Make an extension flag for postfix operators

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.

Trac metadata

Trac field	Value
Version	6.8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown