Project Euler - Problem 2 in F#

Even Fibonacci numbers

Problem 2

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, ...

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

Here is my solution

    // Create a mutable list, so that we can add elements 
    // (this corresponds to standard .NET 'List<T>' type)
    let l = new ResizeArray<_>([1;2])
 
    let rec fabonacci a b upperLimit = 
        match a, b with
            | _, _ when a + b <= upperLimit -> 
                let c = a + b
                l.Add(c)
                fabonacci b c upperLimit
            | _ -> l
 
    let sumOfEven =
        (fabonacci 1 2 4000000)
            |> List.ofSeq
            |> Seq.filter (fun x -> x % 2 = 0) 
            |> Seq.sum 

Since its dynamic seq we are creating which depends on the last two numbers, we need a mutable list which we can populate according to our need and logic. For Fibonacci, we need to have access to previous two numbers in the list to generate new one.

This solution is flexible too with the upper limit, so this logic can be used anywhere to create the Fibonacci sequence with any upper bound. First step is to create Fibonacci sequence with the upper bound and initial number to kick start the sequence.Once seq is created up to desired upper bound, we are left with filtering of the sequence for even numbers only and get their sum.

Please leave your feedback and happy coding.