Pattern matching is one of the killer features of F# that makes it a more attractive language than C# or Visual Basic. It could have been just a nice language feature for the built-in .NET types, but actually it is highly customizable and you can define your own patterns.

Here is how you can define your own active pattern:

// active pattern for sequences
let (|Empty|NotEmpty|) sequence =
    if Seq.isEmpty sequence then Empty
    else NotEmpty sequence

This enables us to use the pattern directly in a match expression:

// example on usage of active pattern
// join ["1"; "2"; "3"] -> "123"
let rec join (s : seq<string>) = 
    match s with
    | Empty -> ""
    | NotEmpty s -> (Seq.head s) + join (Seq.skip 1 s)

The...