Euler, ex. 2 F#.fs

(* Each new term in the Fibonacci sequence is generated by adding the previous two terms.
By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

Find the sum of all the even-valued terms in the sequence which do not exceed four million. *)
#light

let fibonacci = fun(x: int, y: int) -> Some( x, (y, x+y) )
let sequence = Seq.unfold fibonacci <| (1,2) //I like having the tuple at the end better than at the beginning.
let evenFibsUnder4Million = seq {for x in (Seq.takeWhile (fun(x) -> x < 4000000) sequence) do
                                 if x % 2 = 0 then yield x }
let answer = Seq.fold (+) 0 evenFibsUnder4Million


// More compact
let fibonaccis = Seq.unfold( fun(x, y) -> Some( x, (y, x+y)) ) <| (1,2)
let evenFibsUnder4Million = seq {for x in (Seq.takeWhile (fun(x) -> x < 4000000) fibonaccis) do
                                 if x % 2 = 0 then yield x }
let answer = Seq.fold (+) 0 evenFibsUnder4Million

// Or even
let fibonaccis = Seq.unfold( fun(x, y) -> Some( x, (y, x+y)) ) <| (1,2)
let answer = seq {for x in (Seq.takeWhile (fun(x) -> x < 4000000) fibonaccis) do
                                 if x % 2 = 0 then yield x }
             |> Seq.fold (+) 0