arrow-core-data / arrow.typeclasses / Traverse

`interface Traverse<F> : `

`Functor`

`<F>, `

`Foldable`

`<F>`

In functional programming it is very common to encode “behaviors” as data types - common behaviors include Option for possibly missing values, Either and Validated for possible errors, and Ref(/docs/effects/ref/) for asynchronous and concurrent access and modification of its content.

These behaviors tend to show up in functions working on a single piece of data - for instance parsing a single String into an Int, validating a login, or asynchronously fetching website information for a user.

```
import arrow.fx.IO
interface Profile
interface User
fun userInfo(u: User): IO<Profile>
```

Each function asks only for the data it needs; in the case of `userInfo`

, a single `User`

. Indeed, we could write one that takes a `List<User>`

and fetches profile for all of them, but it would be a bit strange. If we just wanted to fetch the profile of only one user, we would either have to wrap it in a List or write a separate function that takes in a single user, nonetheless. More fundamentally, functional programming is about building lots of small, independent pieces and composing them to make larger and larger pieces - does it hold in this case?

Given just `(User) -> IO<Profile>`

, what should we do if we want to fetch profiles for a `List<User>`

? We could try familiar combinators like map.

```
fun profilesFor(users: List<User>): List<IO<Profile>> =
users.map(::userInfo)
```

Note the return type `List<IO<Profile>>`

. This makes sense given the type signatures, but seems unwieldy. We now have a list of asynchronous values, and to work with those values we must then use the combinators on `IO`

for every single one. It would be nicer instead if we could get the aggregate result in a single `IO`

, say a `IO<List<Profile>>`

.

Similar to the latter, you may find yourself with a collection of data, each of which is already in an data type, for instance a `List<Option<A>>`

or `List<IO<Profile>>`

. To make this easier to work with, you want a `Option<List<A>>`

or `IO<List<Profile>>`

. Given `Option`

and `IO`

have an Applicative instance, we can sequence the list to reverse the types.

```
import arrow.core.Option
import arrow.core.extensions.list.traverse.sequence
import arrow.core.extensions.option.applicative.applicative
import arrow.core.none
import arrow.core.some
fun main() {
//sampleStart
val optionList =
listOf(1.some(), 2.some(), 3.some())
.sequence(Option.applicative())
val emptyList =
listOf(1.some(), none(), 3.some())
.sequence(Option.applicative())
//sampleEnd
println("optionList = $optionList")
println("emptyList = $emptyList")
}
```

Traverse provides sequence as a convenience method for `traverse(::identity)`

.

```
import arrow.core.extensions.option.applicative.applicative
import arrow.core.extensions.option.applicative.map
import arrow.core.fix
import arrow.core.identity
import arrow.core.none
import arrow.core.some
import arrow.core.Option
import arrow.core.extensions.list.traverse.traverse
fun main() {
//sampleStart
val optionList: Option<List<Int>> =
listOf(1.some(), 2.some(), 3.some())
.traverse(Option.applicative(), ::identity).fix().map { it.fix() }
val emptyList: Option<List<Int>> =
listOf(1.some(), none(), 3.some())
.traverse(Option.applicative(), ::identity).map { it.fix() }
//sampleEnd
println("optionList = $optionList")
println("emptyList = $emptyList")
}
```

Interestingly enough, traverse is a much more generalized and powerful form of sequence that would allow us to parse a `List<String>`

or validate credentials for a `List<User>`

.

Enter Traverse.

At center stage of Traverse is the traverse method.

```
import arrow.Kind
import arrow.typeclasses.Applicative
import arrow.typeclasses.Foldable
import arrow.typeclasses.Functor
interface Traverse<F> : Functor<F>, Foldable<F> {
fun <G, A, B> Kind<F, A>.traverse(AP: Applicative<G>, f: (A) -> Kind<G, B>): Kind<G, Kind<F, B>>
}
```

In our above example, `F`

is `List`

, and `G`

is `Option`

or `IO`

. For the profile example, traverse says given a `List<User>`

and a function `(User) -> IO<Profile>`

, it can give you a `IO<List<Profile>>`

.

