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

[Simon Marlow]

On 30/11/2009 17:35, Ian Lynagh wrote:
> On Sun, Nov 29, 2009 at 07:40:57PM +0000, Duncan Coutts wrote:
>>
>> 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 don't think offence due to the minimal fix is important, because in
> general one would fix it properly. In my tweaks to get validate working
> I've mostly stuck with minimal fixes so that the difference in the
> layout rule behaviours is clear.
>
>> 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.
>
> Ease of implementation is one motivation for the change, but I think
> ease of understanding/explaining the rule should be another. I'm not
> sure we have a clear winner here yet; the "parse error" case of the H98
> rule is somewhat subtle (and tricky to implement correctly!), but the
> alternative rule has a number of somewhat ad-hoc rules.
>
> What the code ends up looking like is important too, of course, but
> that is somewhat subjective.

So I think the layout rule should read something like

   A layout context can be closed by either
     (a) a token indented less than the current indentation level
     (b) a closing bracket that matches an open bracket outside the
         current layout context

where the set of brackets applying to (b) are (), [], {}, and let..in. 
In particular, not commas.

This will throw out a lot of code that currently parses, including 
Duncan's case/where example.  However, it has the distinct advantage of 
being both easy to understand and easy to implement.

Cheers,
	Simon