[Haskell-cafe] A bug of groupBy implement

7 Dec 2009

      Hi there:

My friend asked me a question, and i suppose he has found a bug of `groupBy'.

Here is the code piece:
...
List.groupBy (\a b -> Foreign.unsafePerformIO (Text.Printf.printf "\t%d <= %d ?: %s\n" a b (show (a<=b)) >> return (a<=b))) [7,3,5,9,6,8,3,5,4]
I have tested it in GHC 6.10.4 (Win XP) and GHC 6.8.3 (Linux), both give the wrong result (categaried):

        7 <= 3 ?: False
        3 <= 5 ?: True
        3 <= 9 ?: True
        3 <= 6 ?: True
        3 <= 8 ?: True
        3 <= 3 ?: True
        3 <= 5 ?: True
        3 <= 4 ?: True
[[7],[3,5,9,6,8,3,5,4]]

    Regards
--------------
L.Guo
2009-12-08