prefix operators

[Simon Marlow]

On 10/07/2010 22:02, John Meacham wrote:
> On Fri, Jul 09, 2010 at 09:33:52AM +0100, Simon Marlow wrote:
>> On 08/07/2010 09:45, John Meacham wrote:
>>> On Thu, Jul 08, 2010 at 07:09:29AM +0000, Simon Peyton-Jones wrote:
>>>> (ie as infix operators) and I have to squizzle around to re-interpret them as prefix operators.  Not very cool.  Something unified would be a Good Thing.
>>>
>>> So, after thinking about it some, I think there may be a somewhat
>>> elegant solution.
>>
>> I like the sound of it.  I put the code for the Haskell 2010 fixity
>> resolver together with a little testing framework in the haskell-prime
>> repo:
>>
>>    http://darcs.haskell.org/haskell-prime
>
>
> There is also my one-pass layout algorithm that requires no interaction
> with the parser that I believe still has promise. It was able to
> properly layout all the wild code I threw at it (all of nofib). With the
> addition of that, we may achieve the holy grail of fully independent
> lexing,layout,parsing,and fixing of haskell code, and a specification
> that has a direct correspondence to an implementable algorithm!
>
> I actually just noticed that my layout code is now implemented in ghc:
> http://hackage.haskell.org/trac/haskell-prime/wiki/AlternativeLayoutRule
> I am curious what the results will be, I know that adding pattern guards
> to it would be complicated, I will have to check out how my algorithm
> was modified.

Yes, Ian Lynagh implemented your algorithm in GHC (with several tweaks 
to implement some of the darker corner cases, I believe).  There's also 
-XAlternativeLayoutRuleTransitional but I'm not sure what that does.

There are cases that you can't reasonably handle this way, e.g.

g = (let x, y :: Int; (x,y) = (1,2) in x, 3)
f xs = [ do x | x <- xs ]

My feeling is that if we were to do layout this way it would have to be 
a simplified version of the current algorithm, so that it is easy to 
explain both to users and in the report.  Perhaps restricting the tokens 
that can prematurely end a layout context to just the important ones, 
like ) ] } 'in'.

Cheers,
	Simon