Abstracting away the `G`

(still imagining `F`

to be `List`

), traverse says given a collection of data, and a function that takes a piece of data and returns an value `B`

wrapped in a container `G`

, it will traverse the collection, applying the function and aggregating the values (in a `List`

) as it goes.

In the most general form, `Kind<F, A>`

is some sort of context `F`

which may contain a value `A`

(or several). While `List`

tends to be among the most general cases, there also exist Traverse instances for Option, Either, and Validated (among others).

In general this holds:

```
map(::f).sequence(AP) == traverse(AP, ::f)
```

where AP stands for an `Applicative<G>`

, which is in prior snippets `Applicative<ForOption>`

or `Applicative<ForIO>`

.

Sometimes our effectful functions return a Unit value in cases where there is no interesting value to return (e.g. writing to some sort of store).

```
import arrow.fx.IO
interface Data
fun writeToStore(data: Data): IO<Unit> = TODO("")
```

If we traverse using this, we end up with a funny type.

```
import arrow.core.ListK
import arrow.fx.IO
import arrow.fx.extensions.io.applicative.applicative
import arrow.fx.fix
interface Data
fun writeToStore(data: Data): IO<Unit> = TODO("")
//sampleStart
fun writeManyToStore(data: ListK<Data>): IO<ListK<Unit>> =
data.traverse(IO.applicative()) { writeToStore(it) }.fix()
//sampleEnd
```

We end up with a `IO<ListK<Unit>>`

! A `ListK<Unit>`

is not of any use to us, and communicates the same amount of information as a single Unit does.

Traversing solely for the sake of the effects (ignoring any values that may be produced, Unit or otherwise) is common, so Foldable (superclass of Traverse) provides traverse_ and sequence_ methods that do the same thing as traverse and sequence but ignore any value produced along the way, returning Unit at the end.

```
import arrow.core.ListK
import arrow.core.extensions.list.foldable.traverse_
import arrow.fx.IO
import arrow.fx.extensions.io.applicative.applicative
import arrow.fx.fix
interface Data
fun writeToStore(data: Data): IO<Unit> = TODO("")
//sampleStart
fun writeManyToStore(data: ListK<Data>): IO<Unit> =
data.traverse_(IO.applicative()) { writeToStore(it) }.fix()
//sampleEnd
```

```
import arrow.core.Either
import arrow.core.extensions.either.applicative.applicative
import arrow.core.extensions.list.foldable.sequence_
import arrow.core.left
import arrow.core.right
fun main() {
//sampleStart
val rightUnit = listOf(1.right(), 2.right(), 3.right())
.sequence_(Either.applicative())
val leftString = listOf(1.right(), "String".left(), 3.right())
.sequence_(Either.applicative())
//sampleEnd
println("someUnit= $rightUnit")
println("noneUnit= $leftString")
}
```

Even though, Foldable and Traverse are related, because both ‘reduce their values to something’, it is not obvious why to consider Traverse over Foldable.

Here is one example:

```
import arrow.core.MapK
import arrow.core.Option
import arrow.core.Some
import arrow.core.extensions.option.applicative.applicative
import arrow.core.fix
import arrow.core.k
fun main(){
//sampleStart
val map: MapK<String, Int> = mapOf("one" to 1, "two" to 2).k()
val optionMapK: Option<MapK<String, String>> = map.traverse(Option.applicative()) { Some("$it") }.fix()
// val optionMapKBoilered = map.foldLeft(Some(emptyMap())) { acc: Option<MapK<String, String>>, i: Int ->
// acc.fold({ emptyMap() }, { /*Some logic to retrieve the key of a value, transform it and add it to the accumulated Map*/})
// }
//sampleEnd
println(optionMapK)
}
```

Both methodologies try to attain the same thing, but what Foldable lacks is that it solely drills down to its `A`

here `Int`

and does not preserve it’s shape `F`

- `MapK`

. Resulting in a tiresome implementation, where we need to come up with an algorithm to resolve a key of a given value in the `Map`

- let alone that this algorithm is not universal over any given `Map`

.

