Building a RESTful Service in F# using Visual Studio 2022 Community Edition

The world of web services is constantly evolving, and Functional First programming languages like F# are gaining popularity in the world of web development. In this blog post, we’ll guide you through creating a RESTful web service in F# using Visual Studio 2022 Community Edition. We’ll also develop an F# client that calls this service to retrieve flight data.

Step 1: Setting up Visual Studio 2022 Community Edition

1. Installation: If you haven’t already, download and install Visual Studio 2022 Community Edition.
2. Creating a New Project: Launch Visual Studio and create a new F# project. Choose „Console App (.NET)“ as the project template.

Step 2: Writing the RESTful Service

1. Adding Dependencies: Right-click your project in the Solution Explorer, select „Manage NuGet Packages,“ and search for Giraffe, a F# web framework. Install it.
2. Creating the Service: Open the Program.fs file in your project and replace the existing code with the following:

open Giraffe
open Giraffe.HttpHandler
open Microsoft.AspNetCore.Builder
open Microsoft.AspNetCore.Hosting
open Microsoft.AspNetCore.Http
open System

let app : HttpHandler =
    choose [
        GET >=> route "/flights" >=> json { flight_number = "FLT123"; destination = "New York"; departure_time = "2023-10-16T12:00:00" }
    ]

[<EntryPoint>]
let main argv =
    let webApp =
        WebHostBuilder()
            .Configure(Action<IApplicationBuilder>(fun app ->
                app.UseGiraffe(app)
            ))
            .UseKestrel()
            .UseUrls("http://localhost:8080")
            .Build()

    webApp.Run()
    0

This code sets up a basic RESTful service using Giraffe, which listens on http://localhost:8080 and provides flight data at the endpoint /flights.

Step 3: Building the F# Client

1. Creating the Client: Add a new F# Console App project to your solution by right-clicking the solution in Solution Explorer and selecting „Add“ -> „New Project.“ Choose „Console App (.NET)“ and name it „FlightDataClient.“
2. Adding Dependencies: In the „FlightDataClient“ project, right-click and select „Manage NuGet Packages.“ Install FSharp.Data for easy JSON handling.
3. Writing the Client Code: Open the Program.fs file in the „FlightDataClient“ project and replace the existing code with the following:

open FSharp.Data

[<Literal>]
let serviceUrl = "http://localhost:8080/flights"

type FlightData = JsonProvider<serviceUrl>

[<EntryPoint>]
let main argv =
    let flightData = FlightData.Load(serviceUrl)

    printfn "Flight Data:"
    printfn "Flight Number: %s" flightData.FlightNumber
    printfn "Destination: %s" flightData.Destination
    printfn "Departure Time: %s" flightData.DepartureTime

    0

This F# client uses FSharp.Data to make a request to the RESTful service we created earlier and displays the retrieved flight data.

Step 4: Running the Service and Client

1. Running the Service: In Visual Studio, set the RESTful service project as the startup project. Press F5 to start the service.
2. Running the Client: Set the client project as the startup project, and press F5 to start the client. You should see the flight data retrieved from the service displayed in the console.

Congratulations! You’ve successfully created a RESTful service in F# using Visual Studio 2022 Community Edition and developed a client to call that service. This demonstrates F#’s versatility in web development, making it an excellent choice for building web services and clients with ease. You can expand on this foundation to develop more complex applications and services using F#.