2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1996
4 \section[Maybes]{The `Maybe' types and associated utility functions}
7 #include "HsVersions.h"
10 -- Maybe(..), -- no, it's in 1.3
20 mkLookupFun, mkLookupFunDef,
31 #if __GLASGOW_HASKELL__ >= 204
32 import Maybe ( catMaybes, mapMaybe )
38 %************************************************************************
40 \subsection[Maybe type]{The @Maybe@ type}
42 %************************************************************************
45 maybeToBool :: Maybe a -> Bool
46 maybeToBool Nothing = False
47 maybeToBool (Just x) = True
50 @catMaybes@ takes a list of @Maybe@s and returns a list of
51 the contents of all the @Just@s in it. @allMaybes@ collects
52 a list of @Justs@ into a single @Just@, returning @Nothing@ if there
56 allMaybes :: [Maybe a] -> Maybe [a]
57 allMaybes [] = Just []
58 allMaybes (Nothing : ms) = Nothing
59 allMaybes (Just x : ms) = case (allMaybes ms) of
61 Just xs -> Just (x:xs)
63 #if __GLASGOW_HASKELL__ < 204
64 -- After 2.04 we get these from the library Maybe
65 catMaybes :: [Maybe a] -> [a]
67 catMaybes (Nothing : xs) = catMaybes xs
68 catMaybes (Just x : xs) = (x : catMaybes xs)
70 mapMaybe :: (a -> Maybe b) -> [a] -> [b]
72 mapMaybe f (x:xs) = case f x of
73 Just y -> y : mapMaybe f xs
74 Nothing -> mapMaybe f xs
78 @firstJust@ takes a list of @Maybes@ and returns the
79 first @Just@ if there is one, or @Nothing@ otherwise.
82 firstJust :: [Maybe a] -> Maybe a
83 firstJust [] = Nothing
84 firstJust (Just x : ms) = Just x
85 firstJust (Nothing : ms) = firstJust ms
89 findJust :: (a -> Maybe b) -> [a] -> Maybe b
90 findJust f [] = Nothing
91 findJust f (a:as) = case f a of
92 Nothing -> findJust f as
97 expectJust :: String -> Maybe a -> a
98 {-# INLINE expectJust #-}
99 expectJust err (Just x) = x
100 expectJust err Nothing = error ("expectJust " ++ err)
106 seqMaybe :: Maybe a -> Maybe a -> Maybe a
107 seqMaybe (Just x) _ = Just x
108 seqMaybe Nothing my = my
110 returnMaybe :: a -> Maybe a
120 @assocMaybe@ looks up in an assocation list, returning
121 @Nothing@ if it fails.
124 assocMaybe :: (Eq a) => [(a,b)] -> a -> Maybe b
130 lookup ((tv,ty):rest) = if key == tv then Just ty else lookup rest
133 @mkLookupFun eq alist@ is a function which looks up
134 its argument in the association list @alist@, returning a Maybe type.
135 @mkLookupFunDef@ is similar except that it is given a value to return
139 mkLookupFun :: (key -> key -> Bool) -- Equality predicate
140 -> [(key,val)] -- The assoc list
142 -> Maybe val -- The corresponding value
144 mkLookupFun eq alist s
145 = case [a | (s',a) <- alist, s' `eq` s] of
149 mkLookupFunDef :: (key -> key -> Bool) -- Equality predicate
150 -> [(key,val)] -- The assoc list
151 -> val -- Value to return on failure
153 -> val -- The corresponding value
155 mkLookupFunDef eq alist deflt s
156 = case [a | (s',a) <- alist, s' `eq` s] of
161 %************************************************************************
163 \subsection[MaybeErr type]{The @MaybeErr@ type}
165 %************************************************************************
168 data MaybeErr val err = Succeeded val | Failed err
172 thenMaB :: MaybeErr val1 err -> (val1 -> MaybeErr val2 err) -> MaybeErr val2 err
178 returnMaB :: val -> MaybeErr val err
179 returnMaB v = Succeeded v
181 failMaB :: err -> MaybeErr val err