Haskell/Traversable
We already have studied four of the five type classes in the Prelude that can be used for data structure manipulation: Functor
, Applicative
, Monad
and Foldable
. The fifth one is Traversable
^{[1]}. To traverse means to walk across, and that is exactly what Traversable
generalises: walking across a structure, collecting results at each stop.
Functors made for walking
editIf traversing means walking across, though, we have been performing traversals for a long time already. Consider the following plausible Functor
and Foldable
instances for lists:
instance Functor [] where
fmap _ [] = []
fmap f (x:xs) = f x : fmap f xs
instance Foldable [] where
foldMap _ [] = mempty
foldMap f (x:xs) = f x <> foldMap f xs
fmap f
walks across the list, applies f
to each element and collects the results by rebuilding the list. Similarly, foldMap f
walks across the list, applies f
to each element and collects the results by combining them with mappend
. Functor
and Foldable
, however, are not enough to express all useful ways of traversing. For instance, suppose we have the following Maybe
encoded test for negative numbers...
deleteIfNegative :: (Num a, Ord a) => a > Maybe a
deleteIfNegative x = if x < 0 then Nothing else Just x
... and we want to use it to implement...
rejectWithNegatives :: (Num a, Ord a) => [a] > Maybe [a]
... which gives back the original list wrapped in Just
if there are no negative elements in it, and Nothing
otherwise. Neither Foldable
nor Functor
on their own would help. Using Foldable
would replace the structure of the original list with that of whatever Monoid
we pick for folding, and there is no way of twisting that into giving either the original list or Nothing
^{[2]}. As for Functor
, fmap
might be attractive at first...
GHCi> let testList = [5,3,2,1,0]
GHCi> fmap deleteIfNegative testList
[Nothing,Just 3,Just 2,Nothing,Just 0]
... but then we would need a way to turn a list of Maybe
into Maybe
a list. If you squint hard enough, that looks somewhat like a fold. Instead, however, of merely combining the values and destroying the list, we need to combine the Maybe
contexts of the values and recreate the list structure within the combined context. Fortunately, there is a type class which is essentially about combining Functor
contexts: Applicative
^{[3]}. Applicative
, in turn, leads us to the class we need: Traversable
.
instance Traversable [] where
 sequenceA :: Applicative f => [f a] > f [a]
sequenceA [] = pure []
sequenceA (u:us) = (:) <$> u <*> sequenceA us
 Or, equivalently:
instance Traversable [] where
sequenceA us = foldr (\u v > (:) <$> u <*> v) (pure []) us
Traversable
is to Applicative
contexts what Foldable
is to Monoid
values. From that point of view, sequenceA
is analogous to fold
− it creates an applicative summary of the contexts within a structure, and then rebuilds the structure in the new context. sequenceA
is the function we were looking for:
GHCi> let rejectWithNegatives = sequenceA . fmap deleteIfNegative
GHCi> :t rejectWithNegatives
rejectWithNegatives
:: (Num a, Ord a, Traversable t) => t a > Maybe (t a)
GHCi> rejectWithNegatives testList
Nothing
GHCi> rejectWithNegatives [0..10]
Just [0,1,2,3,4,5,6,7,8,9,10]
These are the methods of Traversable
:
class (Functor t, Foldable t) => Traversable t where
traverse :: Applicative f => (a > f b) > t a > f (t b)
sequenceA :: Applicative f => t (f a) > f (t a)
 These methods have default definitions.
 They are merely specialised versions of the other two.
mapM :: Monad m => (a > m b) > t a > m (t b)
sequence :: Monad m => t (m a) > m (t a)
If sequenceA
is analogous to fold
, traverse
is analogous to foldMap
. They can be defined in terms of each other, and therefore a minimal implementation of Traversable
just needs to supply one of them:
traverse f = sequenceA . fmap f
sequenceA = traverse id
Rewriting the list instance using traverse
makes the parallels with Functor
and Foldable
obvious:
instance Traversable [] where
traverse _ [] = pure []
traverse f (x:xs) = (:) <$> f x <*> traverse f xs
 Or, equivalently:
instance Traversable [] where
traverse f xs = foldr (\x v > (:) <$> f x <*> v) (pure []) xs
In general, it is better to write traverse
when implementing Traversable
, as the default definition of traverse
performs, in principle, two runs across the structure (one for fmap
and another for sequenceA
).
