F# -- Calculate Max Heartrate based on age input

CalcMaxHR.fs

// Calculates the maximum heartrate based on the age input by the user

(* Strings in F# are by default, immutable. If you want the string to change 
   in any way [ie, assigning values, concantination, etc] you must make the 
   variables mutable. *)

open System
 
let main() =
    let mutable maxHR = 0
    let mutable entries = 0
    let mutable age = String.Empty

    while entries < 5 do
        Console.Write("Enter your age: ")
        entries <- entries + 1
        age <- Console.ReadLine()
        maxHR <- 220 - Int32.Parse(age)
        Console.WriteLine("Your Max Heartrate is " + maxHR.ToString())
 
    Console.ReadKey(true) |> ignore
 
 
main()