foo.fs

type System.Int32 with

    member x.DisplayWithSuffix() =
        let f = x.ToString()
        let defaultVal = f + "th"
        let mutable result = ""
        result <- if f.EndsWith("11") then f + "th" else defaultVal
        result <- if f.EndsWith("12") then f + "th" else defaultVal
        result <- if f.EndsWith("13") then f + "th" else defaultVal
        result <- if f.EndsWith("1") then f + "st" else defaultVal
        result <- if f.EndsWith("2") then f + "nd" else defaultVal
        result <- if f.EndsWith("3") then f + "rd" else defaultVal
        
        result

let (|EndsWith|_|) (ending:string) (input:string) =
    if input.EndsWith(ending) then Some () else None

type System.Int32 with
    member x.DisplayWithSuffix() =
        match x.ToString() with
        | EndsWith "11" | EndsWith "12" | EndsWith "13" -> "th"
        | EndsWith "1" -> "st"
        | EndsWith "2" -> "nd"
        | EndsWith "3" -> "rd"
        | _ -> "th"
        |> fun suffix -> $"{x.ToString()}{suffix}"