Use Option<'T> over null values in F#

option.md

In functional programming languages like F# null values are considered potentially harmful and poor style (non-idiomatic).

Consider this C# code:

    string x = SomeFunction ();
    int    l = x.Length;

x.Length will throw if x is null let's add protection:

    string x = SomeFunction ();
    int    l = x != null ? x.Length : 0;

Or:

    string x = SomeFunction () ?? "";
    int    l = x.Length;

Or:

    string x = SomeFunction ();
    int    l = x?.Length;

In idiomatic F# null values aren't used so our code looks like this:

    let x = SomeFunction ()
    let l = x.Length

However, sometimes there's a need for representing empty or invalid values. Then we can use Option<'T>:

    let SomeFunction () : string option = ...

SomeFunction either returns Some string value or None. We extract the string value using pattern matching

    let v =
      match SomeFunction () with
      | Some x  -> x.Length
      | None    -> 0

The reason this code is less fragile than:

    string x = SomeFunction ();
    int    l = x.Length;

Is because we can't call Length on a string option. We need to extract the string value using pattern matching and by doing so we are guaranteed that the string value is safe to use.