I’m beginning to learn F# and the best way for me to go about that is to start applying it. There are some interesting computational problems over at Project Euler . This post is the continuation of a series of posts each involving a separate Project Euler problem. This post involves solutions to Project Euler problem #10.

The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.

Find the sum of all the primes below two million.

Starting with the prime-generating code from my solution to problem #7:

open System let dividesEvenly dividend divisor = int64(dividend % divisor) = 0L let add x y = x + y let rec isPrime x = let limit = int(Math.Sqrt(float(x))) primes |> LazyList.to_seq |> Seq.takeWhile (fun i -> i <= limit) |> Seq.exists (dividesEvenly x) |> not and primes = let nextPrime prevPrime = Seq.initInfinite (add (prevPrime + 1)) |> Seq.filter isPrime |> Seq.hd let rec PrimesStartingFrom prime = LazyList.consf prime (fun () -> PrimesStartingFrom (nextPrime prime)) LazyList.consf 2 (fun () -> PrimesStartingFrom 3) |

All we need to do is add all of the primes below 2 million. Utilizing F#’s BigInt support we arrive at:

open Microsoft.FSharp.Math let sum = primes |> Seq.takeWhile (fun i -> i < 2000000) |> Seq.map (fun i -> new bigint(i)) |> Seq.fold (+) 0I |