Defining static classes in F#

This is explained in The F# Component Design Guidelines.

[<AbstractClass; Sealed>]
type Demo =
    static member World = "World"
    static member Hello() = Demo.Hello(Demo.World)
    static member Hello(name: string) = sprintf "Hello %s!" name

let s1 = Demo.Hello()
let s2 = Demo.Hello("F#")

It is still possible to define instance methods, but you can't instantiate the class when there is no constructor available.

edit Jan 2021 : See the comment from Abel, and the linked issue. Joel Mueller's answer seems to be the best advice so far, but things will perhaps change in the future.

As Robert Jeppeson pointed out, a "static class" in C# is just short-hand for making a class that cannot be instantiated or inherited from, and has only static members. Here's how you can accomplish exactly that in F#:

[<AbstractClass; Sealed>]
type MyStaticClass private () =
    static member SomeStaticMethod(a, b, c) =
       (a + b + c)

    static member SomeStaticMethod(a, b, c, d) =
       (a + b + c + d)

This might be a little bit of overkill, as both the AbstractClass and the private constructor will prevent you from creating an instance of the class, however, this is what C# static classes do - they are compiled to an abstract class with a private constructor. The Sealed attribute prevents you from inheriting from this class.

This technique won't cause a compiler error if you add instance methods the way it would in C#, but from a caller's point of view there is no difference.