{-# OPTIONS -fno-implicit-prelude #-}
-----------------------------------------------------------------------------
---
+-- |
-- 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
-- Portability : portable
--
--- $Id: Maybe.hs,v 1.2 2001/07/03 11:37:50 simonmar Exp $
---
-- The Maybe type, and associated operations.
--
-----------------------------------------------------------------------------
) where
#ifdef __GLASGOW_HASKELL__
-import GHC.Err ( error )
-import GHC.List
-import GHC.Maybe
+import {-# SOURCE #-} GHC.Err ( error )
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
+
+maybe :: b -> (a -> b) -> Maybe a -> b
+maybe n _ Nothing = n
+maybe _ f (Just x) = f x
+#endif /* __HUGS__ */
+
isJust :: Maybe a -> Bool
isJust Nothing = False
isJust _ = True
Nothing -> rs
Just r -> r:rs
+#endif /* else not __NHC__ */