1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
|
-----------------------------------------------------------------------------
-- Standard Library: Operations on the Maybe datatype
--
-- Suitable for use with Hugs 1.4.
-----------------------------------------------------------------------------
module Maybe(
isJust, fromJust, fromMaybe, listToMaybe, maybeToList,
catMaybes, mapMaybe, unfoldr ) where
isJust :: Maybe a -> Bool
isJust (Just a) = True
isJust Nothing = False
fromJust :: Maybe a -> a
fromJust (Just a) = a
fromJust Nothing = error "Maybe.fromJust: Nothing"
fromMaybe :: a -> Maybe a -> a
fromMaybe d Nothing = d
fromMaybe d (Just a) = a
maybeToList :: Maybe a -> [a]
maybeToList Nothing = []
maybeToList (Just a) = [a]
listToMaybe :: [a] -> Maybe a
listToMaybe [] = Nothing
listToMaybe (a:as) = Just a
catMaybes :: [Maybe a] -> [a]
catMaybes ms = [ m | Just m <- ms ]
mapMaybe :: (a -> Maybe b) -> [a] -> [b]
mapMaybe f = catMaybes . map f
unfoldr :: ([a] -> Maybe ([a], a)) -> [a] -> ([a],[a])
unfoldr f x = case f x of
Nothing -> (x,[])
Just (x',y) -> let (ys,x'') = unfoldr f x'
in (x'',y:ys)
-----------------------------------------------------------------------------
|
