3f1ffad76562cb7ab74e796fff2fc264e8539f43
[packages/base.git] / Data / Maybe.hs
1 {-# OPTIONS -fno-implicit-prelude #-}
2 -----------------------------------------------------------------------------
3 --
4 -- Module : Data.Maybe
5 -- Copyright : (c) The University of Glasgow 2001
6 -- License : BSD-style (see the file libraries/core/LICENSE)
7 --
8 -- Maintainer : libraries@haskell.org
9 -- Stability : experimental
10 -- Portability : portable
11 --
12 -- $Id: Maybe.hs,v 1.3 2001/07/03 14:13:32 simonmar Exp $
13 --
14 -- The Maybe type, and associated operations.
15 --
16 -----------------------------------------------------------------------------
17
18 module Data.Maybe
19 (
20 Maybe(Nothing,Just)-- instance of: Eq, Ord, Show, Read,
21 -- Functor, Monad, MonadPlus
22
23 , maybe -- :: b -> (a -> b) -> Maybe a -> b
24
25 , isJust -- :: Maybe a -> Bool
26 , isNothing -- :: Maybe a -> Bool
27 , fromJust -- :: Maybe a -> a
28 , fromMaybe -- :: a -> Maybe a -> a
29 , listToMaybe -- :: [a] -> Maybe a
30 , maybeToList -- :: Maybe a -> [a]
31 , catMaybes -- :: [Maybe a] -> [a]
32 , mapMaybe -- :: (a -> Maybe b) -> [a] -> [b]
33 ) where
34
35 #ifdef __GLASGOW_HASKELL__
36 import {-# SOURCE #-} GHC.Err ( error )
37 import GHC.Base
38 #endif
39
40 -- ---------------------------------------------------------------------------
41 -- The Maybe type, and instances
42
43 data Maybe a = Nothing | Just a deriving (Eq, Ord)
44
45 instance Functor Maybe where
46 fmap _ Nothing = Nothing
47 fmap f (Just a) = Just (f a)
48
49 instance Monad Maybe where
50 (Just x) >>= k = k x
51 Nothing >>= _ = Nothing
52
53 (Just _) >> k = k
54 Nothing >> _ = Nothing
55
56 return = Just
57 fail _ = Nothing
58
59 -- ---------------------------------------------------------------------------
60 -- Functions over Maybe
61
62 maybe :: b -> (a -> b) -> Maybe a -> b
63 maybe n _ Nothing = n
64 maybe _ f (Just x) = f x
65
66 isJust :: Maybe a -> Bool
67 isJust Nothing = False
68 isJust _ = True
69
70 isNothing :: Maybe a -> Bool
71 isNothing Nothing = True
72 isNothing _ = False
73
74 fromJust :: Maybe a -> a
75 fromJust Nothing = error "Maybe.fromJust: Nothing" -- yuck
76 fromJust (Just x) = x
77
78 fromMaybe :: a -> Maybe a -> a
79 fromMaybe d x = case x of {Nothing -> d;Just v -> v}
80
81 maybeToList :: Maybe a -> [a]
82 maybeToList Nothing = []
83 maybeToList (Just x) = [x]
84
85 listToMaybe :: [a] -> Maybe a
86 listToMaybe [] = Nothing
87 listToMaybe (a:_) = Just a
88
89 catMaybes :: [Maybe a] -> [a]
90 catMaybes ls = [x | Just x <- ls]
91
92 mapMaybe :: (a -> Maybe b) -> [a] -> [b]
93 mapMaybe _ [] = []
94 mapMaybe f (x:xs) =
95 let rs = mapMaybe f xs in
96 case f x of
97 Nothing -> rs
98 Just r -> r:rs
99