let isPrime n =
  let sqrt' = (float >> sqrt >> int) n // square root of integer
  [ 2 .. sqrt' ] // all numbers from 2 to sqrt'
  |> List.forall (fun x -> n % x <> 0) // no divisors

let allPrimes =
  let rec allPrimes' n =
    seq { // sequences are lazy, so we can make them infinite
      if isPrime n then
        yield n
      yield! allPrimes' (n+1) // recursing
    }
  allPrimes' 2 // starting from 2

allPrimes
|> Seq.take 20 // only 20
|> List.ofSeq // forces evaluation of first 20 items