Active Patterns in F#

While learning F# one of the most unique things I’ve come across is active patterns. They allow you to split data into named partitions which can be used when pattern matching. For example:

> let (|Pass|Fail|) sucess =
– if sucess then Pass else Fail
– ;;

val ( |Pass|Fail| ) : bool -> Choice<unit,unit>

> let ExamResult res =
– match res with
– | Pass -> printfn “%s” “You Passed!”
– | Fail -> printfn “%s” “You Failed. Sorry!”
–
– ;;

val ExamResult : bool -> unit

> ExamResult true
– ;;
You Passed!
val it : unit = ()

I’m just starting to really play around with active patterns, but already I can see a significant reduction in code and an increase in readability from having to use discriminate unions instead.