Utils.fs

let randomString len =
    let chars = "ABCDEFGHIJKLMNOPQRSTUVWUXYZ0123456789"
    let charsLength = chars.Length
    let random = System.Random()
    let randomChars = [|for i in 0..len -> chars.[random.Next(charsLength)]|]
    new System.String(randomChars)

Utils2.fs

let randomString =
    let chars = "ABCDEFGHIJKLMNOPQRSTUVWUXYZ0123456789"
    let charsLength = chars.Length
    let random = System.Random()

    fun len -> 
        let randomChars = [|for i in 0..len -> chars.[random.Next(charsLength)]|]
        new System.String(randomChars)

see https://twitter.com/neoeinstein/status/795068069219631105

Perhaps this clarifies - this is equivalent to utils2:

let randomString = 

    let chars = "ABCDEFGHIJKLMNOPQRSTUVWUXYZ0123456789"
    let charsLength = chars.Length
    let random = System.Random()

    // create the function
    let generateString len =
        let randomChars = [|for i in 0..len -> chars.[random.Next(charsLength)]|]
        new System.String(randomChars)
    
    // now return the function
    generateString 

I didn't know String could take an array of char in the constructor, that's a convenient trick :)
Instead of

    // omitted
    let randomChars = [| for i in 0..len -> chars.[random.Next(charsLength)] |]
    new System.String(randomChars)

... you could then go all out and do:

    [| for i in 0..len -> chars.[random.Next(charsLength)] |]
    |> System.String