1 -----------------------------------------------------------------------------
3 -- Module : Data.Traversable
4 -- Copyright : Conor McBride and Ross Paterson 2005
5 -- License : BSD-style (see the LICENSE file in the distribution)
7 -- Maintainer : ross@soi.city.ac.uk
8 -- Stability : experimental
9 -- Portability : portable
11 -- Class of data structures that can be traversed from left to right,
12 -- performing an action on each element.
16 -- * /Applicative Programming with Effects/,
17 -- by Conor McBride and Ross Paterson, online at
18 -- <http://www.soi.city.ac.uk/~ross/papers/Applicative.html>.
20 -- * /The Essence of the Iterator Pattern/,
21 -- by Jeremy Gibbons and Bruno Oliveira,
22 -- in /Mathematically-Structured Functional Programming/, 2006, and online at
23 -- <http://web.comlab.ox.ac.uk/oucl/work/jeremy.gibbons/publications/#iterator>.
25 -- Note that the functions 'mapM' and 'sequence' generalize "Prelude"
26 -- functions of the same names from lists to any 'Traversable' functor.
27 -- To avoid ambiguity, either import the "Prelude" hiding these names
28 -- or qualify uses of these function names with an alias for this module.
30 module Data.Traversable (
38 import Prelude hiding (mapM, sequence, foldr)
39 import qualified Prelude (mapM, foldr)
40 import Control.Applicative
41 import Data.Foldable (Foldable())
42 import Data.Monoid (Monoid)
44 -- | Functors representing data structures that can be traversed from
47 -- Minimal complete definition: 'traverse' or 'sequenceA'.
49 -- Instances are similar to 'Functor', e.g. given a data type
51 -- > data Tree a = Empty | Leaf a | Node (Tree a) a (Tree a)
53 -- a suitable instance would be
55 -- > instance Traversable Tree
56 -- > traverse f Empty = pure Empty
57 -- > traverse f (Leaf x) = Leaf <$> f x
58 -- > traverse f (Node l k r) = Node <$> traverse f l <*> f k <*> traverse f r
60 -- This is suitable even for abstract types, as the laws for '<*>'
61 -- imply a form of associativity.
63 -- The superclass instances should satisfy the following:
65 -- * In the 'Functor' instance, 'fmap' should be equivalent to traversal
66 -- with the identity applicative functor ('fmapDefault').
68 -- * In the 'Foldable' instance, 'Data.Foldable.foldMap' should be
69 -- equivalent to traversal with a constant applicative functor
70 -- ('foldMapDefault').
72 class (Functor t, Foldable t) => Traversable t where
73 -- | Map each element of a structure to an action, evaluate
74 -- these actions from left to right, and collect the results.
75 traverse :: Applicative f => (a -> f b) -> t a -> f (t b)
76 traverse f = sequenceA . fmap f
78 -- | Evaluate each action in the structure from left to right,
79 -- and collect the results.
80 sequenceA :: Applicative f => t (f a) -> f (t a)
81 sequenceA = traverse id
83 -- | Map each element of a structure to a monadic action, evaluate
84 -- these actions from left to right, and collect the results.
85 mapM :: Monad m => (a -> m b) -> t a -> m (t b)
86 mapM f = unwrapMonad . traverse (WrapMonad . f)
88 -- | Evaluate each monadic action in the structure from left to right,
89 -- and collect the results.
90 sequence :: Monad m => t (m a) -> m (t a)
93 -- instances for Prelude types
95 instance Traversable Maybe where
96 traverse f Nothing = pure Nothing
97 traverse f (Just x) = Just <$> f x
99 instance Traversable [] where
100 traverse f = Prelude.foldr cons_f (pure [])
101 where cons_f x ys = (:) <$> f x <*> ys
107 -- | 'for' is 'traverse' with its arguments flipped.
108 for :: (Traversable t, Applicative f) => t a -> (a -> f b) -> f (t b)
112 -- | 'forM' is 'mapM' with its arguments flipped.
113 forM :: (Traversable t, Monad m) => t a -> (a -> m b) -> m (t b)
117 -- | This function may be used as a value for `fmap` in a `Functor` instance.
118 fmapDefault :: Traversable t => (a -> b) -> t a -> t b
119 fmapDefault f = getId . traverse (Id . f)
121 -- | This function may be used as a value for `Data.Foldable.foldMap`
122 -- in a `Foldable` instance.
123 foldMapDefault :: (Traversable t, Monoid m) => (a -> m) -> t a -> m
124 foldMapDefault f = getConst . traverse (Const . f)
128 newtype Id a = Id { getId :: a }
130 instance Functor Id where
131 fmap f (Id x) = Id (f x)
133 instance Applicative Id where
135 Id f <*> Id x = Id (f x)