Project Euler #2 in F#
I discovered an (arguably) better way of doing this in F# using Seq.unfold and tuples. I'll post the solution below. 

open System

(1,1) |&gt; Seq.unfold (fun (a, b) -&gt; Some(a + b, (b, a + b)))
|&gt; Seq.takeWhile (fun x -&gt; x &lt;= 4000000)
|&gt; Seq.sumBy (fun x -&gt; if x % 2 = 0 then x else 0)
|&gt; printf "The sum of Fibonacci terms up until 4,000,000 is: %d"
Console.ReadKey () |&gt; ignore