This is where Traverse shines, whenever you care about the Output `B`

from `(A) -> B`

and the existing shape of `F`

you may use traverse.

Additionally, you’re able to wrap your context `F`

within a `G`

. That is one reason, may among others, why Traverse is strictly more powerful than Foldable.

The Foldable type class abstracts over “things that can be folded over” similar to how Traverse abstracts over “things that can be traversed.” It turns out Traverse is strictly more powerful than Foldable - that is, foldLeft and foldRight can be implemented in terms of traverse by picking the right Applicative. However, arrow’s Traverse does not implement foldLeft and foldRight with traverse as the actual implementation tends to be inefficient.

For brevity and demonstration purposes we’ll implement an isomorphic foldMap method in terms of traverse by using Const. You can then implement foldRight in terms of foldMap, and foldLeft can then be implemented in terms of foldRight, though the resulting implementations may be slow.

```
import arrow.Kind
import arrow.core.Const
import arrow.core.ListK
import arrow.core.extensions.const.applicative.applicative
import arrow.core.extensions.listk.traverse.traverse
import arrow.core.extensions.monoid
import arrow.core.fix
import arrow.core.identity
import arrow.core.k
import arrow.typeclasses.Monoid
import arrow.typeclasses.Traverse
//sampleStart
fun <F, B, A> Kind<F, A>.foldMap(f: (A) -> B, M: Monoid<B>, TF: Traverse<F>): B =
TF.run {
M.run {
traverse(Const.applicative(M)) { a: A -> Const<B, Nothing>(f(a)) }.fix().value()
}
}
val sing = listOf("Hello", " from ", "the", " other ", "side!").k().foldMap(::identity, String.monoid(), ListK.traverse())
//sampleEnd
fun main() {
println("Sing=$sing")
}
```

The type signature of Traverse appears highly abstract, although it’s easier if you think about it as executing operations over collections - what traverse does as it walks the `Kind<F, A>`

depending on the context `F`

of the function. Let’s see some examples where `F`

is taken to be `List`

.

```
import arrow.core.ValidatedNel
import arrow.core.invalidNel
import arrow.core.validNel
import arrow.core.Either
fun parseIntEither(s: String): Either<NumberFormatException, Int> =
if (s.matches(Regex("-?[0-9]+"))) Either.Right(s.toInt())
else Either.Left(NumberFormatException("$s is not a valid integer."))
fun parseIntValidated(s: String): ValidatedNel<NumberFormatException, Int> =
if (s.matches(Regex("-?[0-9]+"))) s.toInt().validNel()
else NumberFormatException("$s is not a valid integer.").invalidNel()
```

We can now traverse structures that contain strings and parse them into integers with either with a `fail-fast`

