# [Haskell-begin] Maximum of a List?

Steve Klabnik steve.klabnik at gmail.com
Sat Jul 26 17:54:57 EDT 2008

```Hello everyone-

I'm working on some project Euler problems today, and I'm stuck on one. It's
not the problem itself that's the problem, it's that finding the maximum of
a list makes me run out of heap space!

http://projecteuler.net/index.php?section=problems&id=14

my code:

import Data.List

f :: Int -> Int -> Int
f acc x
| x == 1 = acc
| even x = f (acc + 1) (x `div` 2)
| otherwise = f (acc + 1) (3 * x + 1)

answer = (foldl' max 0) \$ map (f 1) [1 .. 999999]

I tried using 'foldl' max ' instead of 'maximum' because I thought that
foldl' was supposed to work better than foldl or something...I could be
confused on that point. Anyway, here's what I get...

/ _ \ /\  /\/ __(_)
/ /_\// /_/ / /  | |      GHC Interactive, version 6.4, for Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Prelude> :l ..\test.hs
Compiling Main             ( ..\test.hs, interpreted )
GHC's heap exhausted: current limit is 268435456 bytes;
Use the `-M<size>' option to increase the total heap size.

I also tried writing a simple 'max' function that I thought was tail
recursive, and that that would help. Same error. I'm finding it hard to
believe that finding a maximum of a list of a million small integers would
cause this kind of overflow...is it really that big of a problem?

Any help would be appreciated. I have a feeling I'll run into this problem
again in the future.
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