%
+% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
-\section[Maybes]{The `Maybe' types and associated utility functions}
\begin{code}
module Maybes (
- module Maybe, -- Re-export all of Maybe
+ module Data.Maybe,
- MaybeErr(..), -- Instance of Monad
- failME,
+ MaybeErr(..), -- Instance of Monad
+ failME, isSuccess,
- orElse,
- mapCatMaybes,
- allMaybes,
- firstJust,
- expectJust,
- maybeToBool,
+ orElse,
+ mapCatMaybes,
+ allMaybes,
+ firstJust,
+ expectJust,
+ maybeToBool,
- thenMaybe, seqMaybe, returnMaybe, failMaybe
+ MaybeT(..)
) where
-#include "HsVersions.h"
-
-import Maybe
-
+import Data.Maybe
infixr 4 `orElse`
\end{code}
%************************************************************************
-%* *
+%* *
\subsection[Maybe type]{The @Maybe@ type}
-%* *
+%* *
%************************************************************************
\begin{code}
maybeToBool :: Maybe a -> Bool
maybeToBool Nothing = False
-maybeToBool (Just x) = True
+maybeToBool (Just _) = True
\end{code}
@catMaybes@ takes a list of @Maybe@s and returns a list of
-the contents of all the @Just@s in it. @allMaybes@ collects
+the contents of all the @Just@s in it. @allMaybes@ collects
a list of @Justs@ into a single @Just@, returning @Nothing@ if there
are any @Nothings@.
\begin{code}
allMaybes :: [Maybe a] -> Maybe [a]
allMaybes [] = Just []
-allMaybes (Nothing : ms) = Nothing
-allMaybes (Just x : ms) = case (allMaybes ms) of
- Nothing -> Nothing
- Just xs -> Just (x:xs)
+allMaybes (Nothing : _) = Nothing
+allMaybes (Just x : ms) = case allMaybes ms of
+ Nothing -> Nothing
+ Just xs -> Just (x:xs)
\end{code}
\begin{code}
firstJust :: [Maybe a] -> Maybe a
firstJust [] = Nothing
-firstJust (Just x : ms) = Just x
+firstJust (Just x : _) = Just x
firstJust (Nothing : ms) = firstJust ms
\end{code}
\begin{code}
expectJust :: String -> Maybe a -> a
{-# INLINE expectJust #-}
-expectJust err (Just x) = x
+expectJust _ (Just x) = x
expectJust err Nothing = error ("expectJust " ++ err)
\end{code}
\begin{code}
mapCatMaybes :: (a -> Maybe b) -> [a] -> [b]
-mapCatMaybes f [] = []
+mapCatMaybes _ [] = []
mapCatMaybes f (x:xs) = case f x of
- Just y -> y : mapCatMaybes f xs
- Nothing -> mapCatMaybes f xs
+ Just y -> y : mapCatMaybes f xs
+ Nothing -> mapCatMaybes f xs
\end{code}
-The Maybe monad
-~~~~~~~~~~~~~~~
\begin{code}
-seqMaybe :: Maybe a -> Maybe a -> Maybe a
-seqMaybe (Just x) _ = Just x
-seqMaybe Nothing my = my
+orElse :: Maybe a -> a -> a
+(Just x) `orElse` _ = x
+Nothing `orElse` y = y
+\end{code}
-thenMaybe :: Maybe a -> (a -> Maybe b) -> Maybe b
-thenMaybe ma mb = case ma of
- Just x -> mb x
- Nothing -> Nothing
+%************************************************************************
+%* *
+\subsection[MaybeT type]{The @MaybeT@ monad transformer}
+%* *
+%************************************************************************
-returnMaybe :: a -> Maybe a
-returnMaybe = Just
+\begin{code}
-failMaybe :: Maybe a
-failMaybe = Nothing
+newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)}
+
+instance Functor m => Functor (MaybeT m) where
+ fmap f x = MaybeT $ fmap (fmap f) $ runMaybeT x
+
+instance Monad m => Monad (MaybeT m) where
+ return = MaybeT . return . Just
+ x >>= f = MaybeT $ runMaybeT x >>= maybe (return Nothing) (runMaybeT . f)
+ fail _ = MaybeT $ return Nothing
-orElse :: Maybe a -> a -> a
-(Just x) `orElse` y = x
-Nothing `orElse` y = y
\end{code}
%************************************************************************
-%* *
+%* *
\subsection[MaybeErr type]{The @MaybeErr@ type}
-%* *
+%* *
%************************************************************************
\begin{code}
instance Monad (MaybeErr err) where
return v = Succeeded v
Succeeded v >>= k = k v
- Failed e >>= k = Failed e
+ Failed e >>= _ = Failed e
+
+isSuccess :: MaybeErr err val -> Bool
+isSuccess (Succeeded {}) = True
+isSuccess (Failed {}) = False
failME :: err -> MaybeErr err val
failME e = Failed e