strategy using Either or accumulating failures with ValidatedNel.

```
import arrow.core.Either
import arrow.core.extensions.either.applicative.applicative
import arrow.core.k
import arrow.core.ValidatedNel
import arrow.core.extensions.either.foldable.isEmpty
import arrow.core.invalidNel
import arrow.core.validNel
fun parseIntEither(s: String): Either<NumberFormatException, Int> =
if (s.matches(Regex("-?[0-9]+"))) Either.Right(s.toInt())
else Either.Left(NumberFormatException("$s is not a valid integer."))
fun parseIntValidated(s: String): ValidatedNel<NumberFormatException, Int> =
if (s.matches(Regex("-?[0-9]+"))) s.toInt().validNel()
else NumberFormatException("$s is not a valid integer.").invalidNel()
fun main() {
//sampleStart
val list = listOf("1", "2", "3").k().traverse(Either.applicative(), ::parseIntEither)
val failFastList = listOf("1", "abc", "3", "4s").k().traverse(Either.applicative(), ::parseIntEither)
//sampleEnd
println("list= $list")
println("failFastList= $failFastList")
}
```

```
import arrow.core.Either
import arrow.core.Nel
import arrow.core.ValidatedNel
import arrow.core.extensions.nonemptylist.semigroup.semigroup
import arrow.core.extensions.validated.applicative.applicative
import arrow.core.fix
import arrow.core.invalidNel
import arrow.core.k
import arrow.core.validNel
fun parseIntEither(s: String): Either<NumberFormatException, Int> =
if (s.matches(Regex("-?[0-9]+"))) Either.Right(s.toInt())
else Either.Left(NumberFormatException("$s is not a valid integer."))
fun parseIntValidated(s: String): ValidatedNel<NumberFormatException, Int> =
if (s.matches(Regex("-?[0-9]+"))) s.toInt().validNel()
else NumberFormatException("$s is not a valid integer.").invalidNel()
fun main() {
//sampleStart
val validatedList = listOf("1", "22w", "3", "33s").k()
.traverse(ValidatedNel.applicative(Nel.semigroup<NumberFormatException>()), ::parseIntValidated).fix()
//sampleEnd
println(validatedList)
}
```

Notice that in the Either case, should any string fail to parse the entire traverse is considered a failure. Moreover, once it hits its first bad parse, it will not attempt to parse any others down the line (similar behavior would be found with using `Option`

). Contrast this with `Validated`

where even if one bad parse is hit, it will continue trying to parse the others, accumulating any and all errors as it goes. The behavior of traverse is closely tied with the Applicative behavior of the data type, where computations are run in isolation.

Going back to our `IO`

example from the beginning with concurrency in mind, we can get an Applicative instance for `IO`

, that traverses concurrently, by using `parApplicative`

. Contrary to `IO.applicative()`

, which runs in sequence.

It is worth mentioning that `parApplicative`

does not implement `lazyAp`

in contrast to `IO.applicative()`

, which means `parApplicative`

creates all IO’s upfront and executes them in parallel. More importantly, `parApplicative`

breaks monad-applicative-consistency laws by design. The aforementioned law holds in the case of `IO.applicative()`

, where an effect only runs , when the previous one is successful - hence the monadic nature.

Continuing with the example, we traverse a `List<A>`

with its Traverse instance `ListK.traverse()`

and a function`(A) -> IO<B>`

, we can imagine the traversal as a scatter-gather. Each `A`

creates a concurrent computation that will produce a `B`

(the scatter), and as the `IO`

operations completes they will be gathered back into a `List`

.

Ultimately, we utilize `parTraverse`

that calls traverse with `parApplicative`

in an concurrent environment F - in the following example `IO`

:

```
import arrow.fx.IO
import arrow.fx.extensions.fx
import arrow.fx.extensions.io.concurrent.parTraverse
import arrow.fx.extensions.io.unsafeRun.runBlocking
import arrow.unsafe
interface Profile
interface User
//sampleStart
data class DummyUser(val name: String) : User
data class DummyProfile(val u: User) : Profile
fun userInfo(u: User): IO<Profile> =
IO { DummyProfile(u) } // this can be a call to the DB
fun List<User>.processLogin(): IO<List<Profile>> =
parTraverse { userInfo(it) }
fun program(): IO<Unit> = IO.fx {
val list = listOf(DummyUser("Micheal"), DummyUser("Juan"), DummyUser("T'Challa"))
.processLogin().bind()
println(list)
}
//sampleEnd
fun main() {
program().unsafeRunSync()
}
```

Traverse is not limited to List or Nel, it provides an abstraction over ‘things that can be traversed over’, like a Binary tree, SequenceK, or a `Stream`

, hence the name Traverse.

