heath@B570:~/Sandbox/Project-Euler-Problems/haskell$ ghc four.hs 
[1 of 1] Compiling Main             ( four.hs, four.o )

four.hs:27:8:
    Couldn't match expected type `[b0]' with actual type `IO ()'
    In the return type of a call of `putStrLn'
    In the expression: putStrLn (maximum [pal i * j])
    In the expression:
      do { i <- [100 .. 999];
           j <- [100 .. 999];
           putStrLn (maximum [pal i * j]) }