a stateful bug

Hi all,

If you execute the following commands:

$ wget http://ofb.net/~frederik/futility/futility-0.1.0.tar.gz
$ tar -xvzf futility-0.1.0.tar.gz
$ cd futility-0.1.0/             
$ ./do-build conf=1 

Then if all goes well you should see an error like this:

./Fu/MonadComp.hs:46:0:
    Duplicate instance declarations:
      ./Fu/MonadComp.hs:46:0: instance Monad (Either e)
      Imported from Control.Monad.Error:
        instance (Control.Monad.Error.Error e) => Monad (Either e)

Then, when you try to build again, after having changed nothing:

$ runghc Setup.hs build

the build suddenly succeeds!

Correct me if I'm wrong, but shouldn't a properly working compiler
either fail or succeed on a given package, and not fail one time, and
then succeed the next? I mean, this is the whole referential
transparency deal right? :)

I can reproduce this bug on ghc 6.4 and 6.4.1.20050812 on two i386
computers.

Frederik

-- 
http://ofb.net/~frederik/