'Nothing'.
isNothing :: Maybe a -> Bool
isNothing Nothing = True
isNothing _ = False

-- | The 'fromJust' function extracts the element out of a 'Just' and
-- throws an error if its argument is 'Nothing'.
fromJust :: Maybe a -> a
fromJust Nothing = error "Maybe.fromJust: Nothing" -- yuck
fromJust (Just x) = 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'.
fromMaybe :: a -> Maybe a -> a
fromMaybe d x = case x of {Nothing -> d;Just v -> v}

-- | The 'maybeToList' function returns an empty list when given
-- 'Nothing' or a singleton list when not given 'Nothing'.
maybeToList :: Maybe a -> [a]
maybeToList Nothing = []
maybeToList (Just x) = [x]

-- | The 'listToMaybe' function returns 'Nothing' on an empty list
-- or @'Just' a@ where @a@ is the first element of the list.
listToMaybe :: [a] -> Maybe a
listToMaybe [] = Nothing
listToMaybe (a:_) = Just a

-- | The 'catMaybes' function takes a list of 'Maybe's and returns
-- a list of all the 'Just' values. 
catMaybes :: [Maybe a] -> [a]
catMaybes ls = [x | Just x <- 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 just @'Just' b@, then @b@ is
-- included in the result list.
mapMaybe :: (a -> Maybe b) -> [a] -> [b]
mapMaybe _ [] = []
mapMaybe f (x:xs) =
 let rs = mapMaybe f xs in
 case f x of
 Nothing -> rs
 Just r -> r:rs

#endif /* else not __NHC__ */