`Reader`

Another interesting data type we can use is `Reader`

. Recall that a `Reader<D, A>`

is a type alias for `Kleisli<ForId, D, A>`

which is a wrapper around `(D) -> Id<A>`

.

In other words all three aforementioned representations are isomorphic to `D -> A`

- (Note: In regards to the category of Kotlin, where the morphisms are functions one might consider this signature `D -> A`

rather than wrapper Data types like `Kleisli`

)

If we fix `D`

to be some sort of dependency or configuration and `A`

as the return type, we can use the `Reader`

applicative in our traverse.

```
import arrow.mtl.Reader
interface Context
interface Topic
interface Result
typealias Job<A> = Reader<Context, A>
fun processTopic(t: Topic): Job<Result> = TODO()
```

We can imagine we have a data pipeline that processes a bunch of data, each piece of data being categorized by a topic. Given a specific topic, we produce a `Job`

that processes that topic. (Note that since a `Job`

is just a `Reader`

/`Kleisli`

, one could write many small `Jobs`

and compose them together into one `Job`

that is used/returned by `processTopic`

.)

Corresponding to bunch of topics, a `List<Topic>`

if you will. Since `Reader`

has an Applicative instance, we can traverse over this list.

```
import arrow.core.Id
import arrow.core.ForId
import arrow.core.ListK
import arrow.core.extensions.id.applicative.applicative
import arrow.mtl.KleisliPartialOf
import arrow.mtl.extensions.kleisli.applicative.applicative
import arrow.mtl.fix
val JobForContext = Job.applicative<Context, ForId>(Id.applicative())
fun processTopics(topics: ListK<Topic>): Job<ListK<Result>> =
topics.traverse<KleisliPartialOf<Context, ForId>, Result>(JobForContext) {
processTopic(it)
}.fix()
```

Note the nice return type - `Job<List<Result>>`

. We now have one aggregate `Job`

that, when run, will go through each topic and run the topic-specific job, collecting results as it goes. We say “when run” because a `Job`

is some function that requires a Context before producing the value we want.

One example of a “context” can be found in the Spark project or in Android’s Context. In Spark, information needed to run a Spark job (where the master node is, memory allocated, etc.) resides in a `SparkContext`

. Going back to the above example, we can see how one may define topic-specific Spark jobs `(type Job<A> = Reader<SparkContext, A>)`

and then run several Spark jobs on a collection of topics via traverse. We then get back a `Job<List<Result>>`

, which is equivalent to (`SparkContext) -> List<Result>`

. When finally passed a `SparkContext`

, we can run the job and get our results back.

Moreover, the fact that our aggregate job is not tied to any specific `SparkContext`

allows us to pass in a `SparkContext`

pointing to a production cluster, or (using the exact same job) pass in a test `SparkContext`

that just runs locally across threads. This makes testing our large job nice and easy.

Finally, this encoding ensures that all the jobs for each topic run on the exact same cluster. At no point do we manually pass in or thread a `SparkContext`

through - that is taken care for us by the (applicative) behavior of `Reader`

and therefore by traverse.

As it turns out every Traverse is a lawful Functor. By carefully picking the `G`

to use in traverse we can implement `map`

.

First let’s look at the two signatures.

```
import arrow.Kind
import arrow.core.Id
import arrow.core.extensions.id.applicative.applicative
import arrow.core.value
import arrow.typeclasses.Applicative
import arrow.typeclasses.Foldable
import arrow.typeclasses.Functor
interface Traverse<F> : Functor<F>, Foldable<F> {
fun <G, A, B> Kind<F, A>.traverse(AP: Applicative<G>, f: (A) -> Kind<G, B>): Kind<G, Kind<F, B>>
override fun <A, B> Kind<F, A>.map(f: (A) -> B): Kind<F, B> =
traverse(Id.applicative()) { Id(f(it)) }.value()
}
```

Both have a `Kind<F, A>`

receiver and a similar `f`

parameter. traverse expects the return type of `f`

to be `Kind<G, B>`

whereas `map`

just wants `B`

. Similarly the return type of traverse is `Kind<G, Kind<F, B>>`

whereas for `map`

it’s just `Kind<F, B>`

. This suggests we need to pick a `G`

such that `Kind<G, A>`

communicates exactly as much information as `A`

. We can conjure one up by simply wrapping an `A`

in `arrow.core.Id`

.

