[Haskell-cafe] Looking for suggestions to improve my algorithm

Brent Yorgey byorgey at gmail.com
Wed Aug 29 19:08:51 EDT 2007

On 8/29/07, David Frey <dpfrey at shaw.ca> wrote:
> Hello Haskellers,
> I have been trying to learn a bit about Haskell by solving Project Euler
> problems.  For those of you who don't know what Project Euler is, see
> http://projecteuler.net
> After solving problem 21, which is related to amicable pairs, I decided
> to attempt problem 95 since it is an extension of the same topic.
> The full description of problem 95 is here:
> http://projecteuler.net/index.php?section=problems&id=95
> This is the summary:
> "Find the smallest member of the longest amicable chain with no element
> exceeding one million."
> I have attempted to solve this problem, but my solution is too resource
> hungry to search the full set of 1000000 numbers.
> I am hoping someone reading this list can suggest :
> - How I can improve my algorithm
> - An alternative algorithm that will be more efficient
> - Ways to profile a Haskell program to help me understand why my
>    implementation is performing poorly.
> In addition to the question above, I would also be interested in
> comments on the style of the code.  If there is a more idiomatic way to
> write portions of the code, I would like to see what it is.
> My solution is at the bottom of this e-mail.  The program will probably
> run obscenely slow  or die from stack overflow unless you replace
> [1..999999] with [1..somethingSmaller]  in main.
> Thanks,
> David Frey

Hi David,

Project Euler is a good way to learn some Haskell, although it does tend to
give you a crash course in understanding laziness, efficiency and such in
Haskell (whether that's good or bad depends on your point of view!).

All you need to do to stop your program from overflowing the stack is to
change foldl to foldl' in the definition of main (foldl' is in Data.List so
you'll also have to import that).  The problem with foldl is that since
Haskell is lazy, instead of evaluating things as it goes along, it builds up
a huge string of thunks (=unevaluated expressions) and doesn't even start
evaluating anything until it reaches the end of the list, which can easily
blow the stack.  foldl' is a strict version of foldl which forces evaluation
to take place as it goes along.  For an excellent explanation of all of
this, I suggest you read http://haskell.org/haskellwiki/Stack_overflow.

As soon as I replaced foldl with foldl' in your code, it no longer overflows
the stack -- however, it's still quite slow!  I didn't wait long enough for
it to finish when I ran it up to a million.  I don't have any advice on that
front at the moment, perhaps someone else will come along with a suggestion
or two.  In the meantime, you can poke around
http://haskell.org/haskellwiki/Performance to see if it gives you any ideas.

Hope this helps,
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