As you probably know, the Fibonacci sequence is the infinite sequence of integers where each element is the sum of the previous two (the first two elements being **0** and **1**). Recently, I was inspired by a blog post, Ruby vs. Haskell – project Euler #25 deathmatch. In particular, I enjoyed the Haskell solution for its simplicity and declarativeness.

I decided to try and solve the same problem, but using F#, the functional programming language being introduced as a first class .NET citizen for the first time with Visual Studio 2010. If you have never seen F# code before, the snippets included in this post may be difficult to comprehend, especially if you are used to reading code written in imperative languages like C# or Java.

To declaratively create infinite sequences in F#, the **Seq** module provides the unfold function. This function takes two parameters, a generator function and an initial state. The generator function must take a state parameter and produce an option tuple with a sequence element and a new state. In F# notation, the **unfold** function has the signature `Seq.unfold : ('State -> 'T * 'State option) -> 'State -> seq<'T>`

. Note that if the generator function always returns `Some(_)`

and never `None`

, the resulting sequence will be infinite.

An example of **Seq.unfold** in action is shown in the following one-liner, producing an infinite sequence of all positive integers.

let positiveIntegers = Seq.unfold (fun x -> Some(x, x + 1)) 1

In this example, the generator function takes an integer state as input. The sequence element produced by this function is the current state, while the next state is calculated by incrementing the current state. Thus, each time the generator function is called, the input integer state is one higher than the previous time. The initial state, **1**, is passed as the final parameter to **Seq.unfold**. The result is the sequence** “1, 2, 3, …, ∞”** (or, strictly speaking, as far as 32 bit integers go).

So, how do we go from this sequence to the Fibonacci sequence? First of all, since each element of the Fibonacci sequence is the sum of the previous two, the state cannot consist only of a single integer. Rather, the state has to be a tuple of two integers. By choosing **(0, 1)** as the initial state, the generator function can use the first tuple element as sequence output and construct the next state as **(**[next]**, **[current]** + **[next]**)**, where [current] and [next] are the first and second element, respectively, from the current state tuple.

Translated into F# code, this yields the following definition of the Fibonacci sequence.

let fibonacci = Seq.unfold (fun (current, next) -> Some(current, (next, current + next))) (0, 1)

When enumerating this sequence, however, one problem becomes apparent. Element number 48 is a negative number. This is definitely erroneous behavior, as the Fibonacci sequence consists solely of positive integers. The error is due to the limited value space of 32 bit integers, causing an overflow. To circumvent this problem, we can use the BigInteger type, capable of representing integers of arbitrary size. The only change we need to make to our original Fibonacci definition is to change the initial state tuple to contain **BigInteger** values. The F# type inference system handles the rest.

open System.Numerics let fibonacci = Seq.unfold (fun (current, next) -> Some(current, (next, current + next))) (BigInteger 0, BigInteger 1)

Now to the actual solution to Project Euler #25. Modelled after the previously mentioned Haskell solution, my solution also counts the number of elements in the Fibonacci sequence having a value less than 10^{999}.

Again, translating this into F# results in the following code.

open System.Numerics let limit = BigInteger.Pow(BigInteger 10, 999) let fibonacci = Seq.unfold (fun (current, next) -> Some(current, (next, current + next))) (BigInteger 0, BigInteger 1) let term = fibonacci |> Seq.takeWhile (fun n -> n < limit) |> Seq.length printfn "%d" term

I am intrigued by how this functional solution focuses on **what** the Fibonacci sequence is, rather than **how** it is calculated. Constructing an infinite Fibonacci sequence in C# would typically require an iterator consisting of an infinite loop, representing state with two local variables. Counting elements having a value less than 10^{999}, however, could easily have been accomplished in a functional manner using LINQ.