Project Euler Problem 1 - Functional approach?

Question

I'm trying to learn F# right now coming from C# and I'm finding it a great difficulty to "reconfigure" my mind to the functional programming mindset. So I'm going to attempt a few Project Euler problems to learn how to code functionally.

Here's my attempt at problem 1, now I'm wondering prorimarily, is this an idiomatic approach?

// Project Euler - Problem 1
// If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
// Find the sum of all the multiples of 3 or 5 below 1000.
#light
open System
let multiples = [for n in 1..1000 do if n % 3 = 0 || n % 5 = 0 then yield n]
let sum = List.sum multiples
printfn "The sum of all multiples are: %d" sum
ignore (Console.ReadKey ())

Answer 1

Some changes:

You don't need #light any more. I would also use |> more (this is more useful in more complex examples where it helps type inference.

open System
[for n in 1..1000 do if n % 3 = 0 || n % 5 = 0 then yield n]
|> List.sum 
|> printfn "The sum of all multiples are: %d" 
Console.ReadKey () |> ignore

• Overly Excessive: What exactly does |> do? :)
• John Palmer: @OverlyExcessive = a |> b is exactly the same as b(a)
• Maslow: well no, given b(a) you can do b(a).foo but not a |> b.foo
• 11684: @Maslow I don't know F#, but wouldn't it be possibly to do (a |> b).foo?
• Maslow: yes, but he said exactly the same which is mostly accurate, but not entirely

Answer 2

I think you should use sequence expressions only when you need them. In simple cases like yours, use existing functions, like List.filter. When combined with the pipe operation, your code could look like this:

let sum =
 [1..1000]
 |> List.filter (fun n -> n % 3 = 0 || n % 5 = 0)
 |> List.sum

• Overly Excessive: What is the advantage of this over sequence expression? Is this more idiomatic :) ?
• svick: I think that it's simpler and that the structure of the computation (some initial collection, then filter, then sum) is immediately visible.