Using Shapeless For Programming In Scala To Write A REST API

Shapeless is a library for generic programming in Scala, largely present in the ecosystem, but mostly behind the scenes; it is likely shapeless powers some of the libraries in your project, even though you don’t use it directly.

Trying to solve an everyday problem, I’ve found an use case which I could solve using shapeless. This post doesn’t intend to explain how shapeless works (there is a whole book about it here), but to provide a taste of it instead.

The Challenge

We need to write a REST API consumer using ZIO and http4s. Here is the service definition:

import io.circe.Decoder
import zio.{Has, RIO, Task}

object HttpClient {
  type HttpClient = Has[Service]

  trait Service {
    protected final val rootUrl = "http://localhost:8080"

    def get[T](uri: String, parameters: Map[String, String])
              (implicit d: Decoder[T]): Task[List[T]]
  }

  def get[T](resource: String, parameters: Map[String, String])
            (implicit d: Decoder[T]): RIO[HttpClient, List[T]] =
    RIO.accessM[HttpClient](_.get.get[T](resource, parameters))
}

In case you are not familiar with ZIO (you should, it’s awesome), what you need to know is:

every get requests returns a
```
Task
```
of
```
List
```
the get function outside the
```
Service
```
is just a helper to access the environment of the effect (ZIO stuff)

Learn more about ZIO modules and layers here.

This is the

HttpClient.Service

implementation, using http4s:

import io.circe.Decoder
import org.http4s.Uri
import org.http4s.circe.CirceEntityCodec.circeEntityDecoder
import org.http4s.client.Client
import org.http4s.client.dsl.Http4sClientDsl
import zio._
import zio.interop.catz._

class Http4sClient(client: Client[Task])
  extends HttpClient.Service with Http4sClientDsl[Task] {

  def get[T](resource: String, parameters: Map[String, String])
            (implicit d: Decoder[T]): Task[List[T]] = {
    val uri = Uri(path = rootUrl + resource)
      .withQueryParams(parameters)

    client
      .expect[List[T]](uri.toString())
      .foldM(IO.fail(_), ZIO.succeed(_))
  }
}

Http4sClient.get

adds

resource

to the

uri

and the

parameters

are the query string. Now, to represent the request call, we have a case class called

OrganisationRequest

case class OrganisationRequest(code: Option[String],
                               description: Option[String],
                               page: Integer = 1)

The Problem

Using the client (via

get

helper) is trivial, except for one detail:

import HttpClient.get

def organisations(request: OrganisationRequest):
  get[Organisation]("/organisations", ???)

We need to transform the

request

into

Map[String, String]

, what is an easy task. However, there are many “request” objects, and writing

toMap

methods to every single one of them is a Java-ish solution. Here is the challenge: how can we build this generic transformation?

Spoiler: with shapeless.

A bit of shapeless

This section is a grasp of how shapeless works, so the solution will make more sense when we get there. Shapeless can create an Heterogenous List (or

HList

) as a generic representation of case classes. Let’s do it using Generic:

scala> import shapeless._

scala> val org = OrganisationRequest(Some("acme"), None, 5)
org: OrganisationRequest = OrganisationRequest(Some(org),None,5)

scala> val gen = Generic[OrganisationRequest]
gen: shapeless.Generic[OrganisationRequest]
  {type Repr =
    Option[String]
    :: Option[String]
    :: Integer
    :: shapeless.HNil} = anon$macro$4$1@48f146f2

scala> gen.to(org)
res8: gen.Repr = Some(acme) :: None :: 5 :: HNil

The generic representation of

OrganisationRequest

is an

HList

of type

Option[String] :: Option[String] :: Int :: HNil

. We have the values, but we need the names of the fields for our

Map

. We need

LabelledGeneric

instead of

Generic

scala> val lgen = LabelledGeneric[OrganisationRequest]
lgen: shapeless.LabelledGeneric[OrganisationRequest]
  {type Repr =
    Option[String] with shapeless.labelled.KeyTag[Symbol with shapeless.tag.Tagged[String("code")],Option[String]]
    :: Option[String] with shapeless.labelled.KeyTag[Symbol with shapeless.tag.Tagged[String("description")],Option[String]]
    :: Integer with shapeless.labelled.KeyTag[Symbol with shapeless.tag.Tagged[String("page")],Integer]
    :: shapeless.HNil} = shapeless.LabelledGeneric$$anon$1@55f78c67

As you can see, with

LabelledGeneric

it’s possible to retain the information about the field names as well.

The Solution

Luckily, we don’t need to manipulate

LabelledGeneric

ourselves, shapeless provides us with plenty of useful type classes that can be found in the

shapless.ops

package. We will build our solution using ToMap:

scala> import shapeless.ops.product.ToMap

scala> val toMap = ToMap[OrganisationRequest]
toMap: shapeless.ops.product.ToMap[OrganisationRequest]
  {type K = Symbol
    with shapeless.tag.Tagged[_ >: String("page")
    with String("description")
    with String("code") <: String];
  type V = java.io.Serializable} =
    shapeless.ops.product$ToMap$$anon$5@3bccd311

scala> val map = toMap(org)
map: toMap.Out = Map('page -> 5,
                     'description -> None,
                     'code -> Some(acme))

We can make it even nicer using shapeless syntax:

scala> import shapeless.syntax.std.product._

scala> val map = org.toMap[Symbol, Any]
map: Map[Symbol,Any] = Map('page -> 5,
                           'description -> None,
                           'code -> Some(acme))

For the final solution, let’s create an

implicit class

in order to add a

parameters

method to our

request

class. Besides, we should remove every entry with

null

None

values, flatten the

Options

and turn keys and values into

String

import shapeless.ops.product.ToMap
import shapeless.syntax.std.product._

implicit class RequestOps[A <: Product](val a: A) {
  def parameters(implicit toMap: ToMap.Aux[A, Symbol, Any]): Map[String, String] =
    a.toMap[Symbol, Any]
      .filter {
        case (_, v: Option[Any]) => v.isDefined
        case (_, v) => v != null
      }
      .map {
        case (k, v: Option[Any]) => k.name -> v.get.toString
        case (k, v) => k.name -> v.toString
      }
}

A few comments here:

```
A <: Product
```
needs to be in place so we can use
```
shapeless.ops.product
```
. All case classes implement Product, it’s just a matter of adding the constrain for implicit resolution;
the implicit parameter
```
toMap
```
is a
```
ToMap.Aux
```
instead of just
```
ToMap
```
. Long story short, shapeless defines the
```
Aux
```
alias in order to make some of its internal complexity more readable and usable. Just trust me here ;)

Finally, this brings us to an elegant solution:

import HttpClient.get
import RequestOps

def organisations(request: OrganisationRequest):
  get[Organisation]("/organisations", request.parameters)

Conclusion

Even though shapeless looks almost magical at the first glance, after
dedicating myself to understand it better, I’ve figured it can be very
useful in practical terms. Shapeless provides a broad range of
typeclasses that can be used in all sort of ways, and spending time
learning about how they work is a very interesting exercise, improving
skills related to typeclasses, derivations and bringing clarity about
how some popular libraries that use shapeless work, like circe.

I’ve heard that adding too much shapeless can properly affect the
project’s compile time. I’d like to hear more about it, if you have
experience using shapeless directly, please share in the comments.

Originally published at https://juliano-alves.com on April 6, 2020.