## Project Euler, Problem 10 in F#

12 Jul 2008

Problem’s quite simply stated this time:

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

Find the sum of all the primes below two million.

First typed in solution was simply:

```
[2..1999999]
|> Seq.filter(isprime)
|> Seq.fold1(+)
```

Which got me a number pretty quickly that looked lovely, but was completely wrong. I ran it again, apparently hoping that’d it magically be right the second time. Then I tried for those below 1000000 instead and the sum was negative. Oops! So, the quick fix and working version (changing the “int” range to “bigint” range):

```
[2I..1999999I]
|> Seq.filter(fun x -> isprime(int x))
|> Seq.fold1(+)
```

I wonder if there’s a checked-overflow-arithmetic version of F#? Might be useful for some of these questions, since they’re so computationally light anyway.