[Haskell-beginners] Weird compilation error while doing Euler problems?

[Daniel Fischer]

Am Dienstag, 21. Oktober 2008 05:36 schrieb Ian Duncan:
> 'm trying to compile my file that has my Euler problems in it to output the
> solution to problem four, but I'm getting a compile error. Here's my .hs
> file:
> -------------------------
> module Euler1 where
> import Data.List
> import Data.Ord
>
> main = mapM_ putStrLn problem4
>
> problem1 = foldl1' (+) $ nub $ (takeWhile (< 1000) [3,6..] ++ takeWhile (<
> 1000) [5,10..])
> problem2 = sum $ takeWhile (<= 4000000) [x | x <- fibs, even x]
> where fibs = unfoldr (\(a,b) -> Just (a,(b,a+b))) (0,1)
>
> problem3 z = maximumBy compare (filter (\x -> z `mod` x == 0) (takeWhile
> (<= ceiling (sqrt (fromIntegral z))) primes))
>
> problem4 = nub [ show $ y * z | y <- [100..999], z <- [100..999], show
> (y*z) == reverse (show $ y*z)]
>
> prime p = p `elem` primes
>
> primes = small ++ large
> where
>  1:p:candidates = roll $ mkWheel small
>  small          = [2,3,5,7]
>  large          = p : filter isPrime candidates
>  isPrime n      = all (not . divides n) $ takeWhile (\p -> p*p <= n) large
>  divides n p    = n `mod` p == 0
> mkWheel ds = foldl nextSize w0 ds
> nextSize (Wheel n rs) p =
>  Wheel (p*n) [r' | k <- [0..(p-1)], r <- rs, let r' = n*k+r, r' `mod` p /=
> 0]
> w0 = Wheel 1 [1]
> roll (Wheel n rs) = [n*k+r | k <- [0..], r <- rs]
> data Wheel = Wheel Integer [Integer]
> ------------------------
>
> Here's my output when I try to compile:
>
> ian$ ghc ~/Documents/eulerProblem1.hs -o test
> Undefined symbols:
>  "___stginit_ZCMain", referenced from:
>      ___stginit_ZCMain$non_lazy_ptr in libHSrts.a(Main.o)
>  "_ZCMain_main_closure", referenced from:
>      _ZCMain_main_closure$non_lazy_ptr in libHSrts.a(Main.o)
> ld: symbol(s) not found
> collect2: ld returned 1 exit status
>
> What's going on? I'm running GHC 6.8.3, if that helps.

Your module isn't named Main, so to produce an executable you must pass the 
flag
-main-is ModuleName
on the command line.
Another thing, it is recommendable to develop the habit of passing the --make 
flag to ghc.