[Haskell-cafe] newbie optimization question

[Jaak Randmets]

On 10/28/07, Prabhakar Ragde <plragde at uwaterloo.ca> wrote:
> For the purposes of learning, I am trying to optimize some variation of
> the following code for computing all perfect numbers less than 10000.
>
> divisors i = [j | j<-[1..i-1], i `mod` j == 0]
> main = print [i | i<-[1..10000], i == sum (divisors i)]
>
> I know this is mathematically stupid, but the point is to do a moderate
> nested-loops computation. On my 2.33GHz dual-core MacBookPro, the
> obvious C program takes about .3 seconds, and a compiled OCaML program
> (tail recursion, no lists) about .33 seconds. The above takes about 4
> seconds.
>

You could try giving divisors type signature:
divisors :: Int -> [Int]

> I've tried using foldl', and doing explicit tail recursion with strict
> accumulators, but I can't get the running time below 3 seconds. Is it
> possible to come within striking distance of the other languages?
> Thanks. --PR
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