[Haskell-cafe] Re: Lambda-case / lambda-if

[Simon Marlow]

On 06/10/2010 00:04, Max Bolingbroke wrote:
> On 5 October 2010 17:38, Henning Thielemann
> <schlepptop at henning-thielemann.de>  wrote:
>> Richard O'Keefe schrieb:
>>
>>> I'd prefer to see something like
>>>        \ 1 ->  f
>>>        | 2 ->  g
>>> but I'm sure something could be worked out.
>>
>> In order to be consistent with current case, maybe in layout mode:
>>
>> \1 ->  f
>>   2 ->  g
>>
>> and in non-layout mode
>>
>> \{1 ->  f; 2 ->  g}
>
> Duncan Coutts also suggested this possibility to me - once I saw it
> actually liked it rather better than the lambda-case stuff,
> particularly since it generalises nicely to multiple arguments. I may
> try to write a patch for this extension instead when I get some free
> time.

A slightly different suggestion from Simon PJ and myself (we agreed on 
something syntax-related :-) is the following:

   \case 1 -> f
	2 -> g

where the two-token sequence '\ case' introduces a new optional layout 
context, the body of which is exactly the same as in a case expression. 
  So you could also write

   \case { 1 -> f; 2 -> g }

if you want.  Guards are allowed of course.

The motivation for this syntax is:

  * easy to mentally parse: \ still introduces a function.
    (better than 'case of' in this respect)

  * a bit more noisy than just \:  I'm not sure what the
    ramifications of having \ introduce a layout context
    on its own would be, but I suspect there would be difficulties.
    Certainly some existing code would fail to parse, e.g.

    (case e of [] -> \x -> x+1; (x:xs) -> \x -> x+2)


Cheers,
	Simon