I'm trying to use System.Random in F# but keep getting duplicate values. I'm aware that if I create a new instance of System.Random each time I do .Next i might get duplicate values. What I'm trying to achive is setting a random id on each record when creating them. Here is an example of the problem in code:
[<AutoOpen>]
module Domain =
type Test = {id: int; ....}
module Test =
let r = System.Random()
let t create =
{id = r.Next(); ....}
let a = Test.create
let b = Test.create
In the code above bot a and b gets the same id.
What I'm guessing is happening is that let r = System.Random() doesn't get treated as a variable but as a function which returns a new instance of Random. Is this correct? And how can I Solve this problem?
Thanks!
You need to pass a unit () in your create function otherwise is a value (that's only calculated the first time).
open System
module T =
type Test = {id: int; }
let r = System.Random()
let create () =
{id = r.Next(); }
[<EntryPoint>]
let main _ =
let a = T.create ()
let b = T.create ()
printfn "a = %A" a
printfn "b = %A" b
This yields different values.
a = { id = 1528807395 }
b = { id = 1526175776 }
This solves your immediate problem. However, it's common practice to create stateless functions. Your Random variable contains state thus it must be passed as parameter.
let create (r:Random) =
{id = r.Next(); }