[project @ 1997-11-24 17:49:28 by simonm]

[ghc-hetmet.git] / ghc / lib / ghc / PrelMaybe.lhs

diff --git a/ghc/lib/ghc/PrelMaybe.lhs b/ghc/lib/ghc/PrelMaybe.lhs
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+%
+% (c) The GRASP/AQUA Project, Glasgow University, 1992-1996
+%
+\section[PrelMaybe]{Module @PrelMaybe@}
+
+The @Maybe@ type.
+
+\begin{code}
+{-# OPTIONS -fno-implicit-prelude #-}
+
+module PrelMaybe where
+
+import PrelBase
+
+data  Maybe a  =  Nothing | Just a     deriving (Eq, Ord, Show {- Read -})
+
+maybe :: b -> (a -> b) -> Maybe a -> b
+maybe n f Nothing  = n
+maybe n f (Just x) = f x
+
+instance  Functor Maybe  where
+    map f Nothing       = Nothing
+    map f (Just a)      = Just (f a)
+
+instance  Monad Maybe  where
+    (Just x) >>= k      = k x
+    Nothing  >>= k      = Nothing
+
+    (Just x) >>  k      = k
+    Nothing  >>  k      = Nothing
+
+    return              = Just
+
+instance  MonadZero Maybe  where
+    zero                = Nothing
+
+instance  MonadPlus Maybe  where
+    Nothing ++ ys       = ys
+    xs      ++ ys       = xs
+\end{code}
+
+
+
+