patch applied (ghc): Fix a bug in alternative layout rule

[Duncan Coutts]

On Fri, 2009-11-27 at 10:28 +0000, Simon Marlow wrote:
> On 26/11/2009 11:50, Ian Lynagh wrote:
> > Wed Nov 25 15:19:01 PST 2009  Ian Lynagh<igloo at earth.li>
> >    * Fix a bug in alternative layout rule
> >
> >      M ./compiler/parser/Lexer.x +1
> 
> Great that you've implemented this.  Are you planning to do a proposal 
> for Haskell 2011?

> It doesn't particularly bother me that the simpler rule can't handle 
> some of the obscure cases.

Here's the main one it cannot cope with that bothers me:

foo x = case bar x of
  Pattern1 -> ...
  Pattern1 -> ...

  where
    baz = ...

The new layout requires it to be:

foo x = case bar x of
    Pattern1 -> ...
    Pattern1 -> ...
  where
    baz = ...

or the minimal fix:

foo x = case bar x of
  Pattern1 -> ...
  Pattern1 -> ...
 where
    baz = ...

The minimal fix offends me because it uses the 'where' at an odd column
offset. You would never deliberately write it like that, only "fix" it
to be that way.

I write quite a bit of code that uses the case patterns and where at the
first tab stop (2-spaces). It seems that many of the changes in the
standard libs are also of this pattern.


Another slightly odd one is:

     if p c then do s <- munch p; return (c:s) else pfail

which has to be

     if p c then do s <- munch p; return (c:s)
            else pfail

or presumably it'd also be ok as:

     if p c then (do s <- munch p; return (c:s)) else pfail


So I'm not convinced. If the tradeoff is between ugly code in my modules
or the status quo of ugly code in ghc's lexer/parser then I'm happy to
stick with the status quo.

Duncan