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-----------------------------------------------------------------------------
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-- Module : Data.Traversable
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-- Copyright : Conor McBride and Ross Paterson 2005
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-- License : BSD-style (see the LICENSE file in the distribution)
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-- Maintainer : libraries@haskell.org
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-- Stability : experimental
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-- Portability : portable
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-- Class of data structures that can be traversed from left to right,
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-- performing an action on each element.
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-- * /Applicative Programming with Effects/,
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-- by Conor McBride and Ross Paterson, online at
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-- <http://www.soi.city.ac.uk/~ross/papers/Applicative.html>.
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-- * /The Essence of the Iterator Pattern/,
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-- by Jeremy Gibbons and Bruno Oliveira,
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-- in /Mathematically-Structured Functional Programming/, 2006, and online at
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-- <http://web.comlab.ox.ac.uk/oucl/work/jeremy.gibbons/publications/#iterator>.
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-- Note that the functions 'mapM' and 'sequence' generalize "Prelude"
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-- functions of the same names from lists to any 'Traversable' functor.
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-- To avoid ambiguity, either import the "Prelude" hiding these names
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-- or qualify uses of these function names with an alias for this module.
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module Data.Traversable (
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import Prelude hiding (mapM, sequence, foldr)
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import qualified Prelude (mapM, foldr)
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import Control.Applicative
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import Data.Foldable (Foldable())
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import Data.Monoid (Monoid)
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#if defined(__GLASGOW_HASKELL__)
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#elif defined(__HUGS__)
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#elif defined(__NHC__)
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-- | Functors representing data structures that can be traversed from
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-- Minimal complete definition: 'traverse' or 'sequenceA'.
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-- Instances are similar to 'Functor', e.g. given a data type
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-- > data Tree a = Empty | Leaf a | Node (Tree a) a (Tree a)
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-- a suitable instance would be
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-- > instance Traversable Tree where
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-- > traverse f Empty = pure Empty
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-- > traverse f (Leaf x) = Leaf <$> f x
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-- > traverse f (Node l k r) = Node <$> traverse f l <*> f k <*> traverse f r
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-- This is suitable even for abstract types, as the laws for '<*>'
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-- imply a form of associativity.
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-- The superclass instances should satisfy the following:
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-- * In the 'Functor' instance, 'fmap' should be equivalent to traversal
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-- with the identity applicative functor ('fmapDefault').
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-- * In the 'Foldable' instance, 'Data.Foldable.foldMap' should be
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-- equivalent to traversal with a constant applicative functor
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-- ('foldMapDefault').
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class (Functor t, Foldable t) => Traversable t where
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-- | Map each element of a structure to an action, evaluate
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-- these actions from left to right, and collect the results.
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traverse :: Applicative f => (a -> f b) -> t a -> f (t b)
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traverse f = sequenceA . fmap f
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-- | Evaluate each action in the structure from left to right,
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-- and collect the results.
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sequenceA :: Applicative f => t (f a) -> f (t a)
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sequenceA = traverse id
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-- | Map each element of a structure to a monadic action, evaluate
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-- these actions from left to right, and collect the results.
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mapM :: Monad m => (a -> m b) -> t a -> m (t b)
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mapM f = unwrapMonad . traverse (WrapMonad . f)
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-- | Evaluate each monadic action in the structure from left to right,
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-- and collect the results.
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sequence :: Monad m => t (m a) -> m (t a)
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-- instances for Prelude types
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instance Traversable Maybe where
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traverse _ Nothing = pure Nothing
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traverse f (Just x) = Just <$> f x
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instance Traversable [] where
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{-# INLINE traverse #-} -- so that traverse can fuse
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traverse f = Prelude.foldr cons_f (pure [])
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where cons_f x ys = (:) <$> f x <*> ys
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instance Ix i => Traversable (Array i) where
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traverse f arr = listArray (bounds arr) `fmap` traverse f (elems arr)
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-- | 'for' is 'traverse' with its arguments flipped.
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for :: (Traversable t, Applicative f) => t a -> (a -> f b) -> f (t b)
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-- | 'forM' is 'mapM' with its arguments flipped.
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forM :: (Traversable t, Monad m) => t a -> (a -> m b) -> m (t b)
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-- left-to-right state transformer
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newtype StateL s a = StateL { runStateL :: s -> (s, a) }
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instance Functor (StateL s) where
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fmap f (StateL k) = StateL $ \ s ->
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let (s', v) = k s in (s', f v)
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instance Applicative (StateL s) where
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pure x = StateL (\ s -> (s, x))
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StateL kf <*> StateL kv = StateL $ \ s ->
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-- |The 'mapAccumL' function behaves like a combination of 'fmap'
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-- and 'foldl'; it applies a function to each element of a structure,
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-- passing an accumulating parameter from left to right, and returning
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-- a final value of this accumulator together with the new structure.
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mapAccumL :: Traversable t => (a -> b -> (a, c)) -> a -> t b -> (a, t c)
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mapAccumL f s t = runStateL (traverse (StateL . flip f) t) s
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-- right-to-left state transformer
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newtype StateR s a = StateR { runStateR :: s -> (s, a) }
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instance Functor (StateR s) where
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fmap f (StateR k) = StateR $ \ s ->
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let (s', v) = k s in (s', f v)
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instance Applicative (StateR s) where
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pure x = StateR (\ s -> (s, x))
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StateR kf <*> StateR kv = StateR $ \ s ->
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-- |The 'mapAccumR' function behaves like a combination of 'fmap'
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-- and 'foldr'; it applies a function to each element of a structure,
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-- passing an accumulating parameter from right to left, and returning
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-- a final value of this accumulator together with the new structure.
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mapAccumR :: Traversable t => (a -> b -> (a, c)) -> a -> t b -> (a, t c)
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mapAccumR f s t = runStateR (traverse (StateR . flip f) t) s
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-- | This function may be used as a value for `fmap` in a `Functor` instance.
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fmapDefault :: Traversable t => (a -> b) -> t a -> t b
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fmapDefault f = getId . traverse (Id . f)
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-- | This function may be used as a value for `Data.Foldable.foldMap`
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-- in a `Foldable` instance.
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foldMapDefault :: (Traversable t, Monoid m) => (a -> m) -> t a -> m
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foldMapDefault f = getConst . traverse (Const . f)
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newtype Id a = Id { getId :: a }
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instance Functor Id where
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fmap f (Id x) = Id (f x)
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instance Applicative Id where
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Id f <*> Id x = Id (f x)