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You can refer my <a href="http://fsharp-euler.wikispaces.com/euler+007" rel="nofollow noreferrer">F# for Project Euler Wiki</a>: I got this first version: <pre><code>let isPrime n = if n=1 then false else let m = int(sqrt (float(n))) let mutable p = true for i in 2..m do if n%i =0 then p <- false // ~~ I want to break here! p let rec nextPrime n = if isPrime n then n else nextPrime (n+1) let problem7 = let mutable result = nextPrime 2 for i in 2..10001 do result <- nextPrime (result+1) result </code></pre> In this version, although looks nicer, but I still does not early break the loop when the number is not a prime. In Seq module, exist and forall methods support early stop: <pre><code>let isPrime n = if n<=1 then false else let m = int(sqrt (float(n))) {2..m} |> Seq.exists (fun i->n%i=0) |> not // or equivalently : // {2..m} |> Seq.forall (fun i->n%i<>0) </code></pre> Notice in this version of isPrime, the function is finally mathematically correct by checking numbers below 2. Or you can use a tail recursive function to do the while loop: <pre><code>let isPrime n = let m = int(sqrt (float(n))) let rec loop i = if i>m then true else if n%i = 0 then false else loop (i+1) loop 2 </code></pre> A more functional version of problem7 is to use Seq.unfold to generate an infinite prime sequence and take nth element of this sequence: <pre><code>let problem7b = let primes = 2 |> Seq.unfold (fun p -> let next = nextPrime (p+1) in Some( p, next ) ) Seq.nth 10000 primes </code></pre>
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1. POHow to refactor F# code to not use a mutable accumulator?
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1. USYin Zhu
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1. COThanks, I was unaware of your Wiki. It appears that we both by instinct want to `break` a loop, and that's not possible in F# as far as I know. I like how you used `Seq.unfold`. Did you consider using recursion?
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2. COgenerally a 'while' loop can be substituted by 'tail recursion'. See my `isPrime` example in the answer.
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