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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
\end{code}
%*********************************************************
%* *
\subsection{Standard numeric classes}
%* *
%*********************************************************
\begin{code}
data Maybe a = Nothing | Just a deriving (Eq, Ord)
maybe :: b -> (a -> b) -> Maybe a -> b
maybe n _ Nothing = n
maybe _ f (Just x) = f x
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
\end{code}
%*********************************************************
%* *
\subsection{Standard numeric classes}
%* *
%*********************************************************
\begin{code}
data Either a b = Left a | Right b deriving (Eq, Ord )
either :: (a -> c) -> (b -> c) -> Either a b -> c
either f _ (Left x) = f x
either _ g (Right y) = g y
\end{code}
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