Explore creating robust HTTP services in Scala's functional programming paradigm. Gain insights into pure vs. impure implementations, their pros and cons, and effective tool usage.

scala-platform.tar.gz

Scala-Pure

Scala-Impure

Scala-Tapir

Scala-Impure-Test

Scala-Pure-Test

Scala-Tapir-Test

Test

Scala - Run Code Widget 

Scala-Pure-copy

Scala is one of the most popular general-purpose languages that supports both object-oriented and functional programming. In this course, you will learn how to create back-end HTTP services using Scala's functional programming paradigm.

You will also learn how to develop, test, and document the service using both pure and impure ways. You will be exposed to the differences between pure and impure implementation, their respective pros and cons, and how pure implementation can be more robust in terms of performance, understandability, testability, and type safeness. 

By the end of the course, you will have a clear idea of how functional programming works in Scala and how you can use the available tools more effectively.

Pure Functional HTTP APIs in Scala

val createProduct: Endpoint[Product, StatusCode, Unit, Nothing] = 
    endpoint.post
        .in("products")
        .in(
            jsonBody[Product]
                .description("The product data which should be created.")
        )
        .errorOut(statusCode)
        .out(statusCode(StatusCodes.NoContent))

As we can see, the endpoint definition for creating a product does not differ from the one that was used to update one. The only difference is that we have a different path here and do not need to extract our `Product ID` from the URL path.
# http4s implementation of the create product endpoint

val createRoute: HttpRoutes[F] =
  ProductsRoutes.createProduct.toRoutes { product =>
    for {
      cnt <- repo.saveProduct(product)
      res = cnt match {
        case 0 => StatusCodes.InternalServerError.asLeft[Unit]
        case _ => ().asRight[StatusCode]
      }
    } yield res
}

The implementation is again pretty simple. In case the `saveProduct` function returns a zero, we output a `500 Internal Server Error` because the product has not been saved into the database (**Line 6**). 
 
# Tapir endpoint for loading all products

Finally, we have our streaming endpoint left. Let’s see how we can do this via _tapir_.

// Our type is Endpoint[Unit, StatusCode, Stream[F, Byte], Stream[F, Byte]]
def getProducts[F[_]] =
  endpoint.get
    .in("products")
    .errorOut(statusCode)
    .out(
      streamBody[Stream[F, Byte]](schemaFor[Byte], tapir.MediaType.Json())
    )

The first thing we can see is that we used a `def` instead of a `val` this time (**Line 2**). This is caused by some necessities on the Scala side. 

If we want to abstract over a type parameter, we need to use a `def` here.

We have also set the last type parameter not to `Nothing` but to something concrete this time. This is because we actually want to stream something.

It is a bit annoying to have to define it two times (once for the output type and once for the “stream” type). Something like `Endpoint[I, E, Byte, Stream[F, _]]` would be better, but currently this is not the way we can do it.

So, once again, we specify the `HTTP method` (via get) and the path (which is “products”). The `errorOut` helper again restricts our error output to the status code. Finally, we set the output of the endpoint by declaring a streaming entity (via `streamBody`).

>Currently, the only supported streaming output is `Stream[F, Byte]`, so we are nailed down to just that.

But it is sufficient, and we also directly specify the returned media type to be JSON.

# http4s implementation of the load all products endpoint

Learn how to implement tapir and http4s endpoints for loading and updating products and combining them.


Products Routes - Create and Load all Products

Learn how to implement tapir and http4s endpoints for loading and updating products and combining them.

Introduction

Impure Implementation

Pure Implementation

Testing Impure Services

Testing Pure Services

Benchmarks

Documentation

Epilogue

Products Routes - Create and Load all Products

Tapir endpoint for creating a product

http4s implementation of the create product endpoint