In order to call traverse needs to be Applicative which is straightforward - note that while Id just wraps an `A`

, it is still a type constructor which matches the shape required by Applicative.

```
import arrow.core.ForId
import arrow.core.Id
import arrow.core.IdOf
import arrow.core.extensions.id.comonad.extract
import arrow.typeclasses.Applicative
interface IdApplicative : Applicative<ForId> {
override fun <A, B> IdOf<A>.ap(ff: IdOf<(A) -> B>): Id<B> =
Id(ff.extract().invoke(extract()))
override fun <A> just(a: A): Id<A> =
Id(a)
}
```

We can implement map by wrapping and unwrapping Id as necessary.

```
import arrow.core.Id
import arrow.core.MapK
import arrow.core.extensions.id.applicative.applicative
import arrow.core.fix
import arrow.core.k
import arrow.core.value
//sampleStart
val map: MapK<String, Int> = mapOf("one" to 1, "two" to 2).k()
val result = map.traverse(Id.applicative()) { Id("$it") }.fix().value()
//sampleEnd
fun main() {
println(result)
}
```

Foldable and Traverse act on multiple elements and reduce them into a single value - in category theory - `Catamorphisms`

.

In contrast, `homomorphisms`

such as Monoid and Applicative preserve their structure, hence adding two values of type `List<Int>`

will yield a `List<Int>`

or an Applicative example, where `List<Int>.ap(listOf(Int::inc))`

results in a `List<Int>`

.

We can think of catamorphic operations as:

- the
`if-else`

expression in Kotlin, which models a`fold`

over`Boolean`

- various
`fold`

methods in Arrow, like in`Option`

over`Some`

and`None`

,`Either`

over`Left`

and`Right`

or`ListK`

`fold`

in common ADTs from Computer Science like in a Binary tree- the
`reduce`

method

One among many other usages of `Catamorphisms`

are in Recursion Schemes(/docs/recursion/intro/).

Module | Data types |

- The Essence of the Iterator Pattern - Gibbons, Oliveira. JFP, 2009
- Catamorphisms - Mark Seemann, 2019
- Catamorphisms in 15 minutes - Chris Jones, 2014

The content is heavily inspired by Scala exercise, from examples from the Cats Community and partially adopted from Daniel Shin’s Blog.

flatTraverse | `open fun <G, A, B> Kind<F, A>.flatTraverse(MF: ` `Monad` `<F>, AG: ` `Applicative` `<G>, f: (A) -> Kind<G, Kind<F, B>>): Kind<G, Kind<F, B>>` |

map | Transform the F wrapped value A into B preserving the F structure Kind<F, A> -> Kind<F, B>`open fun <A, B> Kind<F, A>.map(f: (A) -> B): Kind<F, B>` |

sequence | Thread all the G effects through the F structure to invert the structure from F<G<A>> to G<F<A>>.`open fun <G, A> Kind<F, Kind<G, A>>.sequence(AG: ` `Applicative` `<G>): Kind<G, Kind<F, A>>` |

traverse | Given a function which returns a G effect, thread this effect through the running of this function on all the values in F, returning an F<B> in a G context.`abstract fun <G, A, B> Kind<F, A>.traverse(AP: ` `Applicative` `<G>, f: (A) -> Kind<G, B>): Kind<G, Kind<F, B>>` |

TraverseFilter | `interface TraverseFilter<F> : ` `Traverse` `<F>, ` `FunctorFilter` `<F>` |

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