How to pattern-match when something is NOT of a particular type

Question

We're all used to pattern-matching for cases when something is a particular type, e.g.,

match x with
| Y(x) :: tail -> ... // assumes List.head(x) is of type Y(x)

But how can can I match the case when something is not of a particular type? E.g.,

match x with
| Not Y(_) :: tail -> ... // List.head(x) is definitely not a Y

Thanks!

Answer 1

While there is no direct support for Not you can use a partial active pattern.

type Color = | Red | Green | Blue

let (|NotRed|_|) = function
    | Red -> None
    | color -> Some color

let rec print = function
    | (NotRed head) :: tail -> 
        printfn "%A is Not Red" head
        print tail
    | _ :: tail -> print tail
    | _ -> ()

print [Red; Green; Blue]

output

Green is Not Red
Blue is Not Red