2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[Maybes]{The `Maybe' types and associated utility functions}
17 thenMaybe, seqMaybe, returnMaybe, failMaybe, catMaybes,
19 thenMaB, returnMaB, failMaB
23 #include "HsVersions.h"
25 import Maybe( catMaybes, mapMaybe )
31 %************************************************************************
33 \subsection[Maybe type]{The @Maybe@ type}
35 %************************************************************************
38 maybeToBool :: Maybe a -> Bool
39 maybeToBool Nothing = False
40 maybeToBool (Just x) = True
43 @catMaybes@ takes a list of @Maybe@s and returns a list of
44 the contents of all the @Just@s in it. @allMaybes@ collects
45 a list of @Justs@ into a single @Just@, returning @Nothing@ if there
49 allMaybes :: [Maybe a] -> Maybe [a]
50 allMaybes [] = Just []
51 allMaybes (Nothing : ms) = Nothing
52 allMaybes (Just x : ms) = case (allMaybes ms) of
54 Just xs -> Just (x:xs)
58 @firstJust@ takes a list of @Maybes@ and returns the
59 first @Just@ if there is one, or @Nothing@ otherwise.
62 firstJust :: [Maybe a] -> Maybe a
63 firstJust [] = Nothing
64 firstJust (Just x : ms) = Just x
65 firstJust (Nothing : ms) = firstJust ms
69 findJust :: (a -> Maybe b) -> [a] -> Maybe b
70 findJust f [] = Nothing
71 findJust f (a:as) = case f a of
72 Nothing -> findJust f as
77 expectJust :: String -> Maybe a -> a
78 {-# INLINE expectJust #-}
79 expectJust err (Just x) = x
80 expectJust err Nothing = error ("expectJust " ++ err)
86 seqMaybe :: Maybe a -> Maybe a -> Maybe a
87 seqMaybe (Just x) _ = Just x
88 seqMaybe Nothing my = my
90 thenMaybe :: Maybe a -> (a -> Maybe b) -> Maybe b
91 thenMaybe ma mb = case ma of
95 returnMaybe :: a -> Maybe a
101 orElse :: Maybe a -> a -> a
102 (Just x) `orElse` y = x
103 Nothing `orElse` y = y
107 %************************************************************************
109 \subsection[MaybeErr type]{The @MaybeErr@ type}
111 %************************************************************************
114 data MaybeErr val err = Succeeded val | Failed err
118 thenMaB :: MaybeErr val1 err -> (val1 -> MaybeErr val2 err) -> MaybeErr val2 err
124 returnMaB :: val -> MaybeErr val err
125 returnMaB v = Succeeded v
127 failMaB :: err -> MaybeErr val err