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
|
-- |
-- Module : Data.Functor.Identity
-- Copyright : (c) Andy Gill 2001,
-- (c) Oregon Graduate Institute of Science and Technology 2001
-- License : BSD-style (see the file LICENSE)
--
-- Maintainer : ross@soi.city.ac.uk
-- Stability : experimental
-- Portability : portable
--
-- The identity functor and monad.
--
-- This trivial type constructor serves two purposes:
--
-- * It can be used with functions parameterized by functor or monad classes.
--
-- * It can be used as a base monad to which a series of monad
-- transformers may be applied to construct a composite monad.
-- Most monad transformer modules include the special case of
-- applying the transformer to 'Identity'. For example, @State s@
-- is an abbreviation for @StateT s 'Identity'@.
module Data.Functor.Identity (
Identity(..),
) where
import Control.Applicative
import Control.Monad
import Control.Monad.Fix
import Data.Foldable (Foldable(foldMap))
import Data.Traversable (Traversable(traverse))
-- | Identity functor and monad.
newtype Identity a = Identity { runIdentity :: a }
-- ---------------------------------------------------------------------------
-- Identity instances for Functor and Monad
instance Functor Identity where
fmap f m = Identity (f (runIdentity m))
instance Foldable Identity where
foldMap f (Identity x) = f x
instance Traversable Identity where
traverse f (Identity x) = Identity <$> f x
instance Applicative Identity where
pure a = Identity a
Identity f <*> Identity x = Identity (f x)
instance Monad Identity where
return a = Identity a
m >>= k = k (runIdentity m)
instance MonadFix Identity where
mfix f = Identity (fix (runIdentity . f))
|
