Avoid using deprecated flags
[packages/base.git] / Data / Either.hs
1 {-# OPTIONS_GHC -XNoImplicitPrelude #-}
2 -----------------------------------------------------------------------------
3 -- |
4 -- Module : Data.Either
5 -- Copyright : (c) The University of Glasgow 2001
6 -- License : BSD-style (see the file libraries/base/LICENSE)
7 --
8 -- Maintainer : libraries@haskell.org
9 -- Stability : experimental
10 -- Portability : portable
11 --
12 -- The Either type, and associated operations.
13 --
14 -----------------------------------------------------------------------------
15
16 module Data.Either (
17 Either(..),
18 either, -- :: (a -> c) -> (b -> c) -> Either a b -> c
19 lefts, -- :: [Either a b] -> [a]
20 rights, -- :: [Either a b] -> [b]
21 partitionEithers, -- :: [Either a b] -> ([a],[b])
22 ) where
23
24 import Data.Tuple
25
26 #ifdef __GLASGOW_HASKELL__
27 import GHC.Base
28
29 {-
30 -- just for testing
31 import Test.QuickCheck
32 -}
33
34 {-|
35
36 The 'Either' type represents values with two possibilities: a value of
37 type @'Either' a b@ is either @'Left' a@ or @'Right' b@.
38
39 The 'Either' type is sometimes used to represent a value which is
40 either correct or an error; by convention, the 'Left' constructor is
41 used to hold an error value and the 'Right' constructor is used to
42 hold a correct value (mnemonic: \"right\" also means \"correct\").
43 -}
44 data Either a b = Left a | Right b deriving (Eq, Ord )
45
46 -- | Case analysis for the 'Either' type.
47 -- If the value is @'Left' a@, apply the first function to @a@;
48 -- if it is @'Right' b@, apply the second function to @b@.
49 either :: (a -> c) -> (b -> c) -> Either a b -> c
50 either f _ (Left x) = f x
51 either _ g (Right y) = g y
52 #endif /* __GLASGOW_HASKELL__ */
53
54 -- | Extracts from a list of 'Either' all the 'Left' elements
55 -- All the 'Left' elements are extracted in order.
56
57 lefts :: [Either a b] -> [a]
58 lefts x = [a | Left a <- x]
59
60 -- | Extracts from a list of 'Either' all the 'Right' elements
61 -- All the 'Right' elements are extracted in order.
62
63 rights :: [Either a b] -> [b]
64 rights x = [a | Right a <- x]
65
66 -- | Partitions a list of 'Either' into two lists
67 -- All the 'Left' elements are extracted, in order, to the first
68 -- component of the output. Similarly the 'Right' elements are extracted
69 -- to the second component of the output.
70
71 partitionEithers :: [Either a b] -> ([a],[b])
72 partitionEithers = foldr (either left right) ([],[])
73 where
74 left a (l, r) = (a:l, r)
75 right a (l, r) = (l, a:r)
76
77 {-
78 {--------------------------------------------------------------------
79 Testing
80 --------------------------------------------------------------------}
81 prop_partitionEithers :: [Either Int Int] -> Bool
82 prop_partitionEithers x =
83 partitionEithers x == (lefts x, rights x)
84 -}
85