[project @ 2002-04-24 16:31:37 by simonmar]

[ghc-base.git] / Data / Maybe.hs

diff --git a/Data/Maybe.hs b/Data/Maybe.hs
index de5d121..36e684e 100644 (file)
--- a/Data/Maybe.hs
+++ b/Data/Maybe.hs
@@ -1,6 +1,6 @@
 {-# OPTIONS -fno-implicit-prelude #-}
 -----------------------------------------------------------------------------
--- 
+-- |
 -- Module      :  Data.Maybe
 -- Copyright   :  (c) The University of Glasgow 2001
 -- License     :  BSD-style (see the file libraries/core/LICENSE)
@@ -9,7 +9,7 @@
 -- Stability   :  experimental
 -- Portability :  portable
 --
--- $Id: Maybe.hs,v 1.2 2001/07/03 11:37:50 simonmar Exp $
+-- $Id: Maybe.hs,v 1.4 2002/04/24 16:31:39 simonmar Exp $
 --
 -- The Maybe type, and associated operations.
 --
@@ -33,12 +33,36 @@ module Data.Maybe
    ) where
 
 #ifdef __GLASGOW_HASKELL__
-import GHC.Err ( error )
-import GHC.List
-import GHC.Maybe
+import {-# SOURCE #-} GHC.Err ( error )
 import GHC.Base
 #endif
 
+-- ---------------------------------------------------------------------------
+-- The Maybe type, and instances
+
+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
+
 isJust         :: Maybe a -> Bool
 isJust Nothing = False
 isJust _       = True