Got it! Very obscure but easily fixed.
Amazingly, the WriteMutVar prim-op wasn't identified as
having side effects. As a result GHC felt free to omit a
write whose (state) result wasn't used. This meant that
a write followed immediately by raising an exception was
simply discarded! And that meant that the GHC type checker
(which uses side effects) failed in a bizarre way. But only
if you compiled the compiler with lots of -O...
Now fixed in the repository. Thanks for sticking to your guns.
Simon
> -----Original Message-----
> From: Simon Peyton-Jones
> Sent: Friday, August 20, 1999 12:55 PM
> To: 'Sven Panne'; GHC Bugs
> Subject: RE: Instance bug
>
>
> There's something very mysterious going on here. The
> booted compiler does something different to the non-booted one!
> (The latter works ok.)
>
> Will investigate, but perhaps not very fast. This isn't really
> holding you up now, right?
>
> Simon
>
>
> > -----Original Message-----
> > From: Sven Panne
> > Sent: Tuesday, August 17, 1999 8:27 PM
> > To: GHC Bugs
> > Subject: Instance bug
> >
> >
> > Here one of my favourite bugs in larger projects (IIRC, this
> > has partly been reported by me in the "Importing, hiding, and
> > exporting" thread):
> >
> > -- A.hs -------------------------------------------------
> > module A where
> > data Foo = Foo -- Typo! Forgot "deriving Show"
> > -- B.hs -------------------------------------------------
> > module B where
> > import A
> > data Bar = Bar Foo deriving Show
> > data Baz = Baz Int deriving Show
> > -- Main.hs ----------------------------------------------
> > import A
> > import B
> > main :: IO ()
> > main = print (Bar Foo, Baz 42)
> > ---------------------------------------------------------
> >
> > panne@jeanluc:~ > ghc -Wall -O -c A.hs
> > ghc: module version changed to 1; reason: no old .hi file
> > panne@jeanluc:~ > ghc -Wall -O -c B.hs
> > ghc: module version changed to 1; reason: no old .hi file
> >
> > [??? Why does B compile? And why is Baz's Show instance not
> in B.hi? ]
> >
> > panne@jeanluc:~ > ghc -Wall -O -c Main.hs
> >
> > Main.hs:4:
> > No instance for `Show Baz' arising from use of `print'
> > at Main.hs:4
> >
> > Main.hs:4:
> > No instance for `Show Bar' arising from use of `print'
> > at Main.hs:4
> >
> > This one drove me mad several times, looking at the completely
> > wrong place (i.e. B.hs). %-{
> >
> > Cheers,
> > Sven
> > --
> > Sven Panne Tel.:
> > +49/89/2178-2235
> > LMU, Institut fuer Informatik FAX :
> > +49/89/2178-2211
> > LFE Programmier- und Modellierungssprachen
> > Oettingenstr. 67
> > mailto:[EMAIL PROTECTED]
> > D-80538 Muenchen
> > http://www.pms.informatik.uni-muenchen.de/mitarbeiter/panne
> >
>
