backward compatibility

Simon Marlow marlowsd at
Fri Jan 21 11:18:57 CET 2011

On 20/01/2011 20:22, Axel Simon wrote:

> In the case of the layout "bug", I think it might be worth considering
> going the other way: adjusting the standard with what ghc has always
> done. If I understand correctly, all my code using:
> foo = do
> some computation
> trace "I am here" $ do
> some more computation
> will break. I use this style of coding a lot to avoid too much
> indentation and thus I would have to enable this extension everywhere
> (and get warnings (or errors?) for older ghcs). Even if we had 2 or 3
> implementations of Haskell 2010 in a decade, then they might not have
> this extension. Furthermore, if they claim they actually do implement
> the layout extension then they still might get it wrong in some subtle
> way. An extension is never as well exercised as the non-extension part
> of the compiler. I therefore think that keeping the number of extensions
> to a minimum should be a high priority. It seems that the ghc team is
> going overboard with the amount of extensions and their granularity that
> I do not believe that there will ever be another compiler since
> implementing all these extensions is a nightmare. The road of may
> extensions is leading down the road that the Haskell standards aimed to
> avoid: having a single implementation defining what a Haskell program
> can be.
> So, again, for this particular "extension" I suggest that the layout
> rule in the standard(s) should be revised -- if I'm mistaken, this will
> not break other programs.

You've convinced me.  The benefit from "fixing" GHC in this case is 
outweighed by the cost, I think we should revert the change (or at least 
enable RelaxedLayout by default), and propose the change for Haskell 
2011/2012.  Ian, Simon, what do you think?


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