We can cleanly define rejectWithNegatives
directly in terms of traverse
:
rejectWithNegatives :: (Num a, Ord a, Traversable t) => t a > Maybe (t a)
rejectWithNegatives = traverse deleteIfNegative
Exercises 


Interpretations of Traversable
edit
Traversable
structures can be walked over using the applicative functor of your choice. The type of traverse
...
traverse :: (Applicative f, Traversable t) => (a > f b) > t a > f (t b)
... resembles that of mapping functions we have seen in other classes. Rather than using its function argument to insert functorial contexts under the original structure (as might be done with fmap
) or to modify the structure itself (as (>>=)
does), traverse
adds an extra layer of context on the top of the structure. Said in another way, traverse
allows for effectful traversals − traversals which produce an overall effect (i.e. the new outer layer of context).
If the structure below the new layer is recoverable at all, it will match the original structure (the values might have changed, of course). Here is an example involving nested lists:
GHCi> traverse (\x > [0..x]) [0..2]
[[0,0,0],[0,0,1],[0,0,2],[0,1,0],[0,1,1],[0,1,2]]
To understand what is going on here, let's break this down step by step.
traverse (\x > [0..x]) [0..2]
sequenceA $ fmap (\x > [0..x]) [0..2]
sequenceA [[0],[0,1],[0,1,2]]
(:) <$> [0] <*> ((:) <$> [0,1] <*> ((:) <$> [0,1,2] <*> pure []))
(:) <$> [0] <*> ((:) <$> [0,1] <*> ([[0],[1],[2]]))
(:) <$> [0] <*> ([[0,0],[0,1],[0,2],[1,0],[1,1],[1,2]])
[[0,0,0],[0,0,1],[0,0,2],[0,1,0],[0,1,1],[0,1,2]]
The inner lists retain the structure of the original list − all of them have three elements. The outer list is the new layer, corresponding to the introduction of nondeterminism through allowing each element to vary from zero to its (original) value.
We can also understand Traversable
by focusing on sequenceA
and how it distributes context.
GHCi> sequenceA [[1,2,3,4],[5,6,7]]
[[1,5],[1,6],[1,7],[2,5],[2,6],[2,7]
,[3,5],[3,6],[3,7],[4,5],[4,6],[4,7]
]
In this example, sequenceA
can be seen distributing the old outer structure into the new outer structure, and so the new inner lists have two elements, just like the old outer list. The new outer structure is a list of twelve elements, which is exactly what you would expect from combining with (<*>)
one list of four elements with another of three elements. One interesting aspect of the distribution perspective is how it helps making sense of why certain functors cannot possibly have instances of Traversable
(how would one distribute an IO
action? Or a function?).
Exercises 

Having the applicative functors chapter fresh in memory can help with the following exercises.

The Traversable
laws
edit
Sensible instances of Traversable
have a set of laws to follow. There are the following two laws:
traverse Identity = Identity  identity
traverse (Compose . fmap g . f) = Compose . fmap (traverse g) . traverse f  composition
Plus a bonus law, which is guaranteed to hold:
 If t is an applicative homomorphism, then
t . traverse f = traverse (t . f)  naturality
Those laws are not exactly selfexplanatory, so let's have a closer look at them. Starting from the last one: an applicative homomorphism is a function which preserves the Applicative
operations, so that:
 Given a choice of f and g, and for any a,
t :: (Applicative f, Applicative g) => f a > g a
t (pure x) = pure x
t (x <*> y) = t x <*> t y
Note that not only this definition is analogous to the one of monoid homomorphisms which we have seen earlier on but also that the naturality law mirrors exactly the property about foldMap
and monoid homomorphisms seen in the chapter about Foldable
.
The identity law involves Identity
, the dummy functor:
newtype Identity a = Identity { runIdentity :: a }
instance Functor Identity where
fmap f (Identity x) = Identity (f x)
instance Applicative Identity where
pure x = Identity x
Identity f <*> Identity x = Identity (f x)
The law says that all traversing with the Identity
constructor does is wrap the structure with Identity
, which amounts to doing nothing (as the original structure can be trivially recovered with runIdentity
). The Identity
constructor is thus the identity traversal, which is very reasonable indeed.
