[Haskell] arrows

[Sebastian Boldt]

Hello,

I am a bit confused about command stacks. I tried to run the following 
code in ghc:

mshowA :: (Arrow a, Show b) => a (b, String) String
mshowA = proc (x,s) -> returnA -< s ++ show x ++ s

f :: Arrow a => a Int String
f = proc x -> do a <- (mshowA -< x) "***"
                     returnA -< a

To my understanding (this doesn't mean anything :-) this is correct 
because (mshowA -< x) "***" should be equivalent to (mshowA -< (x,"***")),
but somehow ghc is rejecting the code with the error message "Non-empty 
command stack at command: mshowA -< x".

The arrow pre-processor from Ross Paterson compiles the code with no 
problems to ordinary haskell and the resulting code runs as expected in ghc,
so, my question is, what's wrong? Me, ghc, arrowp...

Thanks in advance for every answer or hint you can give me

...Sebastian