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Last time we talked about the simplest ASP.NET Core application in C#. Today we’ll create the same project in a functional way.

Title image

Recently, I’ve been interested in functional programming because it’s a totally different approach with intriguing challenges. Also, I think you noticed that C# sometimes looks functional, so I was wondered how much differences between two elementary web servers would be. Hence, I decided to make a post with a “Hello world” example in F#.

Creation from a template

First of all, create an application from the ASP.NET template.

$ dotnet new web -lang F#

We need only Server.fsproj and Program.fs, other generated files you can safely delete.


There is a library for creating web applications in a functional approach. We’ll use it for our server.

A native functional ASP.NET Core web framework for F# developers.


To add this package to the project let’s modify .fsproj file. It’s similar to the ordinary C# project files.

<Project Sdk="Microsoft.NET.Sdk.Web">


    <Compile Include="Program.fs" />

    <PackageReference Include="Giraffe" Version="4.1.0" />


Now, alter a code from the Program.fs.

namespace SimpleGiraffeApplication

open Giraffe
open Microsoft.AspNetCore.Hosting
open Microsoft.Extensions.Hosting

module Program =
    let webApp =
        choose [
            route "/" >=> text "Hello World!"

    let main args =
                fun webBuilder ->
                        .ConfigureServices(fun services -> services.AddGiraffe() |> ignore)
                        .Configure(fun builder -> builder.UseGiraffe webApp)
                        |> ignore)


In the beginning, we describe our routes. Here we have only one endpoint. The rest of this file will be familiar to you, compare it to the previous example.

That’s all! Finally, we have a functional web server. You can run it and test with a request from a browser.


In this post, I described a straightforward example of a functional web server. With .NET, it’s effortlessy to switch to F# and spend some time learning functional style. Maybe in the future, I’ll publish some new posts about F#.

Link to GitHub Project

Image: Photo by Zdeněk Macháček on Unsplash