Question

I am currently working on Project Euler for fun, and use Haskell for practicing. However, I encountered a problem, and I cannot seem to produce a smaller example, so here are the codes (for project euler 232):

buildNum (x, y) = multNum x y 1 []
  where multNum num mul exp s = if (num > 10 ^ 100) then s else multNum nNum mul nexp ns
          where next = (num * mul) `div` exp
                ns = num:s
                top = next `mod` 10
                nexp = exp * 10
                nNum = num + nexp * top

sumBuild (x, y) = (head (buildNum (x, y))) * length (buildNum (x, y))

Please ignore bad style here:)

If I load this and run sumBuild(7, 5) here, I would get a exception:

*** Exception: Prelude.head: empty list

But if I change sumBuild to:

sumBuild (x, y) = head (buildNum (x, y))

or

sumBuild (x, y) = length (buildNum (x, y))

then it runs fine.

This is really confusing for me, since there is no side effect and two runs produce different results (at least it seems to be such a case). What is the problem here?

If someone could edit this program into a minimum working example, then I am really grateful!

Thanks for any help!

Answer 1

This is the hint:

> :t sumBuild
sumBuild :: (Int, Int) -> Int
> let a = 7 :: Int
> a > 10^100
True

As a suggestion try always explicitly state types of top level declarations. This helps to avoid such bugs.