> The following program (ghc-4.02):
>
> > f :: [Int] -> Int
> > f (i:is) | trace "0" False = error []
> > f is | trace "1" False = error []
> > f is = f $ tail is
>
> > main = putStr $ show $ f [0]
>
> Outputs:
>
> 1
>
> Fail: Prelude.tail: empty list
>
> wheras it should have output:
>
> 0
> 1
>
> Fail: Prelude.tail: empty list
If I rearrange the definition of main a little:
> main = f [0] `seq` return ()
I get
> 0
> 1
> 1
> Fail: Prelude.tail: empty list
with the latest GHC, which seems ok to me.
With the original definition of main, I get a deadlock. This is due to the
locking of stdout: the putStr already has the lock on stdout when the trace
wants to write something, and everything stops. We're looking into a fix
for this.
Cheers,
Simon
