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
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
|
{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude #-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Functor
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : provisional
-- Portability : portable
--
-- Functors: uniform action over a parameterized type, generalizing the
-- 'Data.List.map' function on lists.
module Data.Functor
(
Functor(fmap),
(<$),
($>),
(<$>),
void,
) where
import GHC.Base ( Functor(..), flip )
-- $setup
-- Allow the use of Prelude in doctests.
-- >>> import Prelude hiding ((<$>))
infixl 4 <$>
-- | An infix synonym for 'fmap'.
--
-- The name of this operator is an allusion to '$'.
-- Note the similarities between their types:
--
-- > ($) :: (a -> b) -> a -> b
-- > (<$>) :: Functor f => (a -> b) -> f a -> f b
--
-- Whereas '$' is function application, '<$>' is function
-- application lifted over a 'Functor'.
--
-- ==== __Examples__
--
-- Convert from a @'Maybe' 'Int'@ to a @'Maybe' 'String'@ using 'show':
--
-- >>> show <$> Nothing
-- Nothing
-- >>> show <$> Just 3
-- Just "3"
--
-- Convert from an @'Either' 'Int' 'Int'@ to an @'Either' 'Int'@
-- 'String' using 'show':
--
-- >>> show <$> Left 17
-- Left 17
-- >>> show <$> Right 17
-- Right "17"
--
-- Double each element of a list:
--
-- >>> (*2) <$> [1,2,3]
-- [2,4,6]
--
-- Apply 'even' to the second element of a pair:
--
-- >>> even <$> (2,2)
-- (2,True)
--
(<$>) :: Functor f => (a -> b) -> f a -> f b
(<$>) = fmap
infixl 4 $>
-- | Flipped version of '<$'.
--
-- @since 4.7.0.0
--
-- ==== __Examples__
--
-- Replace the contents of a @'Maybe' 'Int'@ with a constant 'String':
--
-- >>> Nothing $> "foo"
-- Nothing
-- >>> Just 90210 $> "foo"
-- Just "foo"
--
-- Replace the contents of an @'Either' 'Int' 'Int'@ with a constant
-- 'String', resulting in an @'Either' 'Int' 'String'@:
--
-- >>> Left 8675309 $> "foo"
-- Left 8675309
-- >>> Right 8675309 $> "foo"
-- Right "foo"
--
-- Replace each element of a list with a constant 'String':
--
-- >>> [1,2,3] $> "foo"
-- ["foo","foo","foo"]
--
-- Replace the second element of a pair with a constant 'String':
--
-- >>> (1,2) $> "foo"
-- (1,"foo")
--
($>) :: Functor f => f a -> b -> f b
($>) = flip (<$)
-- | @'void' value@ discards or ignores the result of evaluation, such
-- as the return value of an 'System.IO.IO' action.
--
-- ==== __Examples__
--
-- Replace the contents of a @'Maybe' 'Int'@ with unit:
--
-- >>> void Nothing
-- Nothing
-- >>> void (Just 3)
-- Just ()
--
-- Replace the contents of an @'Either' 'Int' 'Int'@ with unit,
-- resulting in an @'Either' 'Int' '()'@:
--
-- >>> void (Left 8675309)
-- Left 8675309
-- >>> void (Right 8675309)
-- Right ()
--
-- Replace every element of a list with unit:
--
-- >>> void [1,2,3]
-- [(),(),()]
--
-- Replace the second element of a pair with unit:
--
-- >>> void (1,2)
-- (1,())
--
-- Discard the result of an 'System.IO.IO' action:
--
-- >>> mapM print [1,2]
-- 1
-- 2
-- [(),()]
-- >>> void $ mapM print [1,2]
-- 1
-- 2
--
void :: Functor f => f a -> f ()
void x = () <$ x
|
