-{-# OPTIONS -fno-implicit-prelude #-}
+{-# LANGUAGE CPP, NoImplicitPrelude, DeriveRepresentable #-}
+
-----------------------------------------------------------------------------
---
+-- |
-- Module : Data.Maybe
-- Copyright : (c) The University of Glasgow 2001
--- License : BSD-style (see the file libraries/core/LICENSE)
+-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
--- Stability : experimental
+-- Stability : stable
-- Portability : portable
--
--- $Id: Maybe.hs,v 1.3 2001/07/03 14:13:32 simonmar Exp $
---
-- The Maybe type, and associated operations.
--
-----------------------------------------------------------------------------
module Data.Maybe
(
Maybe(Nothing,Just)-- instance of: Eq, Ord, Show, Read,
- -- Functor, Monad, MonadPlus
+ -- Functor, Monad, MonadPlus
- , maybe -- :: b -> (a -> b) -> Maybe a -> b
+ , maybe -- :: b -> (a -> b) -> Maybe a -> b
- , isJust -- :: Maybe a -> Bool
- , isNothing -- :: Maybe a -> Bool
- , fromJust -- :: Maybe a -> a
- , fromMaybe -- :: a -> Maybe a -> a
+ , 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]
+ , maybeToList -- :: Maybe a -> [a]
+ , catMaybes -- :: [Maybe a] -> [a]
+ , mapMaybe -- :: (a -> Maybe b) -> [a] -> [b]
) where
#ifdef __GLASGOW_HASKELL__
-import {-# SOURCE #-} GHC.Err ( error )
import GHC.Base
+import GHC.Generics (Representable0)
#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
-data Maybe a = Nothing | Just a deriving (Eq, Ord)
+-- | 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, Representable0)
instance Functor Maybe where
fmap _ Nothing = Nothing
Nothing >> _ = Nothing
return = Just
- fail _ = Nothing
+ 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) =
Nothing -> rs
Just r -> r:rs
+#endif /* else not __NHC__ */
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