Problem 40: If dn represents the nth digit of the fractional part, find the value of the following expression. d1 * d10 * d100 * d1000 * d10000 * d100000 * d1000000

An imperative solution seems appropriate.

let problem40a =
    let sb = System.Text.StringBuilder()
    let mutable n = 1
    while sb.Length <= 1000000 do
        sb.Append(n) |> ignore
        n <- n+1
        
    let mutable prod = 1
    for i in 0..6 do
        prod <- prod * ((sb.[(pown 10 i)-1] |> int) - 48) 
        
    prod