From this thread: http://www.haskell.org/pipermail/cvs-ghc/2007-November/039289.html
> 2 while looking up 8.3.7, i saw this note in 8.3.6, on
> postfix operators:
>
> "(No Haskell 98 programs change their behaviour, of course.)"
>
> like so many "of course" notes, this is wrong, of course;-)
> haskell98 has seq, and that makes eta-expansion observable.
>
> $ /cygdrive/c/ghc/ghc-6.4.1/bin/ghc -e
> 'let (!) a = undefined in (undefined !) `seq` True'
> True
>
> $ /cygdrive/c/ghc/ghc-6.6.1/bin/ghc -e
> 'let (!) a = undefined in (undefined !) `seq` True'
> *** Exception: Prelude.undefined
Good point. We probably ought to make it an extension and give it a -X
flag anyway; I don't think (without having looked at the code) it should
be hard.