The composition law, in turn, is stated in terms of the Compose
functor:
newtype Compose f g a = Compose { getCompose :: f (g a) }
instance (Functor f, Functor g) => Functor (Compose f g) where
fmap f (Compose x) = Compose (fmap (fmap f) x)
instance (Applicative f, Applicative g) => Applicative (Compose f g) where
pure x = Compose (pure (pure x))
Compose f <*> Compose x = Compose ((<*>) <$> f <*> x)
Compose
performs composition of functors. Composing two Functor
s results in a Functor
, and composing two Applicative
s results in an Applicative
^{[4]}. The instances are the obvious ones, threading the methods one further functorial layer down.
The composition law states that it doesn't matter whether we perform two traversals separately (right side of the equation) or compose them in order to walk across the structure only once (left side). It is analogous, for instance, to the second functor law. The fmap
s are needed because the second traversal (or the second part of the traversal, for the left side of the equation) happens below the layer of structure added by the first (part). Compose
is needed so that the composed traversal is applied to the correct layer.
Identity
and Compose
are available from Data.Functor.Identity and Data.Functor.Compose respectively.
The laws can also be formulated in terms of sequenceA
:
sequenceA . fmap Identity = Identity  identity
sequenceA . fmap Compose = Compose . fmap sequenceA . sequenceA  composition
 For any applicative homomorphism t:
t . sequenceA = sequenceA . fmap t  naturality
Though it's not immediately obvious, several desirable characteristics of traversals follow from the laws, including ^{[5]}:
 Traversals do not skip elements.
 Traversals do not visit elements more than once.
traverse pure = pure
 Traversals cannot modify the original structure (it is either preserved or fully destroyed).
Recovering fmap
and foldMap
edit
We still have not justified the Functor
and Foldable
class constraints of Traversable
. The reason for them is very simple: as long as the Traversable
instance follows the laws traverse
is enough to implement both fmap
and foldMap
. For fmap
, all we need is to use Identity
to make a traversal out of an arbitrary function:
fmap f = runIdentity . traverse (Identity . f)
To recover foldMap
, we need to introduce a third utility functor: Const
from ControlApplicative:
newtype Const a b = Const { getConst :: a }
instance Functor (Const a) where
fmap _ (Const x) = Const x
Const
is a constant functor. A value of type Const a b
does not actually contain a b
value. Rather, it holds an a
value which is unaffected by fmap
. For our current purposes, the truly interesting instance is the Applicative
one
instance Monoid a => Applicative (Const a) where
pure _ = Const mempty
Const x <*> Const y = Const (x `mappend` y)
(<*>)
simply combines the values in each context with mappend
^{[6]}. We can exploit that to make a traversal out of any Monoid m => a > m
function that we might pass to foldMap
. Thanks to the instance above, the traversal then becomes a fold:
foldMap f = getConst . traverse (Const . f)
We have just recovered from traverse
two functions which on the surface appear to be entirely different, and all we had to do was pick two different functors. That is a taste of how powerful an abstraction functors are ^{[7]}.
Notes
 ↑ Strictly speaking, we should refer to the five classes in the GHC Prelude, as
Applicative
,Foldable
andTraversable
aren't officially part of the Prelude yet according to the Haskell Report. It is just a matter of time for them to be included, though.  ↑ One thing to attempt would be exploiting the
Monoid a => Monoid (Maybe a)
instance from Data.Monoid. If you try that, however, you will see it can't possibly give the desired results.  ↑ The monoidal presentation of
Applicative
makes that very clear.  ↑ Remarkably, however, composing two
Monad
s does not necessarily result in aMonad
.  ↑ For technical details, check the papers cited by the Data.Traversable documentation.
 ↑ This is a great illustration of how
Applicative
combines contexts monoidally. If we remove the values within the context, the applicative laws in monoidal presentation match the monoid laws exactly.  ↑ A prime example, and one of clear practical relevance at that, is that great ode to functors, the lens library.