You may be wondering what the above code actually does. startWebServer is a function from Suave library of type: SuaveConfig -> WebPart -> unit. This basically means that it takes object of type SuaveConfig as first argument, WebPart as second argument, and returns unit.

defaultConfig is a value from the Suave library of type SuaveConfig, and as the name suggests it determines the default configuration of server. For now we only need to know that among other stuff it configures the HTTP binding, meaning that in our case the server is listening on port 8083 on loopback address

unit in F# is a type for which there is only one applicable value, namely (). It is analogous to C# void, with the difference that you cannot return void. If you'd like to learn more about the unit type and how it differs from void, follow this link.

The most interesting part is the WebPart. WebPart is a type alias for the following type: HttpContext -> Async<HttpContext option> This means that WebPart is actually a function, which takes objects of type HttpContext as its first argument and returns an "asynchronous workflow" of HttpContext option.

HttpContext gathers information about the incoming request, outcoming response and other data. Async, which is often called "asynchronous workflow" is a concept like "promise" or "future" in some other programming languages. C# 5 introduces the Async / Await feature, which is somewhat similar to F# Async. Here is a great F# resource on async workflows and asynchronous programming in general. One of the biggest advantages F# has over C# is that it doesn't allow nulls. F# compiler prevents you from passing null as an argument, and as a result you will no longer have to deal with the infamous NullReferenceException. But what do you do if you want to say that something may have a value or may not have any value at all? That's where the Option type comes in. Object of type Option might be: None or Some x. None means there's no value, Some x means there is a value x and x is not null.

To sum up, one could explain WebPart in these words:

Based on the http context, we give you a promise (async) of optional resulting http context, where the resulting context is likely to have its response set with regards to the logic of the WebPart itself

In our case, (OK "Hello World!") is the simplest WebPart possible. No matter what comes in the HttpContext, we always return an async of Some HttpContext with response of HTTP Result 200 OK and "Hello World!" in the response body.

To some extent you could think of the WebPart as a Filter in ASP.NET MVC application, but there's more to it than Filters can do.

If the above explanation of WebPart doesn't yet make much sense to you, or you don't understand why it has such type signature, bear with me - in next sections we'll try to prove that this type turns out to be really handy when it comes to one of the greatest powers of functional programming paradigm: composition.

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